What are Membrane Absorbers?
Membrane absorbers, also known as panel and diaphragmatic absorbers, utilize the resonant properties of a membrane to absorb sound over a narrow frequency range. Engineers typically employ unperforated, limp panels of wood, pressed wood fibers, plastic or other rigid or semi-rigid materials when constructing membrane absorbers. When mounted on a solid backing, but separated from it by a constricted air space the panel will respond by incident sound waves by vibrating. This results in the flexing of the fibers and a certain amount of frictional loss result in absorption of the sound energy. The mass of the fiber and the springiness of the air constitute a resonant system. In resonant systems, peak absorption occurs at the resonance frequency.
Engineers use succesfully membrane absorbers to control specific resonant modes in small rooms. In order to control room modes, they must be placed on the appropriate surfaces at points of maximum modal pressure. Adding porous absorption, such as mineral fiber panel, to the cavity dampens the resonance. It also effectively broadens the bandwidth or Q factor of the absorber. If the Q factor is broadened, the absorber would be somewhat effective, even if the desired frequency is not precisely attained.
Additionally, care must be taken during the design and construction of membrane absorbers. Changes as small as 1 to 2 mm, for example, the cavity depth can alter the performance significantly.
Since membrane absorbers require high level of precision to perform at the desired frequency, they are often customized for a specific application. Mass Production is often uncommercial, although some companies offer membrane absorbers. They have specifiable one-third-octave- center band frequencies between 40 and 100Hz.
vs Porus Absorbers
Since there have not been many mass production membrane absorbers, there is far less empirical test data available on membrane absorbers relative to porous absorbers. Nonetheless, some formal testing of commercially available membrane traps has been undertaken. The results were mixed, some membrane absorbers performed as designed, others performed well (if not exactly how the designer intended) and some did not work at all.
The sound absorption coefficient is not staggering, perhaps 15-20 percent. But, as the components together constitute a significant area, the effect is significant. Take for example the large areas of glass in many modern buildings. They lead to many problems with the indoor climate. However, as far as the acoustics are concerned, they ensure that the reverberation time does not increase outrageously for the bass tones. Because they are membrane absorbers, the glass sections help to balance the room’s acoustics. Try banging your hand gently on a large windowpane, and notice the depths of the sound. It is in this frequency range that the window is absorbent.
However, bear in mind that membrane absorbers only work for bass tones, and therefore reflect higher frequency sounds. Windows can, therefore, produce uncomfortable reflections or echo effects which have to be counteracted in some other way. It is also useful to know that wood floors on joists, which are, of course, membrane absorbers, can produce additional drum sounds – in other words, noise which occurs when walking in the floor in the same room. If this is a problem, try laying a carpet or placing rugs on part of the floor. This may well be necessary in large, open-plan offices for example.
Handbook for Sound Engineers, Glen Ballou
Different absorber types, Troldekt