It is often thought that soft porous materials such as mineral wool are all that is needed for acoustic treatment. Carpet, drapes and foam also fall into this category of porous absorbers.
But porous absorbers only work for sounds where the wavelength is less than four times the thickness of the absorbent layer. So for instance, a 50 mm (2 inch) layer of mineral wool will absorb frequencies above 1700 Hz effectively; below that frequency it has hardly any effect. A room lined totally with porous absorbers would sound unpleasantly dull.
One way of tackling low frequencies would be to use very thick layers of porous absorption. But to treat frequencies down to 50 Hz, the layer would have to be 1.5 meters (5 feet) thick. Plainly this is a lot of valuable space to lose in this way.
Fortunately there is an alternative, and this is the panel or membrane absorber. Functionally these are the same, except people often use 'panel' to mean a stiff panel such as wood, and 'membrane' to mean a flexible layer.
Although there is scope to look into the science of this elsewhere, a functional low-frequency membrane absorber can be constructed quite easily.
The material of the membrane should be rubbery and fairly thick. Materials such as this are commonly available from builders' merchants - look in the 'roofing' section. If it's rubbery and thick - 3 mm will be OK, then it's good. If it has an additional weatherproofing coating, this is irrelevant.
A rectangular wooden frame should be constructed which is 200 mm to 300 mm deep (8 inches to 12 inches). It can be any size, but a larger absorber will obviously suck out more low frequency energy from the room. It doesn't need a back as it will lay against the wall, with soft flexible strips for a good seal if the wall is uneven. The front of the frame is covered with the membrane, the interior is half filled with mineral wool. You can cover the whole thing with a decorative cloth material.
A typical control room would start to benefit from around four square meters of such absorption. Any less wouldn't really make too much difference; more might be necessary according to how much LF absorption the room needs.
The placing of these absorbers should be in the extreme corners of the room where low frequency energy builds up most.
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