Propagation of sound in open and confined spaces

Perfect reproduction of sound in an open space

In open spaces, the sound waves travel directly from the source to the listener, who hears an almost perfect reproduction of the sound, as illustrated in Figure 7.11. The loudness of the sound

Loudness open space

— words spoken by speaker

sound received by listener


Figure 7.11 Comparison of words 'spoken' and 'heard' in an open space.

depends on the separation of the musician and the listener (except when the two are close together). As the listener moves away from the source, the loud-ness decreases.

In confined spaces, reflection from the surroundings can cause considerable distortion of the sound. A well-known example is the echoing of sound in a large cave. The echo from any one individual word reverberates in the cave and a whole string of words may result in a completely garbled sound.

When we listen to instruments and voices in rooms, halls and other auditoria, we are bombarded by 'surround' sound from all directions due to multiple reflections from walls, ceilings, floors and other surfaces. Figure 7.12 illustrates how individual


confined space

— words spoken by speaker

• —- sound received


by listener




Figure 7.12 Comparison of words 'spoken' and 'heard' in a confined space.

Figure 7.12 Comparison of words 'spoken' and 'heard' in a confined space.

words may be distorted; the sound is louder in an enclosed space and does not fade as quickly as in the open.

The level and type of reflection depend on the nature of the reflecting surface. The law of reflection (angle of incidence =

angle of reflection) holds only for smooth surfaces; rough surfaces reflect sound at all angles. The level of sound which reaches a listener in an auditorium depends on the nature and distribution of sound-absorbing surfaces. We can identify three different parameters — the time to reach a more or less steady level of loudness, the value of this loudness, and the time for the sound to become inaudible.

The decay of sound is called reverberation and is a very important parameter. The reverberant sound from one note (or word) can often be heard when the next one is produced. We quantify reverberation by defining reverberation time as the time taken for the loudness to decrease by 60 dB. Figure 7.13 shows a time sequence of the amplitude of a string of words for two different values of the reverberation time (the left hand image was recorded at the lower value).

Figure 7.13 Increasing the level of reverberation scrambles speech. Courtesy of James Ellis, School of Physics, UCD.
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