The Effect of Air Absorption in Home Cinema Design - December 25th, 2008
Another aspect of reverberation which caught home cinema designers by surprise is an observation. The observation is that, as well as suffering many reflections, the sound energy in a home cinema’s reverberant decay will have travelled through a lot of air. In fact the distance that the sound will have travelled will be directly proportional to the reverberation time, so a one second reverberation time implies that the sound will have travelled 344m by the end of the decay.
Although for low frequencies air absorbs a minimal amount of sound energy, at high frequencies this is not the case. In particular humidity, smoke particles and other impurities will absorb high-frequency energy and so reduce the level of high frequencies in the sound. This is one of the reasons why people sound duller when they are speaking at a distance.
In terms of reverberation time and also the level of the reverberation field, the effect of this extra absorption is to reduce the reverberation time in the home cinema, and the level of the reverberant field, at high frequencies above 2 kHz. Unfortunately though it is dependent on the level of humidity and smoke in the home cinema room and so the high-frequency reverberation time, and the reverberant field level, will change as the audience stays in the space. Note this is an additional dynamic effect over and above the static absorption simply due to the presence of a clothed person in the home cinema seating area, and is due to the fact that people exhale water vapour and perspire. Clearly then the degree of change will be a function of both the physical exertions of the audience and the quality of the ventilation system!
As the effect of air absorption is determined by the distance the sound has travelled (relevant to the size of the dedicated home cinema), rather than its interaction with an acoustic panel, it is difficult to incorporate the effect into the reverberation time equations. An approximation that seems to work in home cinema design is to convert the effect of the air absorption into an equivalent absorption area by scaling an air absorption coefficient by the volume of the home cinema room. This is reasonable because as the volume of the home cinema increases the more is the air that the wave must travel through, and the longer the distance that it travels.
For home cinema installations taking place in spaces smaller than 40 cubic meters, the effect can be ignored because the equivalent absorbing area is less than 1 cubic meter. However, the effect does become significant if one is designing a large high end home cinema, and air absorption would need to be considered.