La place law
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Aug 19, 2012
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La Place's Law
Imagine blood flowing through a blood vessel which has a certain radius and a certain
wall thickness. The blood vessel wall is stretched as a result of the difference between the blood
pressure inside the vessel and the surrounding pressure outside the vessel. La Place's law
describes the relationship between the transmural pressure difference and the tension, radius,
and thickness of the vessel wall. Obviously, the higher the pressure difference the more tension
there will be. On the other hand, the thicker the wall the less tension there is. Also, the larger the
radius the more tension there is. These three rules culminate into one equation:
T = ( P * R ) / M
Where T is the tension in the walls, P is the pressure difference across the wall, R is the
radius of the cylinder, and M is the thickness of the wall. An example of LaPlace Law is Dilated
cardiomyopathy. In this condition heart becomes greatly distended and the radius (R) of ventricle
increases. Therefore to create the same pressure (P) during ejection of the blood much larger
Imagine blood flowing through a blood vessel which has a certain radius and a certain
wall thickness. The blood vessel wall is stretched as a result of the difference between the blood
pressure inside the vessel and the surrounding pressure outside the vessel. La Place's law
describes the relationship between the transmural pressure difference and the tension, radius,
and thickness of the vessel wall. Obviously, the higher the pressure difference the more tension
there will be. On the other hand, the thicker the wall the less tension there is. Also, the larger the
radius the more tension there is. These three rules culminate into one equation:
T = ( P * R ) / M
Where T is the tension in the walls, P is the pressure difference across the wall, R is the
radius of the cylinder, and M is the thickness of the wall. An example of LaPlace Law is Dilated
cardiomyopathy. In this condition heart becomes greatly distended and the radius (R) of ventricle
increases. Therefore to create the same pressure (P) during ejection of the blood much larger
wall tention (T) has be developed by the cardiac muscle. Thus dilated heart requires more energy
to pump the same amount of blood as compared to the heart of normal size. The new surgical
procedure, called ventricular remodeling, uses LaPlace principle to improve the function of
dilated, failing hearts.
Imagine yourself blowing a balloon. The harder you blow the higher the air pressure inside
the balloon and the higher the pressure difference between the outside and inside of the balloon
become. Since the pressure difference rises, the tension in the rubber walls of the balloon also
rises, and this is what causes the balloon to stretch. Now imagine you are blowing a balloon
which is made of much thicker rubber. Now you will notice that the balloon is harder to inflate
because more pressure difference is required to raise the tension in the walls of the balloon.
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to pump the same amount of blood as compared to the heart of normal size. The new surgical
procedure, called ventricular remodeling, uses LaPlace principle to improve the function of
dilated, failing hearts.
Imagine yourself blowing a balloon. The harder you blow the higher the air pressure inside
the balloon and the higher the pressure difference between the outside and inside of the balloon
become. Since the pressure difference rises, the tension in the rubber walls of the balloon also
rises, and this is what causes the balloon to stretch. Now imagine you are blowing a balloon
which is made of much thicker rubber. Now you will notice that the balloon is harder to inflate
because more pressure difference is required to raise the tension in the walls of the balloon.
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See other additions to this page.
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Wall Tension
Pascal's principle requires that the pressure is everywhere the same inside the balloon at equilibrium.
But examination immediately reveals that there are great differences in wall tension on different parts
of the balloon. The variation is described by Laplace's Law.
Once you have established the geometry of the balloon, then the tension, pressure and radius have a
definite relationship and could be used to measure tension or pressure. That is, if you have a gauge to
measure pressure, then you can calculate the wall tension. In the interesting experiment of putting one
end of a balloon into liquid nitrogen, you can collapse one end of it by cooling while the other end
stays essentially at its previous radius. This can be taken to imply that the pressure is not diminishing
significantly since for a given tension, the pressure is related to the radius.
Index
LaPlace's
law
concepts
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Pascal's principle requires that the pressure is everywhere the same inside the balloon at equilibrium.
But examination immediately reveals that there are great differences in wall tension on different parts
of the balloon. The variation is described by Laplace's Law.
Once you have established the geometry of the balloon, then the tension, pressure and radius have a
definite relationship and could be used to measure tension or pressure. That is, if you have a gauge to
measure pressure, then you can calculate the wall tension. In the interesting experiment of putting one
end of a balloon into liquid nitrogen, you can collapse one end of it by cooling while the other end
stays essentially at its previous radius. This can be taken to imply that the pressure is not diminishing
significantly since for a given tension, the pressure is related to the radius.
Index
LaPlace's
law
concepts
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LaPlace's Law
The larger the vessel radius, the larger the wall
tension required to withstand a given internal fluid
pressure.
For a given vessel radius and internal pressure, a
spherical vessel will have half the wall tension of a
cylindrical vessel.
Why does the wall tension increase with radius?
Index
LaPlace's
law
concepts
Balloon
example
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The larger the vessel radius, the larger the wall
tension required to withstand a given internal fluid
pressure.
For a given vessel radius and internal pressure, a
spherical vessel will have half the wall tension of a
cylindrical vessel.
Why does the wall tension increase with radius?
Index
LaPlace's
law
concepts
Balloon
example
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Why does wall tension increase with
radius?
If the upward part of the fluid pressure remains the same, then the downward
component of the wall tension must remain the same. But if the curvature is
less, then the total tension must be greater in order to get that same downward
component of tension.
Index
LaPlace's
law
concepts
Balloon
example
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Alveoli of the Lungs
Index
LaPlace's
law
concepts
Reference
Shier, et
al.
Ch 19
radius?
If the upward part of the fluid pressure remains the same, then the downward
component of the wall tension must remain the same. But if the curvature is
less, then the total tension must be greater in order to get that same downward
component of tension.
Index
LaPlace's
law
concepts
Balloon
example
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Alveoli of the Lungs
Index
LaPlace's
law
concepts
Reference
Shier, et
al.
Ch 19
The oxygen exchange in the lungs takes place
across the membranes of small balloon-like
structures called alveoli attached to the
branches of the bronchial passages. These
alveoli inflate and deflate with inhalation and
exhalation. The behavior of the alveoli is
largely dictated by LaPlace's law and surface
tension. It takes some effort to breathe in
because these tiny balloons must be inflated,
but the elastic recoil of the tiny balloons
assists us in the process of exhalation. If the
elastic recoil of the alveoli is compromised, as
in the case of emphysema, then it is difficult
to exhale forcibly.
The difficulty of inspiration during the baby's first breath is
great because all the balloons must be inflated from a
collapsed state.
Inflation of
alveoli
Respiratory System
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across the membranes of small balloon-like
structures called alveoli attached to the
branches of the bronchial passages. These
alveoli inflate and deflate with inhalation and
exhalation. The behavior of the alveoli is
largely dictated by LaPlace's law and surface
tension. It takes some effort to breathe in
because these tiny balloons must be inflated,
but the elastic recoil of the tiny balloons
assists us in the process of exhalation. If the
elastic recoil of the alveoli is compromised, as
in the case of emphysema, then it is difficult
to exhale forcibly.
The difficulty of inspiration during the baby's first breath is
great because all the balloons must be inflated from a
collapsed state.
Inflation of
alveoli
Respiratory System
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Inflating the Alveoli
Inflating the alveoli in the process of respiration requires an excess pressure
inside the alveoli relative to their surroundings. This is actually accomplished
by making the pressure in the thoracic cavity negative with respect to
atmospheric pressure.
The amount of net pressure required for inflation is dictated by the surface
tension and radii of the tiny balloon-like alveoli. During inhalation the radii of
the alveoli increase from about 0.05 mm to 0.1 mm . The normal mucous
tissue fluid surrounding the alveoli has a nominal surface tension of about 50
dynes/cm so the required net outward pressure is:
The remarkable property of
the surfactant which coats
the alveoli is that it reduces
the surface tension by a
factor of about 15 so that
the 1 mmHg pressure
differential is sufficient to
inflate the alveoli. Other
factors affecting the
remarkable efficiency of
oxygen transport across the
lung membranes is
characterized in Fick's
Law.
Index
LaPlace's
law
concepts
Reference
Shier, et
al.
Ch 19
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Inflating the alveoli in the process of respiration requires an excess pressure
inside the alveoli relative to their surroundings. This is actually accomplished
by making the pressure in the thoracic cavity negative with respect to
atmospheric pressure.
The amount of net pressure required for inflation is dictated by the surface
tension and radii of the tiny balloon-like alveoli. During inhalation the radii of
the alveoli increase from about 0.05 mm to 0.1 mm . The normal mucous
tissue fluid surrounding the alveoli has a nominal surface tension of about 50
dynes/cm so the required net outward pressure is:
The remarkable property of
the surfactant which coats
the alveoli is that it reduces
the surface tension by a
factor of about 15 so that
the 1 mmHg pressure
differential is sufficient to
inflate the alveoli. Other
factors affecting the
remarkable efficiency of
oxygen transport across the
lung membranes is
characterized in Fick's
Law.
Index
LaPlace's
law
concepts
Reference
Shier, et
al.
Ch 19
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Surfactant Role in Respiration
One of the remarkable phenomena in the
process of respiration is the role of the fluid
coating the walls of the alveoli of the lungs.
This fluid, called a surfactant, lowers the
surface tension of the balloon-like alveoli by
about a factor of 15 compared to the normal
mucous tissue fluid in which they are
immersed. There appears to be a nearly
constant amount of this surfactant per
alveolus, so that when the alveoli are deflated
it is more concentrated on the surface. Since
the surface-tension-lowering effect of the
surfactant depends on this concentration, it
diminishes the required pressure for inflation
of the alveoli at their most critical phase. For a
given surface tension, the pressure to inflate a
smaller bubble is greater. It is the surfactant
which makes possible the inflation of the
alveoli with only about 1 mmHg of pressure
excess over their surroundings. The baby's
first breath depends upon this surfactant and is
made more difficult in premature infants by
the incomplete formation of the surfactant.
Index
LaPlace's
law
concepts
Reference
Shier, et
al.
Ch 19
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One of the remarkable phenomena in the
process of respiration is the role of the fluid
coating the walls of the alveoli of the lungs.
This fluid, called a surfactant, lowers the
surface tension of the balloon-like alveoli by
about a factor of 15 compared to the normal
mucous tissue fluid in which they are
immersed. There appears to be a nearly
constant amount of this surfactant per
alveolus, so that when the alveoli are deflated
it is more concentrated on the surface. Since
the surface-tension-lowering effect of the
surfactant depends on this concentration, it
diminishes the required pressure for inflation
of the alveoli at their most critical phase. For a
given surface tension, the pressure to inflate a
smaller bubble is greater. It is the surfactant
which makes possible the inflation of the
alveoli with only about 1 mmHg of pressure
excess over their surroundings. The baby's
first breath depends upon this surfactant and is
made more difficult in premature infants by
the incomplete formation of the surfactant.
Index
LaPlace's
law
concepts
Reference
Shier, et
al.
