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. Here we will consider noise control by absorption, insulation or a combination of both. Damping and vibration control we will treat as a separate method.

Absorption

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

In the field of noise control we are concerned with how much noise is not absorbed – the proportion that is reflected (a materials reflection coefficient and absorption coefficient add up to 1).

Carpet, for example, has an absorption coefficient of 0.50 (at 500Hz) and only reduces the incident wound by 3dB, which is only just perceptible. By contrast, a material with an absorption coefficient of 0.90 (such as acoustic tiling) reduces the incident sound by 10dB which is then perceived by the listener as half as loud.
 
A material’s absorptive properties are frequency dependent; in general it is more difficult to absorb lower than higher frequencies.

Also note that control by absorption does not materially affect the exposure when the listener is positioned between the noise source and the absorptive layer.

Insulation

Typically in noise insulation a barrier is placed between the source and the receive positions. The barrier may be partial or complete.

Good insulators are generally heavy and rigid. Like absorption, the insulating property of materials is frequency dependent; in general it is harder to attenuation low frequencies.

Good absorbers are generally poor insulators and vice versa. Successive layers of insulation are not directly additive. If a single layer of plasterboard provides 10dB insulation at 250Hz then two layers do not provide 20Db; in fact the result is closer to 12dB. For two structures, to be strictly additive then a significant space between them is required.

The actual insulation provided is often limited by both flanking transmission and leakage. Even small gaps or leakage paths can severely degrade the performance of a partition.

With partial partitions, such as non-continuous barriers, the effect is dominated by the height of the barrier and the relative distance of the source and receive points from the barrier, rather than the actual material.

Comparison

Absorption is far less effective than insulation as a means of noise control. Also, good absorbers are poor insulators and vice versa.

Acoustic absorption will do little to improve the insulating property of a wall or partition, for example. However, it can be used to good effect to condition listening conditions in spaces, and is effective means of controlling reverberation time and hence reverberant sound levels in a space.

A prime example of this is fast food establishments which tend to employ hard easy-to-clean surfaces, which also tend to be extremely noisy. A similar effect may be observed in some manufacturing process spaces which have much the same requirements.

Noise control is a diverse science, and generally taken on case-by-case basis. In most industrial applications, where total containment is not possible because access is required, a combination of absorption and insulation is used.

When partial containment is employed the performance of the structure is more a matter of a degree of containment rather than the actual materials employed. Hence even a lightweight barrier can provide a useful degree of screening from, say, a piece of machinery without materially affecting access to the equipment.

Decibels and dB (A)

To comprehend even the rudiments of noise control, it is necessary to have a basic understanding of the nature and size of sound.

Sound is comprised of pressure waves: its amplitude is measured in decibels (dB) and its content and nature is expressed as its frequency. Most adults enjoy an audio frequency range of around 40 Hz to 15 KHz.

Our hearing however, is not equally sensitive to all frequencies. At modest sound levels the ears response can be plotted as a curve. This is applied as a weighting curve to some sound level measurements to give what is known as an A-weighted sound level dB(A). Hence dB(A) is an approximation to our hearing.

In terms of amplitude our hearing has a range from around 0dB (known as the threshold of hearing) to around 140dB (The threshold of pain). A noisy factory is typically in the range 90-100dB(A), while a pneumatic drill registers 120dB(A).

The decibel scale is a logarithmic scale, and as such is not subject to the normal rules of addition and subtraction, for example, two noise sources of 50dB(A) thankfully do not combine to give 100dB(A). In fact the result is 53dB(A): a difference of 3dB is only just perceptible, in terms of a subjective impression we compare loudness; an increase of 10dB is roughly equivalent to a doubling in loudness.

The decibel scale and loudness should be borne in mind when considering noise control.