ACOUSTICS - BACKGROUND INFO
Absorbers
All materials in the room act acoustically even if they are not
Membrane absorbers
This kind of absorber includes wooden floors, windows, doors, etc. This absorber provides absorption in the low end of the frequency range. The efficiency is normally not very high, but under normal conditions large areas are included in the basic room construction. Special designed membrane absorbers can be very effective.
Diffusers
A diffuser provides diffuse reflection of the sound radiated against it. It can be a very useful solution in cases where reflections are disturbing the sound image and it is not advisory to ad further absorption. So in order to reduce flutter echoes, comb filtering etc., special elements can be placed on the "disturbing" surface. These elements must have dimensions comparable to the frequencies at which diffusion is wanted.
Absorbtion or diffusion ?
Resonance absorbers
Resonance absorbers include slit panels, perforated plate, Helmholtz resonators etc. These absorbers are normally used in the frequency midrange. The absorption is medium to high.
Porous absorbers
These absorbers include mineral wool, carpets, curtains, and so on. They can be very effective, but the thickness of the material has to be taken into account. Thin layers will only absorb the highest frequencies. (Think of a rehearsal room in the concrete basement where the only damping is a carpet on the floor: Not good at low frequencies!)
To absorb a given frequency (and all frequencies above) the thickness of the absorber must be the quarter of the wavelength of that frequency. Or: the front of the material must be placed at a distance of one quarter of the wavelength.
A special technique developed by Manfred Schroeder is very capable in making a smooth and controlled diffusion. These diffusers normally referred to as "Schroeder Diffusers" can be found as prefab modules.
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Cross section of one example of the Schroeder Panel.
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