Noise Control   Comparison, Decibels and dB (A)

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.

Insulation
Typically in noise insulation a barrier is placed between the source and receive positions. The barrier may be partial or complete.
Good isolators are generally heavy or rigid. Like absorption, the insulating property of materials is frequency dependent; in general it is harder to attenuate low frequencies.

Good absorbers are generally poor isolators and visa versa
Successive layers of insulation are not directly additive. If a single layer of plasterboard provides 10 dB insulation at 250 Hz then two layers do not provide 20 dB: in fact the result is closer to 12 dB. For two structures to be strictly additive a significant space or space layer is required between them.
The actual insulation provided is often limited by either flanking transmission or leakage. Even small gaps or leakage paths can severely degrade the performance of a partition.
With partial partitions, such as non-continuous barriers, the effects are dominated by the height of the barrier and the distance of the sound source and receive points from the barrier.