Approaches to spatialisation

This article describes some of the basic principles of A true plane wave does not spread out with increasacoustics and psychoacoustics related to the spatialisation ing distance, so it does not decrease in loudness with of sound. It introduces recording and diffusion technologies, distance. In practice, true plane-wave sources do not including binaural, stereo and surround-sound techniques. exist, any more than true point sources. All sound emitters actually consist of a combination of the two which varies with frequency and direction. For 1. SOUND IN SPACE instance, consider the difference between a busy road with many cars on it, which approximates to a line Sound is transmitted through air as longitudinal pressure waves. These expand outwards from their source of sound, and a quiet country road with only one car on it, which is closer to a point source. source and reduce in level as they spread. The objects they encounter will either absorb, reflect or diffract According to Begault (Begault 1993: 87), the loudness of the busy road will decrease with distance at half them. Usually some combination of these processes occurs, resulting in the spectrum of the sound wave the rate of that of the quiet road. This is not, however, the whole story. Consider a flat rectangular changing due to interaction with the physical properties of the objects. The nature of the interaction will sheet of metal, mounted with the short edge closest to the listener. As can be seen from figure 2, when change with the angle of the encounter. Sound waves also interact with the air they travel through, losing this is struck at one end, vibrations passing along the sheet will have an increasing radius of curvature, so higher frequencies progressively with distance as a result of absorption by water vapour in the air. Even will approximate a plane wave when they arrive at the far edge. The sound emitted from that edge will for the simplest sound-emitting object, the purely hypothetical point source which emits simple spherical therefore appear to be a line source, like the busy road. There will, of course, also be radiation of sound wavefronts, the soundfield produced in a space in which there is one or more other objects rapidly from the other edges and from the main surfaces. This will also be heard by the listener, but fractionally becomes very complex both spatially and timbrally. Even in free space, where there is nothing else to after the edge sound, since the vibrations in a metal sheet travel faster than soundwaves in air. These interact with, real sound sources which have extended sound-emitting surfaces have a more complicated differential delays will result in significant spectral modifications caused by cancellation or reinforcebehaviour, since the radiation of sound will vary in a non-simple manner with both position and frement of components of the sound as a result of the differential delays. For a listener directly facing one quency. Intuitively, we tend to expect this complex behaviour from objects which themselves are mechof the two major surfaces of the sheet, these effects will be much less obvious, but the plane wave emitted anically complex, such as a violin, but it is also true for simpler objects like a vibrating flat sheet. To from the surface will not be at right angles to the surface but tilted (figure 3) due to the finite speed at understand why this should be so, note that sounds with wavelengths larger than the size of the body which the wavefront crosses the sheet. In this simplified analysis, the effects of the discontinuity at the which emits them will behave much as if they had been emitted from a point source, with the result that sheet edges has been ignored and it is assumed that the wave in the sheet terminates at the edges. In practheir intensity will drop by 6 dB per doubling of distance. This is a result of the energy emitted by the tice, the wave is reflected back into the sheet which further complicates the emission behaviour. This source being spread over the increasing area of the expanding spherical wavefront (figure 1). In contrast, effect was exploited in the early reverberation systems known as plate reverbs, which originally consisted of an emitting surface which is very much larger than the wavelength of the sound emitted produces a a suspended sheet of steel, about 2B3 m in size, later superseded by a much smaller sheet of gold foil. The wavefront which is more like a plane wave, especially for larger ratios of surface size to sound wavelength. device would be fitted with transducers for injecting