nts Sound waves 8/3 Decibels 8/4 Hearing 8/5 Speech and music 8/8 Acoustics 8/10 Psychoacoustics 8/13 References 8/16 This page intentionally left blank Sound waves 8/3 8.1 Sound waves ever sound wave intensities are frequently so small that uW/m² is frequently more convenient. (Note that in everyday speech intensity Sound consists of longitudinal waves in the air. The term is loosely is often equated with loudness. It is important to realise that the two used also for corresponding vibrations in other media. The term things, although having some relationship, are in fact quite differ- longitudinal means that the mode of transmission is by oscillatory ent.) movements of the air particles (in reality molecules of oxygen, nitrogen, etc.) the direction of oscillation being the same as the 8.1.1 The Inverse Square Law direction of the wave. Alternatively sound waves can be regarded as a succession of compressions followed by rarefactions. It is usual, It is quite obvious that almost any form of wave energy diminishes however, to represent sound waves by graphs of convenient par- in intensity as the distance from the source is increased. In practice ameters. Particle displacement is an obvious one but the most useful this rate of diminution varies greatly with different conditions. An is normally pressure, as shown in Figure 8.1. idealised relationship is given by the Inverse Square Law, as in Particle velocity is another useful parameter for graphical repre- Equation 8.1, which states that, for a point source of waves, capable sentation and this is also shown in Figure 8.1. Note that particle of radiating omnidirectionally and with no obstructions in the vi- velocity and pressure are in phase since pressure arises from the cinity (free field conditions) the intensity, I, decreases with the impact of the moving particles, but pressure and particle displace- square of the distance, d, from the source. ment are 90° out of phase. In general we shall refer to pressure, this being the most important of the parameters as it