Ch 19
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Alveoli and Exhalation
The alveoli of the lungs act much like
balloons in that there is some effort involved
to inflate them, but when the inflating
pressure is released, the recoil of the elastic
walls provides the pressure necessary to
deflate them. The lungs are suspended in the
thoracic cavity which is normally at a slight
negative pressure. When the diaphragm is
lowered, that pressure becomes more negative
and the lungs expand into the cavity. Air from
the atmosphere moves into the resulting
partial vacuum and inflates the alveoli. One is
aware of the effort, but it is not extreme as in
the case of the baby's first breath . Once the
alveoli are fully inflated, exhalation can be
accomplished by merely relaxing the
diaphragm, since the wall tension in all the
tiny alveoli will act to force the air out of
them. By forcing the diaphragm upward, we
can exhale forcefully by adding the diaphragm
effort to the recoil of the elastic alveoli. In
diseases like emphysema, the elasticity of the
alveoli is lost and exhalation becomes a
laborious process.
Index
LaPlace's
law
concepts
Reference
Shier, et
al.
Ch 19
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The alveoli of the lungs act much like
balloons in that there is some effort involved
to inflate them, but when the inflating
pressure is released, the recoil of the elastic
walls provides the pressure necessary to
deflate them. The lungs are suspended in the
thoracic cavity which is normally at a slight
negative pressure. When the diaphragm is
lowered, that pressure becomes more negative
and the lungs expand into the cavity. Air from
the atmosphere moves into the resulting
partial vacuum and inflates the alveoli. One is
aware of the effort, but it is not extreme as in
the case of the baby's first breath . Once the
alveoli are fully inflated, exhalation can be
accomplished by merely relaxing the
diaphragm, since the wall tension in all the
tiny alveoli will act to force the air out of
them. By forcing the diaphragm upward, we
can exhale forcefully by adding the diaphragm
effort to the recoil of the elastic alveoli. In
diseases like emphysema, the elasticity of the
alveoli is lost and exhalation becomes a
laborious process.
Index
LaPlace's
law
concepts
Reference
Shier, et
al.
Ch 19
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The Baby's First Breath
Everyone knows that it is much more difficult to blow up a balloon for the first
time. Why is that? For one thing, the applied pressure does not create much
tension in the walls of a small balloon to start the stretching process necessary
for inflation. According to LaPlace's law, the wall tension will be twice as
large for a balloon of twice the radius. If it takes a certain applied pressure to
overcome the elasticity of the large balloon and cause it to expand further, it
will take twice as much pressure to start to expand the smaller balloon. All this
makes it difficult for the baby to take its first breath -- all the balloons are
small! The alveoli of the lungs are collapsed in the fetus and must be inflated
in the process of inhalation. Thus the traditional spank on the bottom of the
newborn to make him/her mad enough to make the effort for the first breath.
Further difficulties are encountered by premature infants because the
surfactant fluid which coats the alveoli to give them the appropriate wall
tensions is formed in the later stages of pregnacy. Until that point, the alveoli
are coated with fluid which has essentially the surface tension of water, much
higher than that of the normal surfactant.
Index
LaPlace's
law
concepts
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Emphysema
The disease of the lungs called emphysema or chronic obstructive
pulmonary disease (COPD) results in the enlargement of the alveoli of the
lungs as some are destroyed and others either enlarge or combine. The
disease is one of the destructive effects of long-term smoking, but
sometimes occurs in non-smokers. If the normal inhalation process inflates
the alveoli to a larger radius, the implications of LaPlace's law are that the
wall must have lost much of its elasticity. Normally it would take twice the
pressure to inflate a constant tension membrane to twice its radius.
Typically, the wall tension of the healthy alveoli is determined by the
surface tension of the liquid which coats them, and with a uniform coating
(called a surfactant), they will all inflate to a similar radius. The enlarged
alveoli in the emphysema patient imply less elastic recoil during the process
of exhalation. Exhalation requires effort from the diaphragm and in
advanced stages of the disease, a patient will not be able to blow out a
match.
Index
LaPlace's
law
concepts
Reference
Canadian
Lung
Association
Everyone knows that it is much more difficult to blow up a balloon for the first
time. Why is that? For one thing, the applied pressure does not create much
tension in the walls of a small balloon to start the stretching process necessary
for inflation. According to LaPlace's law, the wall tension will be twice as
large for a balloon of twice the radius. If it takes a certain applied pressure to
overcome the elasticity of the large balloon and cause it to expand further, it
will take twice as much pressure to start to expand the smaller balloon. All this
makes it difficult for the baby to take its first breath -- all the balloons are
small! The alveoli of the lungs are collapsed in the fetus and must be inflated
in the process of inhalation. Thus the traditional spank on the bottom of the
newborn to make him/her mad enough to make the effort for the first breath.
Further difficulties are encountered by premature infants because the
surfactant fluid which coats the alveoli to give them the appropriate wall
tensions is formed in the later stages of pregnacy. Until that point, the alveoli
are coated with fluid which has essentially the surface tension of water, much
higher than that of the normal surfactant.
Index
LaPlace's
law
concepts
HyperPhysics***** Mechanics ***** Fluids R Nave
Go Back
Emphysema
The disease of the lungs called emphysema or chronic obstructive
pulmonary disease (COPD) results in the enlargement of the alveoli of the
lungs as some are destroyed and others either enlarge or combine. The
disease is one of the destructive effects of long-term smoking, but
sometimes occurs in non-smokers. If the normal inhalation process inflates
the alveoli to a larger radius, the implications of LaPlace's law are that the
wall must have lost much of its elasticity. Normally it would take twice the
pressure to inflate a constant tension membrane to twice its radius.
Typically, the wall tension of the healthy alveoli is determined by the
surface tension of the liquid which coats them, and with a uniform coating
(called a surfactant), they will all inflate to a similar radius. The enlarged
alveoli in the emphysema patient imply less elastic recoil during the process
of exhalation. Exhalation requires effort from the diaphragm and in
advanced stages of the disease, a patient will not be able to blow out a
match.
Index
LaPlace's
law
concepts
Reference
Canadian
Lung
Association
Besides the loss of elasticity of the alveolar walls, the larger size of the
compartments implies a smaller surface area for a given volume. Because
the oxygen exchange from the air to the blood is proportional to the area of
the exchange membrane, this diminishes the rate of oxygen transfer.
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Tension in Arterial Walls
The tension in the walls of arteries and veins in the human body is a classic
example of LaPlace's law. This geometrical law applied to a tube or pipe says
that for a given internal fluid pressure, the wall tension will be proportional to
the radius of the vessel.
Index
LaPlace's
law
concepts
compartments implies a smaller surface area for a given volume. Because
the oxygen exchange from the air to the blood is proportional to the area of
the exchange membrane, this diminishes the rate of oxygen transfer.
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Tension in Arterial Walls
The tension in the walls of arteries and veins in the human body is a classic
example of LaPlace's law. This geometrical law applied to a tube or pipe says
that for a given internal fluid pressure, the wall tension will be proportional to
the radius of the vessel.
Index
LaPlace's
law
concepts
The implication of this
law for the large
arteries, which have
comparable blood
pressures, is that the
larger arteries must
have stronger walls
since an artery of twice
the radius must be able
to withstand twice the
wall tension. Arteries
are reinforced by
fibrous bands to
strengthen them against
the risks of an
aneurysm. The tiny
capillaries rely on their
small size.
Demonstration with balloon
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Capillary Walls
The walls of the capillaries of the human circulatory system are so thin as to
appear transparent under a microscope, yet they withstand a pressure up to
about half of the full blood pressure. LaPlace's law gives insight into how they
are able to withstand such pressures: their small size implies that the wall
tension for a given internal pressure is much smaller than that of the larger
arteries.
Given a peak blood pressure of about 120 mmHg at the left ventricle, the
pressure at the beginning of the capillary system may be on the order of 50
mmHg. The large radii of the large arteries imply that for pressures in that
range they must have strong walls to withstand the large resulting wall tension.
The larger arteries provide much less resistance to flow than the smaller vessels
according to Poiseuille's law, and thus the drop in pressure across them is only
about half the total drop. The capillaries offer large resistances to flow, but
Index
LaPlace's
law
concepts
law for the large
arteries, which have
comparable blood
pressures, is that the
larger arteries must
have stronger walls
since an artery of twice
the radius must be able
to withstand twice the
wall tension. Arteries
are reinforced by
fibrous bands to
strengthen them against
the risks of an
aneurysm. The tiny
capillaries rely on their
small size.
Demonstration with balloon
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Capillary Walls
The walls of the capillaries of the human circulatory system are so thin as to
appear transparent under a microscope, yet they withstand a pressure up to
about half of the full blood pressure. LaPlace's law gives insight into how they
are able to withstand such pressures: their small size implies that the wall
tension for a given internal pressure is much smaller than that of the larger
arteries.
Given a peak blood pressure of about 120 mmHg at the left ventricle, the
pressure at the beginning of the capillary system may be on the order of 50
mmHg. The large radii of the large arteries imply that for pressures in that
range they must have strong walls to withstand the large resulting wall tension.
The larger arteries provide much less resistance to flow than the smaller vessels
according to Poiseuille's law, and thus the drop in pressure across them is only
about half the total drop. The capillaries offer large resistances to flow, but
Index
LaPlace's
law
concepts
don't require much strength in their walls.
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Danger of Aneurysms
The larger arteries of the body are subject to higher wall tensions than the
smaller arteries and capillaries. This wall tension follows the dictates of
LaPlace's law, a geometrical relationship which shows that the wall tension is
proportional to the radius for a given blood pressure. If an artery wall develops a
weak spot and expands as a result, it might seem that the expansion would
provide some relief, but in fact the opposite is true. In a classic "vicious cycle",
the expansion subjects the weakened wall to even more tension. The weakened
vessel may continue to expand in what is called an aneurysm. Unchecked, this
condition will lead to rupture of the vessel, so aneurysms require prompt
medical attention.
A localized weak spot in an artery might gain some temporary tension relief by
expanding toward a spherical shape, since a spherical membrane has half the
wall tension for a given radius. Minimizing membrane tension is why soap
bubbles tend to form a spherical shape. But for an expanding artery, forming a
near-spherical shape cannot be depended upon to give sufficient tension relief.
Index
LaPlace's
law
concepts
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Danger of Aneurysms
The larger arteries of the body are subject to higher wall tensions than the
smaller arteries and capillaries. This wall tension follows the dictates of
LaPlace's law, a geometrical relationship which shows that the wall tension is
proportional to the radius for a given blood pressure. If an artery wall develops a
weak spot and expands as a result, it might seem that the expansion would
provide some relief, but in fact the opposite is true. In a classic "vicious cycle",
the expansion subjects the weakened wall to even more tension. The weakened
vessel may continue to expand in what is called an aneurysm. Unchecked, this
condition will lead to rupture of the vessel, so aneurysms require prompt
medical attention.
A localized weak spot in an artery might gain some temporary tension relief by
expanding toward a spherical shape, since a spherical membrane has half the
wall tension for a given radius. Minimizing membrane tension is why soap
bubbles tend to form a spherical shape. But for an expanding artery, forming a
near-spherical shape cannot be depended upon to give sufficient tension relief.
Index
LaPlace's
law
concepts
Demonstration with balloon
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Ear and Hearing
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Ear and Hearing
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Hearing
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The Outer Ear
Sound energy spreads out from its sources. For a point source of sound, it
spreads out according to the inverse square law. For a given sound intensity, a
larger ear captures more of the wave and hence more sound energy.
