Some good points regarding room acoustics from Earl Geddes’ book:
http://www.gedlee.com/Books/HomeTheater.aspx
Lets now return to our discussion of the absorption in a small room. I have shown that it is desirable to have large low frequency absorption with little high frequency absorption, where there may be a few exceptions used to control specific early refections. In a practical sense, there is a real prob- lem with this requirement. Virtually all acoustical treatments for rooms have large high frequency absorption dropping to almost nothing at low fre- quencies, which is exactly the opposite of what we want. Clearly, dealing with the absorption aspects of a room by the use of standard materials is not recommended. The use of sound absorption in a small room must be dealt with extremely carefully. It has been my experience that it is almost impossible to make a small room too live at high frequencies. Most typical room construction materials and furniture have significant levels of absorption at high frequencies. Obtaining the right amount of absorption across the frequency band requires different construction techniques and room interior treatments.
I will return to the construction details in a later chapter, however, there are some specific topics that are more relevant here. How absorption actu- ally works is an important issue in our current discussion. There are two principle mechanisms for sound absorption.
The first is to use a porous material such that the sound wave can pene- trate it, and, in doing so, the air moving in and out of this porous medium dissipates energy through friction. This mechanism is by far the most com- mon, and there are some specific features to this kind of absorption. First, it becomes increasingly less effective as the wavelength of sound exceeds the thickness of the material. Thin materials will have no low frequency absorption. The second is that since the porous material works on the acoustic particle velocity, the effectiveness of the material is reduced when it is placed at locations of low particle velocity—places like walls where the velocity must go to zero. A piece of sound absorbing material placed on a wall is 1) not very effective and 2) increases in effectiveness as the fre- quency increases. This most common of all sound treatments is exactly the wrong thing to do.
The second major source of sound absorption is through the actual motion of the room structure—the walls themselves. Of course, if these walls are perfectly rigid—like poured concrete—then this mode of absorp- tion is negligible. But, for a common frame and dry wall construction, wall motion can be quite substantial. Since the wall has mass, its motion will continue to fall as the frequency goes up—that is, unless it has a resilient support structure. All walls must be supported in some way. When the sup- port is resilient (and all supports are to a certain extent, except for maybe a concrete backing), then there will be a resonance frequency and the motion of the wall will fall both above and below this resonance. A wall would typ- ically resonate somewhere below 100Hz—depending on drywall thickness and the method of mounting. When the wall does move, it dissipates energy through friction. (All absorption is friction of some sort.) The main differ- ence with this type of absorption is that it decreases with frequency rather than increase as the porous material method does. This would seem to be the ideal mode of absorption for a small room and indeed it is. In fact, if done properly tremendous absorption can be achieved at low frequencies with almost no high frequency absorption.
Another concept in sound absorption that comes into play in most HTs that I have done has to do with sound absorption on opposing walls. In my book Audio Transducers, I show how, at low frequencies, sound absorption works the same whether it is on one wall of an opposing pair or it is on both of them. By this I mean that the sound absorption is the same whether it is split between two opposing walls or all of it is placed on one wall. This is a good thing to know because it means that if we need to add low frequency absorption to a room, we need only do it on one wall of each of the three opposing pairs. I will show how this is a major advantage when locating a HT in a home.
In small rooms, the obvious preferred mode of sound absorption is to have the walls constructed in such a way that they deliberately move and absorb sound. This technique would never be used in an large auditorium because in those venues we are looking for primarily higher frequency absorption and hanging type materials, such as curtains or drapes are a good choice. In a small HT, the walls should be bare and hard but mounted so that they flex at low frequencies.
5.5 Summary
A complex chapter like this deserves a summary of its main points.
• The first is that the sound field in a room is a random quantity and one must do some form of averaging of samples (frequency responses) to get a valid estimate of the true frequency response.
• The modal region of a room, where there are true resonances, is limited to a relatively low frequency region never more than a few hundred Hertz.
• Damping in a room is effective at smoothing out the modal region but is detrimental to the steady state response at higher frequencies.
• Sound quality is strongly affected by early refections and the level of the reverberant field, and, in a small room, these two require- ments can be in conflict.