Membrane Absorbers

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 to incident sound waves by vibrating. This results in the flexing of the fibers and a certain amount of frictional loss, which results in sound energy absorption. The fiber's mass and the air's springiness constitute a resonant system. In resonant systems, peak absorption occurs at the resonance frequency. 

Small Rooms

Engineers successfully use 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 panels, to the cavity dampens the resonance. It also effectively broadens the bandwidth or Q factor of the absorber. The absorber would be somewhat effective if the Q factor is enlarged, 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, such as the cavity depth, can significantly alter the performance.  

Customization

Since membrane absorbers require high 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 a specifiable one-third-octave center band frequency between 40 and 100Hz.  

VS Porus Absorbers

Since few membrane absorbers have been mass-produced, far less empirical test data is available than 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 as the designer intended), and some did not work.  

Sound Absorption

The sound absorption coefficient is not staggering, perhaps 15-20 percent. But, as the components constitute a significant area, the effect is significant. Take, for example, the large glass areas 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 windowpan, and notice the depths of the sound. It is in this frequency range that the window is absorbent. However, remember that membrane absorbers only work for bass tone and reflect higher-frequency sounds. Thus, Windows can produce uncomfortable reflections or echo effects that must be counteracted in some other way. It is also useful to know that wood floors on joists, which are membrane absorbers, can produce additional drum sounds – in other words, noise that occurs when walking 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 be necessary in large, open-plan offices, for example.

Source Texts

• Handbook for Sound Engineers, Glen Ballou