The outer ear structures act as part of the ear's preamplifier to enhance the
sensitivity of hearing.
The auditory canal acts as a closed tube resonator, enhancing sounds in the
range 2-5 kiloHertz.
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Hearing
concepts
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Sound energy spreads out from its sources. For a point source of sound, it
spreads out according to the inverse square law. For a given sound intensity, a
larger ear captures more of the wave and hence more sound energy.
The outer ear structures act as part of the ear's preamplifier to enhance the
sensitivity of hearing.
The auditory canal acts as a closed tube resonator, enhancing sounds in the
range 2-5 kiloHertz.
Index
Hearing
concepts
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The Tympanic Membrane
The tympanic membrane or "eardrum" receives vibrations traveling up the
auditory canal and transfers them through the tiny ossicles to the oval window,
the port into the inner ear.
The eardrum is
some fifteen
times larger than
the oval window
of the inner ear,
giving an
amplification of
about fifteen
compared to a
case where the
sound pressure
interacted with
the oval window
alone.
The tympanic
membrane is
very thin, about
0.1 mm, but it is
resilient and
strong.(Zemlin)
Active graphic
You may reach information about the nearby structures of
the ear by clicking on the item of interest on the illustration.
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Zemlin
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The tympanic membrane or "eardrum" receives vibrations traveling up the
auditory canal and transfers them through the tiny ossicles to the oval window,
the port into the inner ear.
The eardrum is
some fifteen
times larger than
the oval window
of the inner ear,
giving an
amplification of
about fifteen
compared to a
case where the
sound pressure
interacted with
the oval window
alone.
The tympanic
membrane is
very thin, about
0.1 mm, but it is
resilient and
strong.(Zemlin)
Active graphic
You may reach information about the nearby structures of
the ear by clicking on the item of interest on the illustration.
Index
Hearing
concepts
Zemlin
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Sound Intensity
Sound intensity is defined as the sound power per unit area. The usual
context is the measurement of sound intensity in the air at a listener's
location. The basic units are watts/m
2
or watts/cm
2
. Many sound intensity
measurements are made relative to a standard threshold of hearing
intensity I0 :
The most common approach to sound intensity measurement is to use the
decibel scale:
Decibels measure the ratio of a given intensity I to the threshold of hearing
intensity , so that this threshold takes the value 0 decibels (0 dB). To assess
sound loudness, as distinct from an objective intensity measurement, the
sensitivity of the ear must be factored in.
Index
Sound level
measurement
Loudness
concepts
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Sound intensity is defined as the sound power per unit area. The usual
context is the measurement of sound intensity in the air at a listener's
location. The basic units are watts/m
2
or watts/cm
2
. Many sound intensity
measurements are made relative to a standard threshold of hearing
intensity I0 :
The most common approach to sound intensity measurement is to use the
decibel scale:
Decibels measure the ratio of a given intensity I to the threshold of hearing
intensity , so that this threshold takes the value 0 decibels (0 dB). To assess
sound loudness, as distinct from an objective intensity measurement, the
sensitivity of the ear must be factored in.
Index
Sound level
measurement
Loudness
concepts
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Sound Pressure
Since audible sound consists of pressure waves, one of the ways to
quantify the sound is to state the amount of pressure variation relative to
atmospheric pressure caused by the sound. Because of the great sensitivity
of human hearing, the threshold of hearing corresponds to a pressure
variation less than a billionth of atmospheric pressure.
The standard threshold of hearing can be stated in terms of pressure and
the sound intensity in decibels can be expressed in terms of the sound
pressure:
The pressure P here is to be understood as the amplitude of the pressure
wave. The power carried by a traveling wave is proportional to the square
of the amplitude. The factor of 20 comes from the fact that the logarithm
of the square of a quantity is equal to 2 x the logarithm of the quantity.
Since common microphones such as dynamic microphones produce a
voltage which is proportional to the sound pressure, then changes in sound
intensity incident on the microphone can be calculated from
where V1 and V2 are the measured voltage amplitudes .
Index
Sound level
measurement
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Since audible sound consists of pressure waves, one of the ways to
quantify the sound is to state the amount of pressure variation relative to
atmospheric pressure caused by the sound. Because of the great sensitivity
of human hearing, the threshold of hearing corresponds to a pressure
variation less than a billionth of atmospheric pressure.
The standard threshold of hearing can be stated in terms of pressure and
the sound intensity in decibels can be expressed in terms of the sound
pressure:
The pressure P here is to be understood as the amplitude of the pressure
wave. The power carried by a traveling wave is proportional to the square
of the amplitude. The factor of 20 comes from the fact that the logarithm
of the square of a quantity is equal to 2 x the logarithm of the quantity.
Since common microphones such as dynamic microphones produce a
voltage which is proportional to the sound pressure, then changes in sound
intensity incident on the microphone can be calculated from
where V1 and V2 are the measured voltage amplitudes .
Index
Sound level
measurement
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Threshold of Hearing
Sound level measurements in decibels are generally referenced to a
standard threshold of hearing at 1000 Hz for the human ear which can be
stated in terms of sound intensity:
or in terms of sound pressure:
This value has wide acceptance as a nominal standard threshold and
corresponds to 0 decibels. It represents a pressure change of less than one
billionth of standard atmospheric pressure. This is indicative of the
incredible sensitivity of human hearing. The actual average threshold of
hearing at 1000 Hz is more like 2.5 x 10
-12
watts/m
2
or about 4 decibels,
but zero decibels is a convenient reference. The threshold of hearing varies
with frequency, as illustrated by the measured hearing curves.
Index
Sound level
measurement
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Sound level measurements in decibels are generally referenced to a
standard threshold of hearing at 1000 Hz for the human ear which can be
stated in terms of sound intensity:
or in terms of sound pressure:
This value has wide acceptance as a nominal standard threshold and
corresponds to 0 decibels. It represents a pressure change of less than one
billionth of standard atmospheric pressure. This is indicative of the
incredible sensitivity of human hearing. The actual average threshold of
hearing at 1000 Hz is more like 2.5 x 10
-12
watts/m
2
or about 4 decibels,
but zero decibels is a convenient reference. The threshold of hearing varies
with frequency, as illustrated by the measured hearing curves.
Index
Sound level
measurement
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Threshold of Pain
The nominal dynamic range of human hearing is from the standard
threshold of hearing to the threshold of pain. A nominal figure for the
threshold of pain is 130 decibels, but that which may be considered painful
for one may be welcomed as entertainment by others. Generally, younger
persons are more tolerant of loud sounds than older persons because their
protective mechanisms are more effective. This tolerance does not make
them immune to the damage that loud sounds can produce.
Some sources quote 120 dB as the pain threshold and define the audible
sound frequency range as ending at about 20,000 Hz where the threshold
of hearing and the threshold of pain meet.
Index
Sound level
measurement
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The nominal dynamic range of human hearing is from the standard
threshold of hearing to the threshold of pain. A nominal figure for the
threshold of pain is 130 decibels, but that which may be considered painful
for one may be welcomed as entertainment by others. Generally, younger
persons are more tolerant of loud sounds than older persons because their
protective mechanisms are more effective. This tolerance does not make
them immune to the damage that loud sounds can produce.
Some sources quote 120 dB as the pain threshold and define the audible
sound frequency range as ending at about 20,000 Hz where the threshold
of hearing and the threshold of pain meet.
Index
Sound level
measurement
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Loudness
Loudness is not simply sound intensity!
Sound loudness is a subjective term describing the strength of the ear's
perception of a sound. It is intimately related to sound intensity but can by no
means be considered identical to intensity. The sound intensity must be
factored by the ear's sensitivity to the particular frequencies contained in the
sound. This is the kind of information contained in equal loudness curves for
the human ear. It must also be considered that the ear's response to increasing
sound intensity is a "power of ten" or logarithmic relationship. This is one of
the motivations for using the decibel scale to measure sound intensity. A
general "rule of thumb" for loudness is that the power must be increased by
about a factor of ten to sound twice as loud. To more realistically assess sound
loudness, the ear's sensitivity curves are factored in to produce a phon scale for
loudness. The factor of ten rule of thumb can then be used to produce the sone
scale of loudness. In practical sound level measurement, filter contours such as
the A, B, and C contours are used to make the measuring instrument more
nearly approximate the ear.
Index
Loudness
concepts
Hearing
concepts
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"Rule of Thumb" for Loudness
A widely used "rule of thumb" for the loudness of a particular sound is that the
sound must be increased in intensity by a factor of ten for the sound to be
perceived as twice as loud. A common way of stating it is that it takes 10 violins
to sound twice as loud as one violin. Another way to state the rule is to say that
the loudness doubles for every 10 phon increase in the sound loudness level.
Although this rule is widely used, it must be emphasized that it is an
approximate general statement based upon a great deal of investigation of
average human hearing but it is not to be taken as a hard and fast rule.
Index
Loudness
concepts
Hearing
concepts
Loudness is not simply sound intensity!
Sound loudness is a subjective term describing the strength of the ear's
perception of a sound. It is intimately related to sound intensity but can by no
means be considered identical to intensity. The sound intensity must be
factored by the ear's sensitivity to the particular frequencies contained in the
sound. This is the kind of information contained in equal loudness curves for
the human ear. It must also be considered that the ear's response to increasing
sound intensity is a "power of ten" or logarithmic relationship. This is one of
the motivations for using the decibel scale to measure sound intensity. A
general "rule of thumb" for loudness is that the power must be increased by
about a factor of ten to sound twice as loud. To more realistically assess sound
loudness, the ear's sensitivity curves are factored in to produce a phon scale for
loudness. The factor of ten rule of thumb can then be used to produce the sone
scale of loudness. In practical sound level measurement, filter contours such as
the A, B, and C contours are used to make the measuring instrument more
nearly approximate the ear.
Index
Loudness
concepts
Hearing
concepts
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"Rule of Thumb" for Loudness
A widely used "rule of thumb" for the loudness of a particular sound is that the
sound must be increased in intensity by a factor of ten for the sound to be
perceived as twice as loud. A common way of stating it is that it takes 10 violins
to sound twice as loud as one violin. Another way to state the rule is to say that
the loudness doubles for every 10 phon increase in the sound loudness level.
Although this rule is widely used, it must be emphasized that it is an
approximate general statement based upon a great deal of investigation of
average human hearing but it is not to be taken as a hard and fast rule.
Index
Loudness
concepts
Hearing
concepts
Why is it that doubling the sound intensity to the ear does not produce a
dramatic increase in loudness? We cannot give answers with complete
confidence, but it appears that there are saturation effects. Nerve cells have
maximum rates at which they can fire, and it appears that doubling the sound
energy to the sensitive inner ear does not double the strength of the nerve signal
to the brain. This is just a model, but it seems to correlate with the general
observations which suggest that something like ten times the intensity is
required to double the signal from the innner ear.
One difficulty with this "rule of thumb" for loudness is that it is applicable only
to adding loudness for identical sounds. If a second sound is widely enough
separated in frequency to be outside the critical band of the first, then this rule
does not apply at all.
While not a precise rule even for the increase of the same sound, the rule has
considerable utility along with the just noticeable difference in sound intensity
when judging the significance of changes in sound level.
