Audition by Muwanguzi David, Guluvarsity

About the mechanism of hearing

1. Audition I and II Dr. Ahaisibwe Mackline

2. objectives Nature/ production/movement/speed/periodicity/tone of sound Characteristics of periodic sound waves Sound characteristics as related to the human ear Measurement of sound intensities Range of sound intensities within human comfortable hearing External and middle ear functions

3. Sound production Sound is a form of energy which makes us hear Each sound is special to the object that produces it Every sound has a unique quality which distinguishes it from the rest of the sounds Sound is produced when an object vibrates i.e produced by vibrating objects Energy required to make an object vibrate is provided by an outside source like a vibrating bell, sound is produced by ringing the bell which shakes back and forth, sound by the school bell is produced by vibration of iron plate when hit by a hammer, sound produced while talking is due to vibration of vocal cords

4. Sound produced by the larynx Human beings produce sound using the voice box/larynx Larynx contains 2 ligaments called vocal cords They are attached to muscles which change tension by stretching and creating distance between the cords When in resting state the cords are relaxed and separated and loose so that air from the lungs passes without producing any sound When talking, the muscles of vocal cords contract, the 2 cords become stretched and close together leaving only a narrow slit between them Lungs pass current between them, air makes the vocal cords vibrate hence sound production

5. Pitch of sound When muscles attached to the vocal cords contract and stretch, vocal cords become tight and thin, high frequency sound is produced when they relax the vocal cords become loose and thick, low sound frequency is produced. vocal cords of a man are approximately 25mm long and female 15mm long hence higher pitch

6. Propagation of sound Sound is the sensation produced when longitudinal vibrations of the molecules in the external environment strike the tympanic membrane. When an object moves forth and back in air, molecules of air close to the object vibrate at the same frequency. these pass their motion to the next layer of air molecules which also start to vibrate When the vibrating air molecules fall on our ears, ear feels the vibration as sound A plot of these movements as changes in pressure on the tympanic membrane per unit of time is a series of waves; such movements in the environment are generally called sound waves The waves travel through air at a speed of approximately 344 m/s at 20 °C at sea level. The speed of sound increases with temperature and with altitude.

7. cont The speed of sound is 1450 m/s at 20 °C in fresh water and is even greater in salt water. About 5000m/s in solids It is said that the whistle of the blue whale is as loud as 188 decibels and is audible for 500 miles Note, light movement is 300,000,000m/s

8. Characteristics of sound The loudness of a sound is correlated with the amplitude of a sound wave and its pitch with the frequency (number of waves per unit of time). Brain interprets frequency detected by the ear as a subjective or perceived quality called pitch The greater the amplitude, the louder the sound; and the greater the frequency, the higher the pitch. Sound waves that have repeating patterns, even though the individual waves are complex, are perceived as musical sounds ; aperiodic nonrepeating vibrations cause a sensation of noise . Most musical sounds are made up of a wave with a primary frequency that determines the pitch of the sound plus a number of harmonic vibrations (overtones) that give the sound its characteristic timbre (quality). Single frequency gives pure tone

9. Sound intensity and decibel Variations in timbre permit us to identify the sounds of the various musical instruments even though they are playing notes of the same pitch Sound waves carry energy with them as the move from one place to another Intensity of sound is the energy transported past a given area in a given unit of time When amplitude increases, the energy of the wave increases and therefore intensity is also larger As the wave moves away from source its intensity decreases because the wave is spreading out into larger surface area The amplitude of a sound wave can be expressed in terms of the maximum pressure change at the eardrum, but a relative scale is more convenient. The decibel scale is such a scale. The intensity of a sound in bels is the logarithm of the ratio of the intensity of that sound and a standard sound. A decibel (dB) is 0.1 bel.

10. The standard sound reference level adopted by the Acoustical Society of America corresponds to 0 dB at a pressure level of 0.000204 × dyne/cm2 , a value that is just at the auditory threshold for the average human. A value of 0 dB does not mean the absence of sound but a sound level of an intensity equal to that of the standard.

