Effect of Home Cinema Room Size on Reverberation Time - November 25th, 2008
By looking at Norris-Eyring reverberation formula, allows some broad generalisations to be made about the effect of size of a dedicated home cinema on the reverberation time, irrespective of the quantity of acoustic panels present. The formula shows that the reverberation time is a function of the surface area, which determines the total amount of absorption, and the home cinema room’s volume, which determines the mean time between reflections in conjunction with the surface area.
As the home cinema room size increases, the reverberation time increases proportionally, if the average absorption remains unaltered. In typical home cinema installations the absorption is due to architectural features such as carpets, curtains, people, etc., and so tends to be a constant fraction of the surface area.
The net result is that in general, large dedicated home cinemas have a longer reverberation time than smaller ones, and this is one of the cues we use to ascertain the size of a space, in addition to the initial time delay gap. Thus one often hears people referring to the sound of a ‘big’ or ‘large’ acoustic as opposed to a ’small’ home cinema room, when they are really referring to the reverberation time.
Interestingly, now that it is possible to provide a long reverberation time in small home cinema installations, via electronic reverberation enhancement systems, with good quality, people have found that long reverberation times in a small home cinema room sound ‘wrong’ because the visual cues contradict the audio ones. That is, the listener, on the basis of the apparent size of the space and their experience, expects a shorter reverberation time than they are hearing.
Apparently closing one’s eyes restores the illusion by removing the distracting visual cue!