U2 - Sound.pptx a very brief introduction

It gives a brief insight into the subject of sound

1. SOUND

2. Introduction Sound is both the name given to the sensation of hearing caused by vibrations of the air and the name that describes the vibrations themselves . Generally when we talk of sound, we do so in the context of vibrations at frequencies that are audible to the human ear . The frequency range for hearing is often given as 20 Hz to 20 kHz , it declines quite rapidly with increasing age A bout 15 Hz, however, sound loses its tonal quality. Very low frequency sound is called infrasound . It is created by gunfire

3. Sound at frequencies from 10 kHz to frequencies well beyond the range of human hearing is called ultrasound . It has numerous medical and industrial applications . When an object vibrates, it disturbs the air around it and the disturbance is propagated as a sound wave. The vibrating object is called a source , and the region in which sound waves exist is called a sound field . The compression and expansion is accompanied by a local increase and decrease in pressure relative to the pressure that normally exists in the absence of sound . The latter , called static pressure , is the normal atmospheric pressure. The fluctuating pressure is known as sound pressure

4. Sound Particles The way of describing vibrations in the air caused by the presence of a sound field. N ot only is air invisible , but so far as sound is concerned, we know that the movement we are interested in varies from place to place . Imagining that the air consists of particles that vibrate in response to the passage of sound. Such particles are often called molecules . The particle in acoustics is merely a very small 'piece' of air that shares the vibration of the air as sound is carried . Sound particles do not move haphazardly with thermal energy as do physical molecules. Their movement is determined entirely by the sound vibrations.

5. If we fix our attention on any chosen region within a sound field, we can imagine the particles to move about fixed positions in time with the sound vibrations. particle displacement , particle velocity , and particle acceleration

6. Sound waves We have seen waves on the seashore or ripples on the surface of a pond. Hold one end of a rope and shake it, and a wave is seen to travel towards the other end . A sound wave is a mental or mathematical construction that describes the sound particle displacement in terms of both time and position within a sound field . The waves have two features are immediately obvious. The first is that at any instant the vertical displacement of the surface forms an extended pattern consisting of a rise and fall of the surface relative to its undisturbed level. The second is that the pattern as a whole appears to move outward from the source of the disturbance with a steady speed.

7. The wave system is produced by vertical movements in adjacent areas of the surface so timed as to create the moving pattern (Figure 3.1). Waves that move, that advance from regions where vibrations exist to regions hitherto undisturbed, are called progressive waves .

8. Types of wave motion The types of waves that result from vibratory motion are classified according to the direction of vibration of the medium relative to the direction in which the wave is moving . Waves on the surface of water are called transverse waves , meaning that the displacement of the surface is at right angles to the direction in which the wave is advancing. The vibrations of a string, in a musical instrument for example, can also be treated as waves (standing waves). Again, these are transverse waves. Sound waves in gases and liquids, however, are always longitudinal waves , in which the particles move in the same direction as the wave .

9. A s a sound wave, it is essential that neighbouring parts of the medium should be coupled elastically . The forces on a particle in one small volume have to be communicated to neighbouring particles by what can be imagined as an elastic link. For sound waves in air, the elasticity is associated with compression or expansion as previously stated . Transverse acoustic waves are possible in solids, where they produce a shear deformation . the sound is taken to be sinusoid (a pure tone), and distance from the source in the direction of the sound wave is shown from left to right. Time is shown from top to bottom. Each row of dots represents the positions of sound particles at a given instant.

10. Transverse wave

11. Longitudinal wave motion

12. W avefront Imagine a sound wave properly we should picture it in three dimensions As a sound wave progresses from its source, the boundary between the air that is vibrating and the air that has yet to be disturbed is a surface. This surface is called a wavefront . It follows that the vibrations of all sound particles at the wavefront have the same phase . We can thus extend the definition of a wavefront by using the term to describe any surface within a sound field at which vibrations are in phase . Finite distance- spherical wavefront Infinite distance – plane wavefront (plane progression waves- basic sound wave) Slit illuminated from a lamp- cylindrical wavefront .

13. Properties of Sound EQ: How does intensity, loudness, frequency and pitch affect sound waves?

14. Vocabulary Intensity – the amount of energy an sound wave carries per second through a unit area. Measured in watts per meter squared. W/m ² Example – When a sound wave carries a large amount of energy, the molecules of the medium move a greater distance as the wave pass by, and the sound has a greater amplitude

15. Vocabulary Loudness – sound level, describes what you actually hear. A sound wave of greater intensity sounds louder Decibels (dB) – measures loudness. A sound you can barely hear is 0 dB. 100dB can damage your ears.

16. Intensity determines loudness. A sound wave with a higher amplitude and energy is perceived as a louder sound.

17. Intensity determines loudness. A sound wave with a lower amplitude and energy is perceived as a softer sound.

18. Vocabulary Frequency – the number of vibrations that occur per second in a sound wave. Measured in Hertz. A frequency of 50Hz means 50 vibrations per second. We hear sounds with frequencies that are between 20Hz and 20,000Hz Ultrasound – sound waves with frequencies above the normal range of hearing Infrasound – sound waves with frequencies below the human range of hearing

19. Vocabulary Pitch – a description of how high or how low a sound seems to a person. Pitch depends on the frequency of a sound wave High frequency means high pitch Low frequency means low pitch

20. Frequency determines pitch. A sound wave with a higher frequency and shorter wavelength is perceived to have a higher pitch. A sound wave with a lower frequency and longer wavelength is perceived to have a lower pitch.

21. Piano Example of Pitch Tension - A tighter string produces a higher frequency. Therefore you hear the sound at a higher pitch. Length of the string - right side of the piano has higher pitch, shorter string. Left side of piano has lower pitch, longer string Thickness of a string – thicker means lower pitch

22. Vocabulary Resonance – when the frequency of the sound wave exactly matches the frequency of an object

23. Doppler Effect The apparent change in frequency as a wave source moves in relation to the listener

24. Doppler Effect As a sound source moves toward the listener, the waves reach the listener with a higher frequency. The pitch appears to increase because of the Doppler effect