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Department of Linguistics


Basic Acoustics

Robert Mannell

What is Sound?

Sound is a wave-like distortion of a physical medium.

There are two classes of wave that can distort a physical medium, transverse waves and longitudinal waves.

In transverse waves, the movement of the elements of the medium move orthogonally (at 90°) to the direction of movement of the wave. A typical example of a transverse wave is a wave pattern on the surface of a body of water (eg. on a pond after a stone has been thrown in or ocean waves before they reach the breaking zone). In such a wave the molecules of water move up and down whilst the wave front moves along the surface of the water.

Figure 1: An example of a transverse wave; a wave induced in a piece of string.

In figure 1 a hand induces a transverse wave in a string by periodically moving up and down. This causes the string to move up and down. This movement propagates through the string producing a series of wavefronts which move towards the fixed wall with a velocity v. Obviously, individual parts of the string only move up and down (as indicated by the vertical arrows).

In longitudinal waves the elements of the medium move back and forth in line with the direction of propagation of the wave fronts.

Figure 2: An example of a longitudinal wave; a wave induced in a spring.

In figure 2 a hand induces a longitudinal wave in a spring by periodically moving back and forth in line with the direction of the spring. This causes the regions of high and low spring compression to move along the spring. This movement propagates through the spring producing a series of wavefronts which move towards the fixed wall with a velocity v. Individual parts of the spring only move backwards and forwards short distances in the direction of wave propagation. This causes the coils to periodically come closer to and further from adjacent coils than would be the case for the spring at rest. A longitudinal wave is a compression wave in which particles move back and forth in the direction of wavefront movement (ie. longitudinally).

Sound is a longitudinal compression wave.

Sound is a longitudinal compression wave which distorts a medium by creating moving fronts of high and low particle compression. Sound can occur in any medium (solid, liquid and gas). Sound cannot occur in a vacuum as there is no medium to compress.

Individual particles only move short distances backward and forward in the direction of wave propagation whilst the compression wave front can move considerable distances.

Wave cycles consist of regions of high compression separated by regions of low compression (rarefaction) relative to the ambient or resting state.

Sound in air consists of consecutive regions of higher and lower air pressure relative to ambient air pressure (typically 1 atmosphere at sea level). These fluctuations in air pressure are extremely small relative to ambient air pressure.

A Perceptual Dimension?

Does a longitudinal compression wave need to be perceivable in order to be regarded as sound?

(For convenience, I will refer to longitudinal compression waves as "acoustic waves" or AWs in the following discussion.)

In some texts it is claimed that perceivability is a prerequisite for regarding an AW as sound. This doesn't mean that it must be heard, merely that it could be heard if an observer was present. In its simple form, this approach would claim that sound is defined by the limits of human hearing. That is, AWs between 20 Hz and 20,000 Hz and between the threshold of hearing and the threshold of pain would be considered sound.

This view is actually quite problematic.

Firstly, are AWs that possess these (human) characteristics but that are being conducted in hostile media (eg. in lava flows, on other planets) sound, as they are not perceivable to humans.

Are sounds outside my range of hearing (ie. above about 15,000 Hz) not sound for me even though they are perceivable by younger ears? Can sound be said to exist for the profoundly deaf?

What about "sounds" that are not perceivable to humans but that are perceivable to other species (eg. echolocation sounds up to 100-200 kHz in some bats). Are they sounds? Are they only sounds to bats?

Perhaps the problem is better approached from another perspective. It seems that the word sound has two meanings. One meaning is simply the physical definition of sound as a longitudinal compression wave. The other definition is that "sound" is the perceptual correlate of longitudinal compression waves.

In this course I will take the view that sound is any longitudinal compression wave. This would include very high frequency compression waves (including bat echolocation signals) and very low frequency compression waves (including extremely long wavelength compression waves observed on the surface of the sun).

On the other hand, when we are dealing with speech sounds I will generally assume that we are talking about AWs in the range of human hearing being conducted through normal atmospheric air within temperature and pressure ranges acceptable to humans. The only exception to this that I can imagine would be deep sea divers' speech in Heliox (Helium/Oxygen) and similar exotic gas mixtures.