The absorption level required for reflection-free zones
7.1.5 The absorption level required for reflection-free zones
In order to achieve a reflection-free zone it is necessary to suppress early reflections, but by how much? Figure 7.9 shows a graph of the average level that an early reflection has to be at in order to disturb the direction of a stereo image.
FIGURE 7.9 The degree of reflection suppression required to assure a reflection-free zone (data from Toole, 1990).
From this we can see that the level of the reflections must be less than about 15 dB to be subjectively inaudible. Allowing for some reduction due to the inverse square law, this implies that there must be about 10 dB, or α = 0.9 of absorption on the surfaces contributing to the first reflections.
In a domestic setting it is possible to get close to the desired target using carpets and curtains, and bookcases can form effective diffusers, although persuading the other occupants of the house that carpets, or curtains, on the ceiling is chic can be difficult. In a studio more extreme treatments can be used. However, it is important to realize that the overall acoustic must still be good and comfortable, that it is not anechoic, and that, due to the wavelength range of audible sound, this technique is only applicable at mid to high frequencies where small patches of treatment are significant with respect to the wavelength.
7.1.6 The absorption position for reflection-free zones
Figure 7.10 shows one method of working out where absorption should be placed in a room to control early reflections.
FIGURE 7.10 The image method for controlled reflection room absorption placement.
By imagining the relevant walls to be mirrors it is possible to create "image rooms" that show the direction of the early reflections. By defining a reflection-free space around the listening position, and by drawing "rays" from the image speaker sources, one can see which portions of the wall need to be made absorbent, as shown in Figure 7.11.
FIGURE 7.11 Non-environment room principles.
This is very straightforward for rectangular rooms, but a little more complicated for rooms with angled walls. Nevertheless, this technique, can still be used. It is applicable for both stereo and surround systems, the only real difference being the number of sources.
In Figure 7.11 the rear wall is not treated because normally some form of diffusing material would be placed there. However, absorbing material could be so placed, in the places determined by another image room created by the rear wall, if these reflections were to be suppressed.
One advantage of this technique is that it also shows places where absorption is unnecessary. This is useful because it shows you where to place doors and windows that are difficult to make absorptive.
To minimize the amount of absorption needed one should make the listening area as small as possible because larger reflection free volumes require larger absorption patches. The method is equally applicable in the vertical as well as the horizontal direction.