[Part 1discusses pressure waves and sound transmission. Part 2 covers sound intensity, power and pressure level.]
1.3 Adding sounds together
So far we have only considered the amplitude of single sources of sound. However, in most practical situations more than one source of sound is present; these may result from other musical instruments or reflections from surfaces in a room.
There are two different situations which must be considered when adding sound levels together.
Correlated sound sources: in this situation the sound comes from several sources which are related. In order for this to happen the extra sources must be derived from a single source. This can happen in two ways. Firstly, the different sources may be related by a simple reflection, such as might arise from a nearby surface. If the delay is short then the delayed sound will be similar to the original and so it will be correlated with the primary sound source. Secondly, the sound may be derived from a common electrical source, such as a recording or a microphone, and then may be reproduced using several loudspeakers. Because the speakers are being fed the same signal, but are spatially disparate, they act like several related sources and so are correlated. Figure 1.8 shows two different situations.
Uncorrelated sound sources: in this situation the sound comes from several sources which are unrelated. For example, it may come from two different instruments, or from the same source but with a considerable delay due to reflections. In the first case the different instruments will be generating different waveforms and at different frequencies. Even when the same instruments play in unison, these differences will occur. In the second case, although the additional sound source comes from the primary one and so could be expected to be related to it, the delay will mean that the waveform from the additional source will no longer be the same. This is because in the intervening time, due to the delay, the primary source of the sound will have changed in pitch, amplitude and waveshape. Because the delayed wave is different it appears to be unrelated to the original source and so is uncorrelated with it. Figure 1.9 shows two possibilities.