Noise Control

There are at present three methods available for the control of noise: insulation, absorption, and active noise control. Active noise control is not applicable in all cases. The focus will be on noise control through absorption, insulation, or a combination of both. Damping and vibration control should be treated as a separate method.

Absorption

The physical mechanism of absorption is the conversion of sound energy to heat. This generally involves a light density material. Acoustically, materials are described by their absorption coefficient which ranges between 0 (totally reflective) and 1 (totally absorptive).

In the field of noise control it is a question of how much noise is not absorbed – the proportion that is reflected. (A material’s reflection coefficient and absorption coefficient add up to 1). Carpet, for example, has an absorption coefficient of 0.50 (at 500 Hz) and only reduces the incident sound by 3 dB, which is only just perceptible. By contrast, a material with an absorption coefficient of 0.90 (such as acoustic slab) reduces the incident sound by 10dB which is then perceived by the listener as half as loud.

A material’s absorption properties are frequency dependent; in general it is more difficult to absorb lower frequencies. It should also be noted that, control by absorption does not materially affect the exposure when the listener is positioned between the noise source and the absorption layer.