Sound Absorption

Reduction of sound pressure level through sound absorption.

Examples of porous absorbers are carpets, curtains, mineral wool and foam.

The absorption of low frequency sound increases with the thickness of the absorber. The absorption will be more effective where the particle velocity is high. Close to the boundary of the room the particle velocity will be zero and so this is not an ideal location for sound absorption. The absorption furthest away from the backing surface will be the most effective and this is why thick layers absorb at lower frequencies / longer wavelengths.

At low frequencies / long wavelengths the absorption must be placed a considerable distance from the wall to reach a point where the particle velocity is significant. As a rule of thumb this distance is 1/10th of a wavelength for any significant absorption and of a wavelength to absorb all of the incident sound wave. One method used to improve the absorption of a given layer of absorption is to move the absorption away from the surface.
Membranes
It is often necessary to cover the porous absorption with a thin membrane to prevent it from being damaged or to reduce dirt ingress or from fibres being lost. This membrane may be in the form of a thin plastic covering in which case the low frequency performance will remain largely unchanged and may even be enhanced due to the added mass. However, at higher frequencies the absorption will be reduced as the membrane will inhibit the progress of the acoustic wave through the absorptive material. One option is to use a porous membrane such as a thin cloth or perforated plastic.

See also: Absorbent Material, Absorption Coefficient, Acoustic Wedges, Average Room Absorption Coefficient, Room Absorption, Sound Absorption Coefficient.

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Subjects: Noise & Vibration