Sound is produced when an object (the source) vibrates and causes the air around it to move. It is a pulsating sphere which could be imagined as something like a squash ball, and it is pulsating regularly so that its size oscillates between being slightly larger than normal and then slightly smaller than normal. As it pulsates it will alternately compress and then rarefy the surrounding air, resulting in a series of compressions and rarefactions traveling away from the sphere, rather like a three-dimensional version of the ripples which travel away from a stone dropped into a pond. These are known as longitudinal waves since the air particles move in the same dimension as the direction of wave travel. The alternative to longitudinal wave motion is transverse wave motion, such as is found in vibrating strings, where the motion of the string is at right angles to the direction of apparent wave travel.
CHARACTERISTICS OF A SOUND WAVE
The rate at which the source oscillates is the frequency of the sound wave it produces and is quoted in hertz (Hz) or cycles per second (cps). 1000 hertz is termed 1 kilohertz (1 kHz). The amount of compression and rarefaction of the air which results from the sphere’s motion is the amplitude of the sound wave and is related to the loudness of the sound when it is finally perceived by the ear (see Chapter 2). The distance between two adjacent peaks of compression or rarefaction as the wave travels through the air is the wavelength of the sound wave and is often represented by the Greek letter lambda ( λ). The wavelength depends on how fast the sound wave travels, since a fast-traveling wave would result in a greater distance between peaks than a slow-traveling wave, given a fixed time between compression peaks (i.e., a fixed frequency of oscillation of the source). The sound wave’s characteristics can be represented on a graph, with amplitude plotted on the vertical axis and time plotted on the horizontal axis. It will be seen that both positive and negative ranges are shown on the vertical axis: these represent compressions ( + ) and rarefactions ( - ) of the air. This graph represents the waveform of the sound. For a moment, a source vibrating in a very simple and regular manner is assumed, in so-called simple harmonic motion, the result of which is a simple sound wave known as a sine wave. The simplest vibrating systems oscillate in this way, such as a mass suspended from a spring, or a swinging pendulum (see also ‘Phase ’below). It will be seen that the frequency ( f) is the inverse of the time between peaks or troughs of the wave ( f x 1/ t). So the shorter the time between oscillations of the source, the higher the frequency. The human ear is capable of perceiving sounds with frequencies between approximately 20 Hz and 20 kHz, this is known as the audio frequency range or audio spectrum.
HOW SOUND TRAVELS IN AIR
Airis made up of gas molecules and has an elastic property (imagine putting a thumb over the end of a bicycle pump and compressing the air inside – the air is springy). Longitudinal sound waves travel in air in somewhat the same fashion as a wave travel down a row of up-ended dominoes after the first one is pushed over. The half-cycle of compression created by the vibrating source causes successive air particles to be moved in a knock-on effect, and this is normally followed by a balancing rarefaction which causes a similar motion of particles in the opposite direction. It may be appreciated that the net effect of this is that individual air particles do not actually travel –they oscillate about a fixed point –but the result is that a wave is formed which appears to move away from the source. The speed at which it moves away from the source depends on the density and elasticity of the substance through which it passes, and in air the speed is relatively slow compared with the speed at which sound travels through most solids. In air the speed of sound is approximately 340 meters per second (ms - 1), although this depends on the temperature of the air. At freezing point, the speed is reduced to nearer 330ms - 1. In steel, to give an example of a solid, the speed of sound is approximately 5100ms - 1 .
The frequency and wavelength of a sound wave are related very simply if the speed of the wave (usually denoted by the letter c) is known:
To show some examples, the wavelength of sound in air at 20 Hz (the low-frequency or LF end of the audio spectrum), assuming normal room temperature, would be:
whereas the wavelength of 20 kHz (at the high-frequency or HF end of the audio spectrum) would be 1.7 cm. Thus, it is apparent that the wavelength of sound ranges from being very long in relation to most natural objects at low frequencies, to quite short at high frequencies. This is important when considering how sound behaves when it encounters objects –whether the object acts as a barrier or whether the sound bends around it.
Bibliography:
Sound and Recording, Sixth Edition, Francis Rumsey and Tim McCormick.
Designing Sound, MIT
Sound Design, Maurizio Giri.
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