Home Cinema Design - Early Decay Time - December 3rd, 2008
It has already been discussed that in home cinema installations the measure of reverberation time is actually the time it takes the sound to fall by 60 dB. This is not particularly relevant psychoacoustically, and it is also very difficult to measure in situ. This is due to the presence of background noise in the home cinema room, either unwanted or the music being played, which often results in less than 60 dB of energy decay before the decay sound becomes less than the residual noise in the environment.
Even in the quieter environment of a Victorian town in the days before road traffic, Sabine had to do measurements, using his ears, at night to avoid the results being affected by the level of background noise. Because we rarely hear a full reverberant decay, our ears and brains have adapted, quite logically, to focus on what can be heard. Thus we are more sensitive to the effects of the first 20 to 30 dB of the reverberant decay curve, the 60 dB reverberation is directly proportional to the earlier curves and so this should not cause any problems when measuring in custom dedicated home cinema installations.
However, if the curve of a home cinema room is of the double-slope form, then this simple relationship is broken. The net result is that, although the T60 reverberation time of the home cinema may be an appropriate value, because of the faster early decay to below 30 dB we perceive the cinema’s reverberation as being shorter than it really is. The psychoacoustic effect of this is that the dedicated home cinema sounds ‘drier’ than one would expect from a simple measurement of T60.