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dramatic increase in loudness? We cannot give answers with complete
confidence, but it appears that there are saturation effects. Nerve cells have
maximum rates at which they can fire, and it appears that doubling the sound
energy to the sensitive inner ear does not double the strength of the nerve signal
to the brain. This is just a model, but it seems to correlate with the general
observations which suggest that something like ten times the intensity is
required to double the signal from the innner ear.
One difficulty with this "rule of thumb" for loudness is that it is applicable only
to adding loudness for identical sounds. If a second sound is widely enough
separated in frequency to be outside the critical band of the first, then this rule
does not apply at all.
While not a precise rule even for the increase of the same sound, the rule has
considerable utility along with the just noticeable difference in sound intensity
when judging the significance of changes in sound level.
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Adding Loudness
When one sound is produced and another sound is added, the increase in
loudness perceived depends upon its frequency relation to the first sound.
Insight into this process can be obtained from the place theory of pitch
perception. If the second sound is widely separated in pitch from the first, then
they do not compete for the same nerve endings on the basilar membrane of the
inner ear. Adding a second sound of equal loudness yields a total sound about
twice as loud. But if the two sounds are close together in frequency, within a
critical band, then the saturation effects in the organ of Corti are such that the
perceived combined loudness is only slightly greater than either sound alone.
This is the condition which leads to the commonly used rule of thumb for
loudness addition.
Index
Loudness
concepts
Hearing
concepts
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When one sound is produced and another sound is added, the increase in
loudness perceived depends upon its frequency relation to the first sound.
Insight into this process can be obtained from the place theory of pitch
perception. If the second sound is widely separated in pitch from the first, then
they do not compete for the same nerve endings on the basilar membrane of the
inner ear. Adding a second sound of equal loudness yields a total sound about
twice as loud. But if the two sounds are close together in frequency, within a
critical band, then the saturation effects in the organ of Corti are such that the
perceived combined loudness is only slightly greater than either sound alone.
This is the condition which leads to the commonly used rule of thumb for
loudness addition.
Index
Loudness
concepts
Hearing
concepts
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Critical Band
When two sounds of equal loudness when sounded separately are close
together in pitch, their combined loudness when sounded together will be only
slightly louder than one of them alone. They may be said to be in the same
critical band where they are competing for the same nerve endings on the
basilar membrane of the inner ear. According the the place theory of pitch
perception, sounds of a given frequency will excite the nerve cells of the
organ of Corti only at a specific place. The available receptors show saturation
effects which lead to the general rule of thumb for loudness by limiting the
increase in neural response.
If the two sounds are widely separated in pitch, the perceived loudness of the
combined tones will be considerably greater because they do not overlap on
the basilar membrane and compete for the same hair cells. The phenomenon
of the critical band has been widely investigated.
Backus reports that this critical band is about 90 Hz wide for sounds below
200 Hz and increases to about 900 Hz for frequencies around 5000 Hertz. It is
suggested that this corresponds to a roughly constant length on the basilar
membrane of length about 1.2 mm and involving some 1300 hair cells. If the
tones are far apart in frequency (not within a critical band), the combined
sound may be perceived as twice as loud as one alone.
Illustration of critical band
Index
Hearing
concepts
References
Rossing,
Science of
Sound
Backus
Zwicker, et
al.
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When two sounds of equal loudness when sounded separately are close
together in pitch, their combined loudness when sounded together will be only
slightly louder than one of them alone. They may be said to be in the same
critical band where they are competing for the same nerve endings on the
basilar membrane of the inner ear. According the the place theory of pitch
perception, sounds of a given frequency will excite the nerve cells of the
organ of Corti only at a specific place. The available receptors show saturation
effects which lead to the general rule of thumb for loudness by limiting the
increase in neural response.
If the two sounds are widely separated in pitch, the perceived loudness of the
combined tones will be considerably greater because they do not overlap on
the basilar membrane and compete for the same hair cells. The phenomenon
of the critical band has been widely investigated.
Backus reports that this critical band is about 90 Hz wide for sounds below
200 Hz and increases to about 900 Hz for frequencies around 5000 Hertz. It is
suggested that this corresponds to a roughly constant length on the basilar
membrane of length about 1.2 mm and involving some 1300 hair cells. If the
tones are far apart in frequency (not within a critical band), the combined
sound may be perceived as twice as loud as one alone.
Illustration of critical band
Index
Hearing
concepts
References
Rossing,
Science of
Sound
Backus
Zwicker, et
al.
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Critical Band Measurement
For low frequencies the critical band is about 90 Hz wide. For higher
frequencies, it is between a whole tone and 1/3 octave wide.
Center
Freq (Hz)
Critical
bandwidth (Hz)
100 90
200 90
500 110
1000 150
2000 280
5000 700
10000 1200
Rossing 2nd Ed p74
Index
Hearing
concepts
Reference
Rossing,
Science of
Sound
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Pure Tone Audiometry
The testing of hearing is most often carried out by establishing the threshold of
hearing, the softest sound which can be perceived in a controlled environment.
It is typical to do this testing with pure tones by providing calibrated tones to a
person via earphones, allowing that person to increase the level until it can just
be heard. Various strategies are used, but pure tone audiometry with tones
starting at about 125 Hz and increasing by octaves, half-octaves, or third-
octaves to about 8000 Hz is typical. Hearing tests of right and left ears are
generally done independently. The results of such tests are summarized in
audiograms.
Index
Hearing
concepts
Dangers
of Loud
Sounds
For low frequencies the critical band is about 90 Hz wide. For higher
frequencies, it is between a whole tone and 1/3 octave wide.
Center
Freq (Hz)
Critical
bandwidth (Hz)
100 90
200 90
500 110
1000 150
2000 280
5000 700
10000 1200
Rossing 2nd Ed p74
Index
Hearing
concepts
Reference
Rossing,
Science of
Sound
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Pure Tone Audiometry
The testing of hearing is most often carried out by establishing the threshold of
hearing, the softest sound which can be perceived in a controlled environment.
It is typical to do this testing with pure tones by providing calibrated tones to a
person via earphones, allowing that person to increase the level until it can just
be heard. Various strategies are used, but pure tone audiometry with tones
starting at about 125 Hz and increasing by octaves, half-octaves, or third-
octaves to about 8000 Hz is typical. Hearing tests of right and left ears are
generally done independently. The results of such tests are summarized in
audiograms.
Index
Hearing
concepts
Dangers
of Loud
Sounds
Audiograms compare
hearing to the normal
threshold of hearing, which
varies with frequency as
illustrated by the hearing
curves. The audiogram is
normalized to the hearing
curve so that a straight
horizontal line at 0 represents
normal hearing.
Click on illustration for further details.
Hearing loss
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Audiogram Showing Presbycusis
The progressive loss of high frequency sensitivity with aging is typical, and
is called presbycusis. The loss of the high frequencies can make it difficult to
understand speech, since the intelligible differences in speech sounds are
often in the range above 2000 Hz.
When hearing
aids are used, it
is important to
amplify the
high
frequencies,
since it is
uncommon for
Speak up! Quit mumbling!
Older persons may have difficulty understanding speech
clearly because of progressive loss of high frequency
hearing.
Index
Hearing
concepts
References
Nave &
Nave
Ch. 18
Backus
Ch. 5
hearing to the normal
threshold of hearing, which
varies with frequency as
illustrated by the hearing
curves. The audiogram is
normalized to the hearing
curve so that a straight
horizontal line at 0 represents
normal hearing.
Click on illustration for further details.
Hearing loss
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Audiogram Showing Presbycusis
The progressive loss of high frequency sensitivity with aging is typical, and
is called presbycusis. The loss of the high frequencies can make it difficult to
understand speech, since the intelligible differences in speech sounds are
often in the range above 2000 Hz.
When hearing
aids are used, it
is important to
amplify the
high
frequencies,
since it is
uncommon for
Speak up! Quit mumbling!
Older persons may have difficulty understanding speech
clearly because of progressive loss of high frequency
hearing.
Index
Hearing
concepts
References
Nave &
Nave
Ch. 18
Backus
Ch. 5
there to be
significant loss
at low
frequencies.
Audiograms are
important for
the prescribing
of hearing aids.
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significant loss
at low
frequencies.
Audiograms are
important for
the prescribing
of hearing aids.
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Audiograms Showing Hearing Loss
Audiograms can help with the diagnosis of various types of hearing
disorders. Specific geometries of curves are found to be typical of
presbycusis, and a characteristic notch in the hearing curve may be the
signature of damage by a sudden loud sound like a gunshot or a firecracker
explosion close to the ear.
The curves are
normalized so
that a straight
horizontal line
represents equal
loudness.
Index
Hearing
concepts
References
Nave &
Nave
Ch. 18
Backus
Ch. 5
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Audiograms can help with the diagnosis of various types of hearing
disorders. Specific geometries of curves are found to be typical of
presbycusis, and a characteristic notch in the hearing curve may be the
signature of damage by a sudden loud sound like a gunshot or a firecracker
explosion close to the ear.
The curves are
normalized so
that a straight
horizontal line
represents equal
loudness.
Index
Hearing
concepts
References
Nave &
Nave
Ch. 18
Backus
Ch. 5
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Hearing Loss
Hearing loss is typically described as being conductive, sensorineural, or
mixed.
Conductive hearing loss refers to an impairment of one's ability to conduct
airborne sound through the middle ear to the inner ear. Scar tissue or
otosclerosis, the abnormal growth of bone within the middle ear, can lead to
restricted movement of the ossicles. Recently it has been shown that there can
also be conductive problems with the basilar membrane of the inner ear that
reduce the efficiency of energy transfer to the hair cells (Holt).
Sensorineural hearing loss refers to impairment of the sensory unit
consisting of the auditory nerve and the hair cells that excite it.
Sometimes the distinction between these two types of hearing loss can be
made with a simple tuning fork test. If the tuning fork cannot be heard when
sounded in air, then the base of the tuning fork is placed against the hard bone
behind the ear. If the person can now hear it by conduction through the bone,
then conductive hearing loss is indicated. It in cannot be heard by either air or
bone conduction, then sensorineural loss is indicated.
Hearing Loss
0 to -15 dBNormal range
-16 to -40 dBMinimal loss
-26 to -15 dBMild loss
-41 to -55 dBModerate loss
-56 to -70 dBModerate/severe loss
-71 to -90 dBSevere loss
> -91 dB Profound loss
American Speech and Hearing
Association
The "power of ten" or logarithmic
nature of hearing response is
evident in the fact that a loss in
sensitivity by a factor of 10,000, or
-40 decibels, is still at the edge of
"minimal loss". By the admittedly
simplistic "rule of thumb" for
loudness, this -40dB sound would
still be 1/16 as loud as the 0 dB
reference. 0 dB in this table
represents the normal hearing
threshold, or 0 dB Hearing Level.
The categories of hearing loss are
based on measurements at 500,
1000 and 2000 Hz.
Assessment of hearing lossHearing Aids
Index
Hearing
concepts
Dangers of
Loud
Sounds
Reference
Holt
ASHA
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Hearing loss is typically described as being conductive, sensorineural, or
mixed.
