Dedicated Home Cinemas with Short Reverberation Times - November 28th, 2008
Quite often we come across home cinema installations where too many acoustic panels are being used. The very short reverberation times that occur pose an interesting problem. One of the assumptions behind the derivation of the reverberation time calculation of dedicated home cinemas was that the sound energy visited all the surfaces in the room with equal probability.
For example, a home cinema room whose surface area is 75 sq. meters and whose volume is 42 cubic meters, would have a mean time between reflection of 6.51 ms. By applying a lot of absorption (let’s say that the average absorption coefficient is 0.9), the reverberation time can be calculated to be 0.43 s, and the average number of reflections that have occurred during the reverberation time can be calculated to be 6.45.
These are barely enough reflections to have hit each surface once. In such a home cinema installation, the reverberant field does not really exist; instead the decay of sound in the home cinema room is really a series of early reflection to which the concept of reverberant field or reverberation does not really apply. In order to have a reverberant field in dedicated home cinema installations there must be more than six reflections. A suitable number of reflections, in order to have a reverberant field, might be nearer 20, although this is clearly a hard boundary to accurately define.
Home cinema designs that are not properly engineered tend to be over-treated with absorptive acoustic panels so that they are very ‘dead’ and do not support a reverberant field.