11. A range of 120 to 160 dB ( eg , firearms, jackhammer, jet plane on take off) is classified as painful; 90 to 110 dB ( eg , subway, bass drum, chain saw, lawn mower) is classified as extremely high; 60 to 80 dB ( eg , alarm clock, busy traffic, dishwasher, conversation) is classified as very loud; 40 to 50 dB ( eg , moderate rainfall, normal room noise) is moderate; 30 dB ( eg , whisper, library) is faint

12. Audible and inaudible sounds The sound frequencies audible to humans range from about 20 to a maximum of 20,000 cycles per second (Hz).( audio spectrum ) Infrasonic sounds are those below normal range of normal hearing and they occur below 20Hz, ultrasonic frequencies occur above the range of normal human hearing which is above 20kHz In bats and dogs, much higher frequencies are audible. The threshold of the human ear varies with the pitch of the sound, the greatest sensitivity being in the 1000- to 4000-Hz range. The pitch of the average male voice in conversation is about 120 Hz and that of the average female voice about 250 Hz. The number of pitches that can be distinguished by an average individual is about 2000, but trained musicians can improve on this figure considerably. Pitch discrimination is best in the 1000- to 3000-Hz range and is poor at high and low pitches

13. masking The presence of one sound decreases an individual’s ability to hear other sounds, a phenomenon known as masking . It is believed to be due to the relative or absolute refractoriness of previously stimulated auditory receptors and nerve fibers to other stimuli. The degree to which a given tone masks others is related to its pitch

14. SOUND TRANSMISSION The ear converts sound waves in the external environment into action potentials in the auditory nerves. The waves are transformed by the eardrum and auditory ossicles into movements of the foot plate of the stapes. These movements set up

15. EXTERNAL & MIDDLE EAR The external ear funnels sound waves to the external auditory meatus. From the external auditory meatus, sound waves pass inward to the tympanic membrane (eardrum). The middle ear is an air-filled cavity in the temporal bone that opens via the auditory (eustachian) tube into the nasopharynx and through the oropharynx to the exterior.

16. The tube is usually closed, but during swallowing, chewing, and yawning it opens, keeping the air pressure on the two sides of the eardrum equalized. The three auditory ossicles, the malleus, incus, and stapes, are located in the middle ear. The manubrium (handle of the malleus) is attached to the back of the tympanic membrane. Its head is attached to the wall of the middle ear, and its short process is attached to the incus, which in turn articulates with the head of the stapes.

17. The stapes is named for its resemblance to a stirrup. Its foot plate is attached by an annular ligament to the walls of the oval window. Two small skeletal muscles, the tensor tympani and the stapedius, are also located in the middle ear The contraction of the tensor tympani pulls the manubrium of the malleus medially and decreases the vibrations of the tympanic membrane; contraction of the stapedius pulls the foot plate of the stapes out of the oval window.

18. Attenuation of Sound by Contraction of the Tensor Tympani and Stapedius Muscles. When loud sounds are transmitted through the ossicular system and from there into the central nervous system, a reflex occurs after a latent period of only 40 to 80 milliseconds to cause contraction of the stapedius muscle and, to a lesser extent, the tensor tympani muscle. The tensor tympani muscle pulls the handle of the malleus inward while the stapedius muscle pulls the stapes outward. These two forces oppose each other and thereby cause the entire ossicular system to develop increased rigidity, thus greatly reducing the ossicular conduction of low-frequency sound, mainly frequencies below 1000 cycles per second

19. This attenuation reflex can reduce the intensity of lower-frequency sound transmission by 30 to 40 decibels, which is about the same difference as that between a loud voice and a whisper. The function of this mechanism is believed to be twofold: 1. To protect the cochlea from damaging vibrations caused by excessively loud sound. 2. To mask low-frequency sounds in loud environments. This usually removes a major share of the background noise and allows a person to concentrate on sounds above 1000 cycles per second, where most of the pertinent information in voice communication is transmitted.