Conductive hearing loss refers to an impairment of one's ability to conduct
airborne sound through the middle ear to the inner ear. Scar tissue or
otosclerosis, the abnormal growth of bone within the middle ear, can lead to
restricted movement of the ossicles. Recently it has been shown that there can
also be conductive problems with the basilar membrane of the inner ear that
reduce the efficiency of energy transfer to the hair cells (Holt).
Sensorineural hearing loss refers to impairment of the sensory unit
consisting of the auditory nerve and the hair cells that excite it.
Sometimes the distinction between these two types of hearing loss can be
made with a simple tuning fork test. If the tuning fork cannot be heard when
sounded in air, then the base of the tuning fork is placed against the hard bone
behind the ear. If the person can now hear it by conduction through the bone,
then conductive hearing loss is indicated. It in cannot be heard by either air or
bone conduction, then sensorineural loss is indicated.
Hearing Loss
0 to -15 dBNormal range
-16 to -40 dBMinimal loss
-26 to -15 dBMild loss
-41 to -55 dBModerate loss
-56 to -70 dBModerate/severe loss
-71 to -90 dBSevere loss
> -91 dB Profound loss
American Speech and Hearing
Association
The "power of ten" or logarithmic
nature of hearing response is
evident in the fact that a loss in
sensitivity by a factor of 10,000, or
-40 decibels, is still at the edge of
"minimal loss". By the admittedly
simplistic "rule of thumb" for
loudness, this -40dB sound would
still be 1/16 as loud as the 0 dB
reference. 0 dB in this table
represents the normal hearing
threshold, or 0 dB Hearing Level.
The categories of hearing loss are
based on measurements at 500,
1000 and 2000 Hz.
Assessment of hearing lossHearing Aids
Index
Hearing
concepts
Dangers of
Loud
Sounds
Reference
Holt
ASHA
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Hearing Aids
Sometimes a satisfactory level of hearing can be restored by a hearing aid -
a combination of a microphone to sense ambient sound, an amplifier, and a
tiny speaker that projects the amplified sound into the ear canal. A typical
modern hearing aid would employ an electret condenser microphone - small
and rugged with a high signal-to-noise ratio. The frequency range of
application is typically 100-10,000 Hz. While some assistance may be
rendered by bone conduction, this discussion will be limited to hearing aids
that operate by sounds produced in the air.
Wearing Styles
ITEIn-the-ear
BTE
Behind-the-
ear
ITCIn-the-canal
CIC
Completely
in-the-canal
Body
Worn on
body
(profound
loss)
A basic hearing aid may be called a linear circuit aid,
implying that it simply amplifies any ambient sound
that reaches it. It is important for such a hearing aid
to contour the amplification to the nature of the
hearing loss of the individual, which typically means
amplifying high frequencies more than low
frequencies. Presbycusis, the progressive loss of high
frequency hearing with age, often calls for
amplification of high frequencies with little or no
bass boost. A basic hearing aid may have three
frequency bands to permit the amplification to be
adjusted based on the audiogram.
The next step up in sophistication of the hearing aid would be to employ
some kind of audio "compression". Compression implies the adjustment of
the "gain" or degree of amplification based on the input level, it being a
practical fact that louder sounds wouldn't need as much amplification. This
compression would reduce the amplification for loud sounds either at the
microphone end or at the speaker end. Some types of compression are called
"adaptive compression" in that some logic is used to compress some kinds
of sounds more than others.
For those hearing aids that use adaptive compression, but not digital logic,
some are classified under the headings ASP and K-AMP circuits. The ASP
Index
Hearing
concepts
Dangers of
Loud
Sounds
References
Holt
Goldenberg
Sometimes a satisfactory level of hearing can be restored by a hearing aid -
a combination of a microphone to sense ambient sound, an amplifier, and a
tiny speaker that projects the amplified sound into the ear canal. A typical
modern hearing aid would employ an electret condenser microphone - small
and rugged with a high signal-to-noise ratio. The frequency range of
application is typically 100-10,000 Hz. While some assistance may be
rendered by bone conduction, this discussion will be limited to hearing aids
that operate by sounds produced in the air.
Wearing Styles
ITEIn-the-ear
BTE
Behind-the-
ear
ITCIn-the-canal
CIC
Completely
in-the-canal
Body
Worn on
body
(profound
loss)
A basic hearing aid may be called a linear circuit aid,
implying that it simply amplifies any ambient sound
that reaches it. It is important for such a hearing aid
to contour the amplification to the nature of the
hearing loss of the individual, which typically means
amplifying high frequencies more than low
frequencies. Presbycusis, the progressive loss of high
frequency hearing with age, often calls for
amplification of high frequencies with little or no
bass boost. A basic hearing aid may have three
frequency bands to permit the amplification to be
adjusted based on the audiogram.
The next step up in sophistication of the hearing aid would be to employ
some kind of audio "compression". Compression implies the adjustment of
the "gain" or degree of amplification based on the input level, it being a
practical fact that louder sounds wouldn't need as much amplification. This
compression would reduce the amplification for loud sounds either at the
microphone end or at the speaker end. Some types of compression are called
"adaptive compression" in that some logic is used to compress some kinds
of sounds more than others.
For those hearing aids that use adaptive compression, but not digital logic,
some are classified under the headings ASP and K-AMP circuits. The ASP
Index
Hearing
concepts
Dangers of
Loud
Sounds
References
Holt
Goldenberg
units monitor incoming sounds and automatically change the gain, output
and frequency response. The K-AMP approach detects and amplifies only
quiet sounds while leaving louder ones unaltered.
Currently under very active development are the digital programmable
hearing aids that use a digital signal processor (dsp). They can be
programmed to more nearly fit the detailed needs of an individual user and
open the door to more sophisticated approaches to assisting the user. Since
the understanding of human speech is often the highest priority, and since
speech has identifiable characteristics like vocal formants, some steps can
be made to program the hearing aid to amplify speech sounds more than
some distinctly different other types of sounds. A friend with a digital
hearing aid told me something like "I leaned over an expressway bridge and
listened to the traffic noise. After a short time there was a kind of burbling
sound like the hearing aid was trying to make voices out of this sound." An
intriguing idea, that we might get enough sophistication into hearing aids to
recognize and selectively amplify the sounds of meaningful human
communication.
Another approach to hearing assistance is the cochlear implant. Currently
very expensive and in the experimental stage, it is one of the future
possibilities.
(Tal Berkowitz is acknowledged for investigative work on this topic.)
Assessment of hearing loss
HyperPhysics***** Sound
R
Nave
Go Back
and frequency response. The K-AMP approach detects and amplifies only
quiet sounds while leaving louder ones unaltered.
Currently under very active development are the digital programmable
hearing aids that use a digital signal processor (dsp). They can be
programmed to more nearly fit the detailed needs of an individual user and
open the door to more sophisticated approaches to assisting the user. Since
the understanding of human speech is often the highest priority, and since
speech has identifiable characteristics like vocal formants, some steps can
be made to program the hearing aid to amplify speech sounds more than
some distinctly different other types of sounds. A friend with a digital
hearing aid told me something like "I leaned over an expressway bridge and
listened to the traffic noise. After a short time there was a kind of burbling
sound like the hearing aid was trying to make voices out of this sound." An
intriguing idea, that we might get enough sophistication into hearing aids to
recognize and selectively amplify the sounds of meaningful human
communication.
Another approach to hearing assistance is the cochlear implant. Currently
very expensive and in the experimental stage, it is one of the future
possibilities.
(Tal Berkowitz is acknowledged for investigative work on this topic.)
Assessment of hearing loss
HyperPhysics***** Sound
R
Nave
Go Back
Sensitivity of Human Ear
The human ear can respond to minute pressure variations in the air if they are in
the audible frequency range, roughly 20 Hz - 20 kHz.
It is capable of detecting pressure variations of less than one billionth of
atmospheric pressure. The threshold of hearing corresponds to air vibrations on
the order of a tenth of an atomic diameter. This incredible sensitivity is
enhanced by an effective amplification of the sound signal by the outer and
middle ear structures. Contributing to the wide dynamic range of human hearing
are protective mechanisms that reduce the ear's response to very loud sounds.
Sound intensities over this wide range are usually expressed in decibels.
Index
Hearing
concepts
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The human ear can respond to minute pressure variations in the air if they are in
the audible frequency range, roughly 20 Hz - 20 kHz.
It is capable of detecting pressure variations of less than one billionth of
atmospheric pressure. The threshold of hearing corresponds to air vibrations on
the order of a tenth of an atomic diameter. This incredible sensitivity is
enhanced by an effective amplification of the sound signal by the outer and
middle ear structures. Contributing to the wide dynamic range of human hearing
are protective mechanisms that reduce the ear's response to very loud sounds.
Sound intensities over this wide range are usually expressed in decibels.
Index
Hearing
concepts
HyperPhysics***** Sound R Nave
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Dynamic Range of Hearing
In addition to its remarkable sensitivity, the human ear is capable of
responding to the widest range of stimuli of any of the senses. The practical
dynamic range could be said to be from the threshold of hearing to the
threshold of pain:
Threshold of
Hearing
Threshold of Pain
I0 10
13
I0 = 10,000,000,000,000 I0
0 decibels 130 decibels
This remarkable dynamic range is enhanced by an effective amplification
structure which extends its low end and by a protective mechanism which
extends the high end.
Dynamic levels of music
Index
Hearing
concepts
HyperPhysics***** Sound R Nave
Go Back
In addition to its remarkable sensitivity, the human ear is capable of
responding to the widest range of stimuli of any of the senses. The practical
dynamic range could be said to be from the threshold of hearing to the
threshold of pain:
Threshold of
Hearing
Threshold of Pain
I0 10
13
I0 = 10,000,000,000,000 I0
0 decibels 130 decibels
This remarkable dynamic range is enhanced by an effective amplification
structure which extends its low end and by a protective mechanism which
extends the high end.
Dynamic levels of music
Index
Hearing
concepts
HyperPhysics***** Sound R Nave
Go Back
Pitch Resolution
The extremely small size of the cochlea and the
extremely high resolution of human pitch
perception cast doubt on the sufficiency of the
place theory to completely account for the human
ear's pitch resolution. Some typical data:
Cochlea:
turns,
about 3.2 cm length.
Resolves about 1500 separate pitches
with 16,000-20,000 hair cells.
This would require a separate detectable pitch for every 0.002 cm, which is
physically unreasonable for a simple peaking action on the membrane.
The normal human ear can detect the difference between 440 Hz and 441 Hz.
It is hard to believe it could attain such resolution from selective peaking of
the membrane vibrations. Some pitch sharpening mechanism must be
operating.
Index
Hearing
concepts
Place
theory
concepts
HyperPhysics***** Sound R Nave
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The extremely small size of the cochlea and the
extremely high resolution of human pitch
perception cast doubt on the sufficiency of the
place theory to completely account for the human
ear's pitch resolution. Some typical data:
Cochlea:
turns,
about 3.2 cm length.
Resolves about 1500 separate pitches
with 16,000-20,000 hair cells.
This would require a separate detectable pitch for every 0.002 cm, which is
physically unreasonable for a simple peaking action on the membrane.
The normal human ear can detect the difference between 440 Hz and 441 Hz.
It is hard to believe it could attain such resolution from selective peaking of
the membrane vibrations. Some pitch sharpening mechanism must be
operating.
