Mass-spring type absorber

It is often difficult to obtain absorption at low frequencies with porous textile absorbers because the required thickness of the material is large and the sound absorbing layers are often placed at room boundaries where the absorbers are inefficient due to the low particle velocity. Resonant absorbers might be a solution. There are two common forms of resonant absorbers manbrane/panel absorbers and Helmholtz absorbers. A membrane/panel absorber is a sheet of vinyl or plywood, which is free to vibrate while for a Helmholtz absorber, the mass is a plug of air in the opening of a perforated sheet. The spring in both cases is provided by air enclosed in the cavity. Best performance can be obtained by placing a porous textile absorbent in the neck of the Helmholtz resonator or just behind the membrane in the panel absorber. The resonant frequency of this type of absorber can be tuned to the frequency of interest. 

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