Reverberation Time Variation with Frequency - November 30th, 2008
By simply looking at Sabine’s equation, one can clearly see that the reverberation time of home cinema installations depends on the volume, surface area and the average absorption coefficient in the room. However, the absorption coefficients of any acoustic wall panels are not constant with frequency. This means that, assuming that the dedicated home cinema has a constant surface area and volume, which is not an unreasonable assumption, the reverberation time in the room will also vary with frequency.
This will subjectively alter the timbre of the sound in the home cinema due to both the effect on the level of the reverberant field, and the change in timbre as the sound in the room decays away. As an extreme example, if a particular frequency has a much slower rate of decay compared with other frequencies, then as the sound decays away this frequency will ultimately dominate and the room will ‘ring’ at that particular frequency.
The sound power for steady-state sounds will also have a strong peak at that frequency because of the effect on the reverberant field level. Materials used for constructing acoustic wall panels for all custom home cinema installations will always have some absorption coefficients, shown as a function of frequency. These are usually presented over octave bands. One could argue that third octave band measurements would be more appropriate psychoacoustically, as the octave measurement will tend to blur variations within the octave which might be perceptually noticeable.
In many cases, because the absorption coefficient of the acoustic treatment varies smoothly with frequency, octave measurements are sufficient. However, especially when considering resonant structures in dedicated home cinema installations, more resolution would be helpful. Note also that there are often no measurements of the absorption coefficient below 125 Hz, this is due to both the difficulty in making such measurements and the fact that below 125 Hz other factors in the home cinema room become more important.