Index
Hearing
concepts
Place
theory
concepts
HyperPhysics***** Sound R Nave
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The structures of the outer and middle ear contribute to both the remarkable
sensitivity and the wide dynamic range of human hearing. They can be
considered to be both a pre-amplifier and a limiter for the human hearing
process.
The outer ear
(pinna) collects
more sound
energy than the
ear canal
would receive
without it and
thus
contributes
some area
amplification.
The numbers
here are just
representative .
.. not precise
data.
Closed tube
resonance of
the auditory
canal
enhances
2000-5000 Hz
Tympanic
membrane
(eardrum) has some
15x area of oval
window
contributing an area
amplification.
Ossicles (hammer,
anvil and stirrup)
contribute a lever-
type amplification
when listening to
soft sounds.
Outer ear
2x
Tympanic
membrane
15x
Ossicles
3x
The outer and middle ears contribute something like a factor of 100 or about
20 decibels of amplification under optimum conditions.
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound
R
Nave
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sensitivity and the wide dynamic range of human hearing. They can be
considered to be both a pre-amplifier and a limiter for the human hearing
process.
The outer ear
(pinna) collects
more sound
energy than the
ear canal
would receive
without it and
thus
contributes
some area
amplification.
The numbers
here are just
representative .
.. not precise
data.
Closed tube
resonance of
the auditory
canal
enhances
2000-5000 Hz
Tympanic
membrane
(eardrum) has some
15x area of oval
window
contributing an area
amplification.
Ossicles (hammer,
anvil and stirrup)
contribute a lever-
type amplification
when listening to
soft sounds.
Outer ear
2x
Tympanic
membrane
15x
Ossicles
3x
The outer and middle ears contribute something like a factor of 100 or about
20 decibels of amplification under optimum conditions.
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound
R
Nave
Go Back
Audible Sound
Usually "sound" is used to mean sound which can be perceived by the human
ear, i.e., "sound" refers to audible sound unless otherwise classified. A
reasonably standard definition of audible sound is that it is a pressure wave
with frequency between 20 Hz and 20,000 Hz and with an intensity above the
standard threshold of hearing. Since the ear is surrounded by air, or perhaps
under water, the sound waves are constrained to be longitudinal waves.
Normal ranges of sound pressure and sound intensity may also be specified.
Frequency: 20 Hz - 20,000 Hz(corresponds with pitch)
Intensity: 10
-12
- 10 watts/m
2
(0 to 130 decibels)
Pressure: 2 x 10
-5
- 60 Newtons/m
2
2 x 10
-10
- .0006 atmospheres
For an air temperature of 20°C where the sound speed is 344 m/s, the audible
sound waves have wavelengths from 0.0172 m (0.68 inches) to 17.2 meters
(56.4 feet).
Ultrasonic sound
Index
Hearing
concepts
HyperPhysics***** Sound R Nave
Go Back
Usually "sound" is used to mean sound which can be perceived by the human
ear, i.e., "sound" refers to audible sound unless otherwise classified. A
reasonably standard definition of audible sound is that it is a pressure wave
with frequency between 20 Hz and 20,000 Hz and with an intensity above the
standard threshold of hearing. Since the ear is surrounded by air, or perhaps
under water, the sound waves are constrained to be longitudinal waves.
Normal ranges of sound pressure and sound intensity may also be specified.
Frequency: 20 Hz - 20,000 Hz(corresponds with pitch)
Intensity: 10
-12
- 10 watts/m
2
(0 to 130 decibels)
Pressure: 2 x 10
-5
- 60 Newtons/m
2
2 x 10
-10
- .0006 atmospheres
For an air temperature of 20°C where the sound speed is 344 m/s, the audible
sound waves have wavelengths from 0.0172 m (0.68 inches) to 17.2 meters
(56.4 feet).
Ultrasonic sound
Index
Hearing
concepts
HyperPhysics***** Sound R Nave
Go Back
In response to sustained loud sounds, muscle tension tightens the tympanic
membrane and, acting through the tendon connecting the hammer and anvil,
repositions the ossicles to pull the stirrup back, lessening the transfer of force to
the oval window of the inner ear. This contributes to the ear's wide dynamic
range.
The stapedius muscle and the tensor tympani muscle act in response to loud
sounds.(DeBonis & Donohue)
More detail
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
DeBonis &
Donohue
HyperPhysics***** Sound Go Back
membrane and, acting through the tendon connecting the hammer and anvil,
repositions the ossicles to pull the stirrup back, lessening the transfer of force to
the oval window of the inner ear. This contributes to the ear's wide dynamic
range.
The stapedius muscle and the tensor tympani muscle act in response to loud
sounds.(DeBonis & Donohue)
More detail
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
DeBonis &
Donohue
HyperPhysics***** Sound Go Back
Loud Sound Response
In response to loud
sounds, the tensor
tympani muscle
tightens the eardrum
and through the
tendon between the
hammer and anvil and
shifts the stirrup
backward from the
oval window of the
inner ear. This shifting
of the ossicles reduces
the transmitted force
to the inner ear,
protecting it.
However, it is a
relatively slow action
and cannot protect the
ear from sudden loud
sounds like a gunshot.
The process is less
effective in older ears.
Dynamic levels of music
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound Go Back
In response to loud
sounds, the tensor
tympani muscle
tightens the eardrum
and through the
tendon between the
hammer and anvil and
shifts the stirrup
backward from the
oval window of the
inner ear. This shifting
of the ossicles reduces
the transmitted force
to the inner ear,
protecting it.
However, it is a
relatively slow action
and cannot protect the
ear from sudden loud
sounds like a gunshot.
The process is less
effective in older ears.
Dynamic levels of music
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound Go Back
Young and Old Ears
A young person's ear
can provide a limited
amount of protection
from sustained loud
sounds by shifting the
stirrup backward so
that it doesn't exert as
much force on the oval
window. In the very
young, the stirrup is
thought to be capable
of actually breaking
contact with the oval
window, breaking the
direct link to the inner
ear. In an older ear,
the structures become
stiffer and cannot
adjust backward as
much. Older persons
are generally less
tolerant of loud
sounds.
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound Go Back
A young person's ear
can provide a limited
amount of protection
from sustained loud
sounds by shifting the
stirrup backward so
that it doesn't exert as
much force on the oval
window. In the very
young, the stirrup is
thought to be capable
of actually breaking
contact with the oval
window, breaking the
direct link to the inner
ear. In an older ear,
the structures become
stiffer and cannot
adjust backward as
much. Older persons
are generally less
tolerant of loud
sounds.
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound Go Back
In response to sustained loud sounds, muscle tension tightens the tympanic
membrane and, acting through the tendon connecting the hammer and anvil,
repositions the ossicles to pull the stirrup back, lessening the transfer of force to
the oval window of the inner ear. This contributes to the ear's wide dynamic
range.
The stapedius muscle and the tensor tympani muscle act in response to loud
sounds.(DeBonis & Donohue)
More detail
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
DeBonis &
Donohue
HyperPhysics***** Sound Go Back
membrane and, acting through the tendon connecting the hammer and anvil,
repositions the ossicles to pull the stirrup back, lessening the transfer of force to
the oval window of the inner ear. This contributes to the ear's wide dynamic
range.
The stapedius muscle and the tensor tympani muscle act in response to loud
sounds.(DeBonis & Donohue)
More detail
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
DeBonis &
Donohue
HyperPhysics***** Sound Go Back
Loud Sound Response
In response to loud
sounds, the tensor
tympani muscle
tightens the eardrum
and through the
tendon between the
hammer and anvil and
shifts the stirrup
backward from the
oval window of the
inner ear. This shifting
of the ossicles reduces
the transmitted force
to the inner ear,
protecting it.
However, it is a
relatively slow action
and cannot protect the
ear from sudden loud
sounds like a gunshot.
The process is less
effective in older ears.
Dynamic levels of music
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound Go Back
In response to loud
sounds, the tensor
tympani muscle
tightens the eardrum
and through the
tendon between the
hammer and anvil and
shifts the stirrup
backward from the
oval window of the
inner ear. This shifting
of the ossicles reduces
the transmitted force
to the inner ear,
protecting it.
However, it is a
relatively slow action
and cannot protect the
ear from sudden loud
sounds like a gunshot.
The process is less
effective in older ears.
Dynamic levels of music
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound Go Back
Young and Old Ears
A young person's ear
can provide a limited
amount of protection
from sustained loud
sounds by shifting the
stirrup backward so
that it doesn't exert as
much force on the oval
window. In the very
young, the stirrup is
thought to be capable
of actually breaking
contact with the oval
window, breaking the
direct link to the inner
ear. In an older ear,
the structures become
stiffer and cannot
adjust backward as
much. Older persons
are generally less
tolerant of loud
sounds.
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound Go Back
A young person's ear
can provide a limited
amount of protection
from sustained loud
sounds by shifting the
stirrup backward so
that it doesn't exert as
much force on the oval
window. In the very
young, the stirrup is
thought to be capable
of actually breaking
contact with the oval
window, breaking the
direct link to the inner
ear. In an older ear,
the structures become
stiffer and cannot
adjust backward as
much. Older persons
are generally less
tolerant of loud
sounds.
Index
Hearing
concepts
Reference
Stevens &
Warshofsky
HyperPhysics***** Sound Go Back
Spectral Colors
In a rainbow or the separation of colors by a prism we see the continuous
range of spectral colors (the visible spectrum). A spectral color is composed of
a single wavelength and can be correlated with wavelength as shown in the
chart below ( a general guide and not a precise statement about color). It is safe
enough to say that monochromatic light like the helium-neon laser is red (632
nm) or that the 3-2 transition from the hydrogen spectrum is red ( 656 nm)
because they fall in the appropriate wavelength range. But most colored
objects give off a range of wavelengths and the characterization of color is
much more than the statement of wavelength. Perceived colors can be mapped
on a chromaticity diagram.
Index
Vision
concepts
Color
vision
Visible
spectrum
HyperPhysics***** Light and Vision R Nave
Go Back
In a rainbow or the separation of colors by a prism we see the continuous
range of spectral colors (the visible spectrum). A spectral color is composed of
a single wavelength and can be correlated with wavelength as shown in the
chart below ( a general guide and not a precise statement about color). It is safe
enough to say that monochromatic light like the helium-neon laser is red (632
nm) or that the 3-2 transition from the hydrogen spectrum is red ( 656 nm)
because they fall in the appropriate wavelength range. But most colored
objects give off a range of wavelengths and the characterization of color is
much more than the statement of wavelength. Perceived colors can be mapped
on a chromaticity diagram.
Index
Vision
concepts
Color
vision
Visible
spectrum
HyperPhysics***** Light and Vision R Nave
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Color
It is common practice to define pure colors in terms of the wavelengths of light
as shown. This works well for spectral colors but it is found that many
different combinations of light wavelengths can produce the same perception
of color.
This progression from left to right is from long wavelength to short
wavelength, and from low frequency to high frequency light. The wavelengths
are commonly expressed in nanometers (1 nm = 10
-9
m). The visible spectrum
is roughly from 700 nm (red end) to 400 nm (violet end). The letter I in the
sequence above is for indigo - no longer commonly used as a color name. It is
included above strictly for the reason of making the sequence easier to say as a
mnemonic, like a person's name: Roy G. Biv - a tradition in the discussion of
color.
