Sound Waves

No account of waves and their properties can be complete without the inclusion of a discussion of sound waves. Sounds in the frequency range of 20-20,000 Hz are associated with human hearing. Many animals and birds can produce and detect sounds above and below this frequency range, providing them with a means of communication and an aid to survival.

The perception of sound follows general biological laws which relate stimulus to perception. We will look at the relationship between real and perceived changes and how loudness, pitch and tone quality relate to measurable properties of sound waves.

Music at an open-air concert sounds very different from the same music played indoors, where reflections can modify the sound by distorting the wave form. The design of good auditoria is both a science and an art.

The apparent change in the frequency of sound due to motion of the source or the listener — the Doppler effect — provides useful and sometimes vital information. It is used by some animals and birds to navigate and locate food. The familiar 'sonic boom' heard when an aircraft moves faster than the local speed of sound in air is also associated with the Doppler effect.

7.1 Sound and hearing 7.1.1 Sound as a pressure wave

Sound waves in air are longitudinal. They can be produced, for example, by vibrations of the membrane of a loudspeaker. When

Compression Rarefaction

Compression Rarefaction

Particles vibrate m-►

Particles vibrate m-►

Low High pressure pressure

Figure 7.1 Pressure waves travel from source to listener.

Low High pressure pressure

Figure 7.1 Pressure waves travel from source to listener.

a membrane starts to vibrate, it produces to-and-fro motions of neighbouring air molecules, creating alternate regions of high pressure (compressions) and low pressure (rarefactions), as illustrated in Figure 7.1. The compressions and rarefactions are pushed along according to the principles of propagation of any mechanical disturbance. High pressure regions are shown as being more densely populated with dots than low pressure regions. (The mean pressure is usually atmospheric pressure.)

The amplitudes of vibration of the diaphragm, the air and the eardrum are all different. In low pressure regions, particles are more easily displaced than in high pressure regions, so low pressure means high displacement, and the converse.

Sound waves exert a varying pressure on the eardrum, causing it to vibrate. The vibrations are communicated to a chamber called the auditory canal, then transmitted along a series of small, interlinked bones, and eventually received as nerve impulses by the brain.

7.1.2 The speed of sound

The velocity of sound varies from one material to another. Contrary to what one might expect, sound can travel faster in

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g 3000

1000

Figure 7.2 Measured values of the speed of sound.

6000

g 3000

1000

Figure 7.2 Measured values of the speed of sound.

liquids and in solids than in gases, because the particles in solids and liquids are closer together and can respond more rapidly. Some measured values may be seen in Figure 7.2. Sound propagates as a longitudinal wave in gases but can also propagate as a transverse wave in solids because solids resist any form of shearing stress.

The speed of sound in a given medium is found to be independent of its frequency.

7.1.3 Ultrasound and infrasound

Sound waves with frequencies in the range of 20-20,000 Hz are audible to humans. Frequencies above 20,000 Hz are called ultrasound. Ultrasonic waves can be produced when a quartz crystal is subjected to an oscillating electric field and changes shape, rather like a diaphragm (piezoelectric effect). Many animals use ultrasonic waves as a means of communication. Ultrasound is also used extensively in medicine and in industry. Infrasound is the name given to sound waves with frequencies below about 20 Hz. They are usually accompanied by low frequency sound audible to humans. Natural sources include ocean waves, lightning and large mammals; man-made sources include motors, supersonic aircraft and explosions. Infrasound can cause unpleasant effects such as seasickness and driver fatigue and can penetrate protective equipment such as ear-muffs. Infrasonic waves can travel for thousands of miles through the atmosphere without significant attenuation.

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