In the bass frequency range, it is necessary to consider other types of absorbers, in particular membrane absorbers which are proper bass absorbers. They constitute an important group absorbers for several reasons, but they are often overlooked, perhaps because they do not look as though they can absorb sound. Basically, the membrane absorber is a flat box, 100-200 millimetres deep, mounted on the wall with a thin sheet of plywood or similar on the front and with a light mineral wool filling the box cavity. See Figure 12.
Figure 12: Membrane absorber, plasterboard wall
If you softly bang the front panel with your hand, you will hear a deep tone, a bit like a bass drum, though much weaker. The tone you hear is the resonance frequency of the oscillating system which consists of the front panel with a certain mass combined with the spring that is formed by the trapped air.
The resonance frequency is also the frequency at which the membrane absorber absorbs, as sound energy causes the membrane to oscillate. In other words there is an energy transformation, but this time from sound energy to mechanical oscillatory energy. Thus, the first significant characteristic of the membrane absorber is that it absorbs sound energy at low frequencies. See Figure 13.
Figure 13: Membrane absorber, parquet floor on joists
The second key characteristic of the membrane absorber is that it is a common feature, as it were, of our day-to-day life, as ordinary building components such as doors, windows, wood floors (on joists) and plaster walls all function as membrane absorbers. The sound absorption coefficient is not staggering, perhaps 15-20 per cent, 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, but 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 how deep the sound is which is produced. 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.