The inherently distinguishable characteristics of color are hue, saturation, and
brightness. Color measurement systems characterize colors in various
parameters which relate to hue, saturation, and brightness. They include the
subjective Munsell and Ostwald systems and the quantitative CIE color
system.
White light, or nearly white light from the Sun, contains a continuous
distribution of wavelengths. The light from the Sun is essentially that of a
blackbody radiator at 5780 K. The wavelengths (spectral colors) of white light
can be separated by a dispersive medium like a prism. Even more effective
separation can be achieved with a diffraction grating.
Index
Vision
concepts
Color
vision
Visible
spectrum
Go Back
It is common practice to define pure colors in terms of the wavelengths of light
as shown. This works well for spectral colors but it is found that many
different combinations of light wavelengths can produce the same perception
of color.
This progression from left to right is from long wavelength to short
wavelength, and from low frequency to high frequency light. The wavelengths
are commonly expressed in nanometers (1 nm = 10
-9
m). The visible spectrum
is roughly from 700 nm (red end) to 400 nm (violet end). The letter I in the
sequence above is for indigo - no longer commonly used as a color name. It is
included above strictly for the reason of making the sequence easier to say as a
mnemonic, like a person's name: Roy G. Biv - a tradition in the discussion of
color.
The inherently distinguishable characteristics of color are hue, saturation, and
brightness. Color measurement systems characterize colors in various
parameters which relate to hue, saturation, and brightness. They include the
subjective Munsell and Ostwald systems and the quantitative CIE color
system.
White light, or nearly white light from the Sun, contains a continuous
distribution of wavelengths. The light from the Sun is essentially that of a
blackbody radiator at 5780 K. The wavelengths (spectral colors) of white light
can be separated by a dispersive medium like a prism. Even more effective
separation can be achieved with a diffraction grating.
Index
Vision
concepts
Color
vision
Visible
spectrum
Go Back
HyperPhysics***** Light and Vision R Nave
Refraction of Light
Refraction is the bending of a wave when it enters a medium where it's speed
is different. The refraction of light when it passes from a fast medium to a
slow medium bends the light ray toward the normal to the boundary between
the two media. The amount of bending depends on the indices of refraction of
the two media and is described quantitatively by Snell's Law.
Refraction is
responsible for
image
formation by
lenses and the
eye.
As the speed of light is reduced in the slower medium, the wavelength is
shortened proportionately. The frequency is unchanged; it is a characteristic of
the source of the light and unaffected by medium changes.
Refraction and the eyeRefraction of sound
Refraction of light by water
Index
Lens
concepts
HyperPhysics***** Light and Vision R Nave
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Refraction of Light
Refraction is the bending of a wave when it enters a medium where it's speed
is different. The refraction of light when it passes from a fast medium to a
slow medium bends the light ray toward the normal to the boundary between
the two media. The amount of bending depends on the indices of refraction of
the two media and is described quantitatively by Snell's Law.
Refraction is
responsible for
image
formation by
lenses and the
eye.
As the speed of light is reduced in the slower medium, the wavelength is
shortened proportionately. The frequency is unchanged; it is a characteristic of
the source of the light and unaffected by medium changes.
Refraction and the eyeRefraction of sound
Refraction of light by water
Index
Lens
concepts
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Index of Refraction
The index of refraction is defined as the speed of light in vacuum divided by
the speed of light in the medium.
The indices of refraction of some common substances are given below with a
more complete description of the indices for optical glasses given elsewhere.
The values given are approximate and do not account for the small variation of
index with light wavelength which is called dispersion.
Refraction and the eyeRefraction of sound
Table of refractive indices
Index
Lens
concepts
HyperPhysics***** Light and Vision R Nave
Go Back
The index of refraction is defined as the speed of light in vacuum divided by
the speed of light in the medium.
The indices of refraction of some common substances are given below with a
more complete description of the indices for optical glasses given elsewhere.
The values given are approximate and do not account for the small variation of
index with light wavelength which is called dispersion.
Refraction and the eyeRefraction of sound
Table of refractive indices
Index
Lens
concepts
HyperPhysics***** Light and Vision R Nave
Go Back
Snell's Law
Snell's Law relates the indices of refraction n of the two media to the
directions of propagation in terms of the angles to the normal. Snell's law can
be derived from Fermat's Principle or from the Fresnel Equations.
Enter data below, then click the symbol of the quantity you wish to calculate.
Indices of refraction:
=
=
Angles with surface normal:
= °
= °
Enter data and then click on the symbol for the quantity you wish to calculate
in the active equation above. The numbers will not be forced to be consistent
until you click on the quantity to calculate. Indices of refraction must be
greater than or equal to 1, so values less than 1 do not represent a physically
possible system.
If the incident medium has the larger index of refraction, then the angle with
the normal is increased by refraction. The larger index medium is commonly
called the "internal" medium, since air with n=1 is usually the surrounding or
"external" medium. You can calculate the condition for total internal reflection
by setting the refracted angle = 90° and calculating the incident angle. Since
you can't refract the light by more than 90°, all of it will reflect for angles of
incidence greater than the angle which gives refraction at 90°.
Index
Lens
concepts
HyperPhysics***** Light and Vision R Nave
Go Back
Snell's Law relates the indices of refraction n of the two media to the
directions of propagation in terms of the angles to the normal. Snell's law can
be derived from Fermat's Principle or from the Fresnel Equations.
Enter data below, then click the symbol of the quantity you wish to calculate.
Indices of refraction:
=
=
Angles with surface normal:
= °
= °
Enter data and then click on the symbol for the quantity you wish to calculate
in the active equation above. The numbers will not be forced to be consistent
until you click on the quantity to calculate. Indices of refraction must be
greater than or equal to 1, so values less than 1 do not represent a physically
possible system.
If the incident medium has the larger index of refraction, then the angle with
the normal is increased by refraction. The larger index medium is commonly
called the "internal" medium, since air with n=1 is usually the surrounding or
"external" medium. You can calculate the condition for total internal reflection
by setting the refracted angle = 90° and calculating the incident angle. Since
you can't refract the light by more than 90°, all of it will reflect for angles of
incidence greater than the angle which gives refraction at 90°.
Index
Lens
concepts
HyperPhysics***** Light and Vision R Nave
Go Back
Heat Transfer
The transfer of heat is normally from a high temperature object to a lower
temperature object. Heat transfer changes the internal energy of both systems
involved according to the First Law of Thermodynamics.
Heat transfer from a cold to a hotter region
Radiation cooling time
Index
HyperPhysics***** Thermodynamics R Nave
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The transfer of heat is normally from a high temperature object to a lower
temperature object. Heat transfer changes the internal energy of both systems
involved according to the First Law of Thermodynamics.
Heat transfer from a cold to a hotter region
Radiation cooling time
Index
HyperPhysics***** Thermodynamics R Nave
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Heat Conduction
Conduction is heat transfer by means of molecular agitation within a material
without any motion of the material as a whole. If one end of a metal rod is at a
higher temperature, then energy will be transferred down the rod toward the
colder end because the higher speed particles will collide with the slower ones
with a net transfer of energy to the slower ones. For heat transfer between two
plane surfaces, such as heat loss through the wall of a house, the rate of
conduction heat transfer is:
Calculation
= heat transferred in time =
= thermal conductivity of the barrier
= area
= temperature
= thickness of barrier
Thermal conductivity table
Discussion of thermal conductivity
Home heat loss by conduction.
Index
Heat
transfer
concepts
Heat
transfer
examples
HyperPhysics***** Thermodynamics R Nave
Go Back
Conduction is heat transfer by means of molecular agitation within a material
without any motion of the material as a whole. If one end of a metal rod is at a
higher temperature, then energy will be transferred down the rod toward the
colder end because the higher speed particles will collide with the slower ones
with a net transfer of energy to the slower ones. For heat transfer between two
plane surfaces, such as heat loss through the wall of a house, the rate of
conduction heat transfer is:
Calculation
= heat transferred in time =
= thermal conductivity of the barrier
= area
= temperature
= thickness of barrier
Thermal conductivity table
Discussion of thermal conductivity
Home heat loss by conduction.
Index
Heat
transfer
concepts
Heat
transfer
examples
HyperPhysics***** Thermodynamics R Nave
Go Back
Heat Convection
Convection is heat transfer by mass motion of a fluid such as air or water
when the heated fluid is caused to move away from the source of heat,
carrying energy with it. Convection above a hot surface occurs because hot air
expands, becomes less dense, and rises (see Ideal Gas Law). Hot water is
likewise less dense than cold water and rises, causing convection currents
which transport energy.
Convection can also lead to
circulation in a liquid, as in
the heating of a pot of water
over a flame. Heated water
expands and becomes more
buoyant. Cooler, more dense
water near the surface
descends and patterns of
circulation can be formed,
though they will not be as
regular as suggested in the
drawing.
Convection cells are visible in the
heated cooking oil in the pot at left.
Heating the oil produces changes in
the index of refraction of the oil,
making the cell boundaries visible.
Circulation patterns form, and
presumably the wall-like structures
visible are the boundaries between
the circulation patterns.
Index
Heat
transfer
concepts
Heat
transfer
examples
Convection is heat transfer by mass motion of a fluid such as air or water
when the heated fluid is caused to move away from the source of heat,
carrying energy with it. Convection above a hot surface occurs because hot air
expands, becomes less dense, and rises (see Ideal Gas Law). Hot water is
likewise less dense than cold water and rises, causing convection currents
which transport energy.
Convection can also lead to
circulation in a liquid, as in
the heating of a pot of water
over a flame. Heated water
expands and becomes more
buoyant. Cooler, more dense
water near the surface
descends and patterns of
circulation can be formed,
though they will not be as
regular as suggested in the
drawing.
Convection cells are visible in the
heated cooking oil in the pot at left.
Heating the oil produces changes in
the index of refraction of the oil,
making the cell boundaries visible.
Circulation patterns form, and
presumably the wall-like structures
visible are the boundaries between
the circulation patterns.
Index
Heat
transfer
concepts
Heat
transfer
examples
Convection is thought to play a
major role in transporting energy
from the center of the Sun to the
surface, and in movements of the
hot magma beneath the surface of
the earth. The visible surface of
the Sun (the photosphere) has a
granular appearance with a
typical dimension of a granule
being 1000 kilometers. The
image at right is from the NASA
Solar Physics website and is
credited to G. Scharmer and the
Swedish Vacuum Solar
Telescope. The granules are
described as convection cells
which transport heat from the
interior of the Sun to the surface.
In ordinary heat transfer on the Earth, it is difficult to quantify the effects of
convection since it inherently depends upon small nonuniformities in an
otherwise fairly homogeneous medium. In modeling things like the cooling of
the human body, we usually just lump it in with conduction.
HyperPhysics***** Thermodynamics R Nave
Go Back
major role in transporting energy
from the center of the Sun to the
surface, and in movements of the
hot magma beneath the surface of
the earth. The visible surface of
the Sun (the photosphere) has a
granular appearance with a
typical dimension of a granule
being 1000 kilometers. The
image at right is from the NASA
Solar Physics website and is
credited to G. Scharmer and the
Swedish Vacuum Solar
Telescope. The granules are
described as convection cells
which transport heat from the
interior of the Sun to the surface.
