At frequencies above the diffusion region, rooms behave according to the high frequency model described here. This model is valid for all types of large rooms. In most small rooms this model will only be valid above a few hundred Hz.

Sound behaves as rays and sound rays reflect from room surfaces similar to the way in which billiard balls reflect from the sides of a billiard table during a billiards game. We can use a trick known as the mirror trick to find the reflecting regions of walls, ceilings and floors to determine where energy is reflected from loudspeakers to the listening location.

Accurate multichannel sound depends on the direction and timing of sound coming from the loudspeakers. Interfering reflections will upset the stereo or multichannel auditory image that is presented by the system. For this reason it is advantageous to remove these harmful reflections. Excessive use of absorption may lead to a loss of ambience leading to a very "dry" sounding environment. For this reason, only specific reflections should be absorbed. Taking ETF measurements before and after absorber placement with trial and error will lead to the optimum placement with very little loss of natural room ambience.

The mirror trick works as follows: The listener is successively seated in each location of the room deemed to be a likely position for a listener. At each location, the listener observes a second participant move a mirror along the ceiling and walls of the room as well as any other suspect hard surfaces.

When a loudspeaker can be seen in the mirror, the mirror is in a spot where sound waves can reflect from the surface to the listening position. These locations require absorption. After the placement of each absorber, an ETF impulse response measurement should be taken to verify the correct position of the absorber and to verify that the absorber is actually reducing the level of the reflection. If the absorber is not neccesary, it should be removed.

The measurements below illustrate the effect of a single absorber being placed on a side wall in a room. The blue curve is the measurement before absorber placement, the green curve shows the effect of the absorber placement.

Notice the reduction in level at approximately 5 ms after the foam treatment has been added. Note that this also has the effect of reducing secondary reflections at approximately 10 ms. In this case the side wall reflection would reach the listener at 5 ms, reflect from the ceiling and reach the listener once again at approximately 8 ms. The side wall absorber eliminates both of these reflections. The effect of reflections is much more visible in the envelope (Energy Time curves) than in the impulse response.

Absorbers should be very thick, in the order of 6 inches or greater. The absorber should be at least 8 times its thickness in width and height. Do not be fooled by advertisers peddling 2 inch thick absorbers as having great effectiveness at low frequencies. The methods use to derive this specification are in no way similar to this application. The lowest frequency of absorption is dependent apon absorber thickness rather than "exotic high end technologies". A 6 inch thick absorber can be very effective down to approximately 600 Hz - 800 Hz, a 3 inch thick absorber can be effective to frequencies as low as 1200 Hz - 1600 Hz.

Reflections that reach the listener after the direct sound cause a phenomenon known as comb filtering. The appearance a single reflection in a frequency response looks similar to the teeth in a hair comb. Comb filtering due to a single 2 ms reflection is illustrated below.

The corresponding impulse response of an ideal loudspeaker with a single 2 ms reflection is shown below.

The frequency response deviates from the flat ideal because the single 2 ms reflection generates the comb filtering pattern.

The comb filtering nulls in the frequency response due to a 2 ms reflection will be spaced 1/0.002 = 500 Hz. The first null will appear at 500/2 = 250 Hz. An ideal absorber will eliminate the comb filtering nulls above its frequency of operation.

Before and after measurements for absorber placement are shown below.

Fig 3: Before Absorber placement

Absorbers can be tested by placing a loudspeaker a small distance from a hard wall and directly facing the wall in an otherwise reflection free environment. An ETF measurement of the linear frequency response with the microphone placed half way between the loudspeaker & wall will show a clear comb filtering pattern. Reflections that occur in time after the wall reflection can be removed from the frequency response measurement effectively using ETF gating.

The absorber under test can then be placed against the wall and the measurement repeated. The absorber lowest frequency of operation can be determined by comparing the new measured frequency response with the one that had no absorber. Comb filtering nulls will be reduced greatly over the effective range of the absorber.