In ordinary heat transfer on the Earth, it is difficult to quantify the effects of
convection since it inherently depends upon small nonuniformities in an
otherwise fairly homogeneous medium. In modeling things like the cooling of
the human body, we usually just lump it in with conduction.
HyperPhysics***** Thermodynamics R Nave
Go Back
Greenhouse Effect
The greenhouse effect refers to circumstances where the short wavelengths of
visible light from the sun pass through a transparent medium and are absorbed,
but the longer wavelengths of the infrared re-radiation from the heated objects
are unable to pass through that medium. The trapping of the long wavelength
radiation leads to more heating and a higher resultant temperature. Besides the
heating of an automobile by sunlight through the windshield and the namesake
example of heating the greenhouse by sunlight passing through sealed,
transparent windows, the greenhouse effect has been widely used to describe
the trapping of excess heat by the rising concentration of carbon dioxide in the
atmosphere. The carbon dioxide strongly absorbs infrared and does not allow
as much of it to escape into space.
Sunlight warms your
car
Increasing atmospheric
carbon dioxide
Global warming
Role in the absence of
water on Venus?
A major part of the efficiency of the heating of an actual greenhouse is the
trapping of the air so that the energy is not lost by convection. Keeping the hot
air from escaping out the top is part of the practical "greenhouse effect", but it
is common usage to refer to the infrared trapping as the "greenhouse effect" in
atmospheric applications where the air trapping is not applicable.
Index
HyperPhysics***** Thermodynamics R Nave
Go Back
The greenhouse effect refers to circumstances where the short wavelengths of
visible light from the sun pass through a transparent medium and are absorbed,
but the longer wavelengths of the infrared re-radiation from the heated objects
are unable to pass through that medium. The trapping of the long wavelength
radiation leads to more heating and a higher resultant temperature. Besides the
heating of an automobile by sunlight through the windshield and the namesake
example of heating the greenhouse by sunlight passing through sealed,
transparent windows, the greenhouse effect has been widely used to describe
the trapping of excess heat by the rising concentration of carbon dioxide in the
atmosphere. The carbon dioxide strongly absorbs infrared and does not allow
as much of it to escape into space.
Sunlight warms your
car
Increasing atmospheric
carbon dioxide
Global warming
Role in the absence of
water on Venus?
A major part of the efficiency of the heating of an actual greenhouse is the
trapping of the air so that the energy is not lost by convection. Keeping the hot
air from escaping out the top is part of the practical "greenhouse effect", but it
is common usage to refer to the infrared trapping as the "greenhouse effect" in
atmospheric applications where the air trapping is not applicable.
Index
HyperPhysics***** Thermodynamics R Nave
Go Back
Greenhouse Effect Example
Bright sunlight will effectively warm your car on a cold, clear day by the
greenhouse effect. The longer infrared wavelengths radiated by sun-warmed
objects do not pass readily through the glass. The entrapment of this energy
warms the interior of the vehicle. The trapping of the hot air so that it cannot
rise and lose the energy by convection also plays a major role.
Short wavelengths
of visible light are
readily transmitted
through the
transparent
windshield.
(Otherwise you
wouldn't be able to
see through it!)
Shorter wavelengths of ultraviolet light are largely blocked by glass since
they have greater quantum energies which have absorption mechanisms in
the glass. Even though you may be uncomfortably warm with bright sunlight
streaming through, you will not be sunburned.
Index
Blackbody
radiation
concepts
HyperPhysics***** Thermodynamics
R
Nave
Go Back
Bright sunlight will effectively warm your car on a cold, clear day by the
greenhouse effect. The longer infrared wavelengths radiated by sun-warmed
objects do not pass readily through the glass. The entrapment of this energy
warms the interior of the vehicle. The trapping of the hot air so that it cannot
rise and lose the energy by convection also plays a major role.
Short wavelengths
of visible light are
readily transmitted
through the
transparent
windshield.
(Otherwise you
wouldn't be able to
see through it!)
Shorter wavelengths of ultraviolet light are largely blocked by glass since
they have greater quantum energies which have absorption mechanisms in
the glass. Even though you may be uncomfortably warm with bright sunlight
streaming through, you will not be sunburned.
Index
Blackbody
radiation
concepts
HyperPhysics***** Thermodynamics
R
Nave
Go Back
Increase in Greenhouse Gases
The increase in the concentration of carbon dioxide, one of the three major
atmospheric contributers to the greenhouse effect has been carefully
documented at the Mauna Loa Observatory in Hawaii. The 1990 rate of increase
was about 0.4% per year. The interesting cyclic variations represent the
reduction in carbon dioxide by photosynthesis during the growing season in the
northern hemisphere.
Current analysis suggests that the combustion of fossil fuels is a major
contributer to the increase in the carbon dioxide concentration, such
contributions being 2 to 5 times the effect of deforestation (Kraushaar &
Ristinen).
Increase in Atmospheric Carbon Dioxide
The Mauna Loa monitoring station reports the carbon dioxide level in the
atmosphere today as about 380 parts per million compared to 315 ppm in 1958
when modern measurements were initiated. Measurements of air bubbles
trapped in the Greenland ice sheet indicate concentrations of 270 ppm in
preindustrial times.
Index
References
Kraushaar
& Ristinen
Trefil
The increase in the concentration of carbon dioxide, one of the three major
atmospheric contributers to the greenhouse effect has been carefully
documented at the Mauna Loa Observatory in Hawaii. The 1990 rate of increase
was about 0.4% per year. The interesting cyclic variations represent the
reduction in carbon dioxide by photosynthesis during the growing season in the
northern hemisphere.
Current analysis suggests that the combustion of fossil fuels is a major
contributer to the increase in the carbon dioxide concentration, such
contributions being 2 to 5 times the effect of deforestation (Kraushaar &
Ristinen).
Increase in Atmospheric Carbon Dioxide
The Mauna Loa monitoring station reports the carbon dioxide level in the
atmosphere today as about 380 parts per million compared to 315 ppm in 1958
when modern measurements were initiated. Measurements of air bubbles
trapped in the Greenland ice sheet indicate concentrations of 270 ppm in
preindustrial times.
Index
References
Kraushaar
& Ristinen
Trefil
These are sketches of
the graphs produced in
the IPCC 2007 report
of the increase in key
greenhouse gases. They
make clear that most of
the increase of the last
thousand years has
occurred in the past 200
years. The radiative
forcing of these gases is
related to their
concentration .
HyperPhysics***** Thermodynamics R Nave
Go Back
the graphs produced in
the IPCC 2007 report
of the increase in key
greenhouse gases. They
make clear that most of
the increase of the last
thousand years has
occurred in the past 200
years. The radiative
forcing of these gases is
related to their
concentration .
HyperPhysics***** Thermodynamics R Nave
Go Back
Contributers to Greenhouse Effect
Those gas molecules in the Earth's atmosphere with three or more atoms are
called "greenhouse gases" because they can capture outgoing infrared energy
from the Earth, thereby warming the planet. The greenhouse gases include
water vapor with three atoms (H2O), ozone (O3), carbon dioxide (CO2), and
methane (CH4). Also, trace quantities of chloro-fluoro-carbons (CFC's) can
have a disproportionately large effect.
To attempt to quantify the effects of greenhouse gases on the global
temperature, climatologists use the "radiative forcing" of the current
atmospheric content of these gases.
Increase in greenhouse gasesGreenhouse effect
Index
Reference
Kraushaar
& Ristinen
HyperPhysics***** Thermodynamics
R
Nave
Go Back
Those gas molecules in the Earth's atmosphere with three or more atoms are
called "greenhouse gases" because they can capture outgoing infrared energy
from the Earth, thereby warming the planet. The greenhouse gases include
water vapor with three atoms (H2O), ozone (O3), carbon dioxide (CO2), and
methane (CH4). Also, trace quantities of chloro-fluoro-carbons (CFC's) can
have a disproportionately large effect.
To attempt to quantify the effects of greenhouse gases on the global
temperature, climatologists use the "radiative forcing" of the current
atmospheric content of these gases.
Increase in greenhouse gasesGreenhouse effect
Index
Reference
Kraushaar
& Ristinen
HyperPhysics***** Thermodynamics
R
Nave
Go Back
Global Warming
An issue of major concern is the possible effect of the burning of fossil fuels
and other contributers to the increase of carbon dioxide in the atmosphere. The
action of carbon dioxide and other greenhouse gases in trapping infrared
radiation is called the greenhouse effect. It may measurably increase the overall
average temperature of the Earth, which could have disastrous consequences.
Sometimes the effects of the greenhouse effect are stated in terms of the albedo
of the Earth, the overall average reflection coefficient.
This graphic of the global air temperature was posted by Phil Jones on behalf of
the Climatic Research Unit, UK. The key reference used was Brohan, et al.
Another depiction of the mean temperatures in the northern hemisphere was
drawn from NOAA.
Index
References
Kraushaar
& Ristinen
Brohan, et
al.
Schneider
An issue of major concern is the possible effect of the burning of fossil fuels
and other contributers to the increase of carbon dioxide in the atmosphere. The
action of carbon dioxide and other greenhouse gases in trapping infrared
radiation is called the greenhouse effect. It may measurably increase the overall
average temperature of the Earth, which could have disastrous consequences.
Sometimes the effects of the greenhouse effect are stated in terms of the albedo
of the Earth, the overall average reflection coefficient.
This graphic of the global air temperature was posted by Phil Jones on behalf of
the Climatic Research Unit, UK. The key reference used was Brohan, et al.
Another depiction of the mean temperatures in the northern hemisphere was
drawn from NOAA.
Index
References
Kraushaar
& Ristinen
Brohan, et
al.
Schneider
Essentially any kind of tabulation you access will tell the same story. The
temperature has gradually risen over the last 150 years.
Because the potential consequences of global warming in terms of loss of snow
cover, sea level rise, change in weather patterns, etc are so great, it is a major
societal concern. On the other hand, proposed measures to reduce human
contributions to greenhouse gases can also have great consequences. The large
potential impact combined with the ambiguities of the science has given rise to
many passionate extremes.
Stephen Schneider of Stanford seems to me to be one of the more balanced
temperature has gradually risen over the last 150 years.
Because the potential consequences of global warming in terms of loss of snow
cover, sea level rise, change in weather patterns, etc are so great, it is a major
societal concern. On the other hand, proposed measures to reduce human
contributions to greenhouse gases can also have great consequences. The large
potential impact combined with the ambiguities of the science has given rise to
many passionate extremes.
Stephen Schneider of Stanford seems to me to be one of the more balanced
voices. His website is a good source for relevant data. He discusses the
problems in the context of the Earth's energy balance and the changes in the
concentrations of greenhouse gases.
Increase in greenhouse gasesGreenhouse effect
Modeling the human impact on global worming
Skeptical views of global warming
Longer term temperature variations
HyperPhysics***** Thermodynamics R Nave
Go Back
problems in the context of the Earth's energy balance and the changes in the
concentrations of greenhouse gases.
Increase in greenhouse gasesGreenhouse effect
Modeling the human impact on global worming
Skeptical views of global warming
Longer term temperature variations
HyperPhysics***** Thermodynamics R Nave
Go Back
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