There are essentially three methods currently available for the control of noise: absorption, insulation and active noise control.
We have summarised these methods below.
Noise Control in a Nutshell
Damping and vibration control are treated as separate methods.
The predominant methods are insulation, absorption or a combination of the two, and the fundamentals of how these work are explained below.
Absorption
The physical mechanism of absorption is the conversion of sound energy to heat - generally involving a light fibrous 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 material's reflection coefficient and absorption coefficient add up to 1.)
A material's absorptive properties are frequency dependent. In general, it is more difficult to absorb lower than higher frequencies.
It is also worth noting 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 receive positions. The barrier may be partial or complete, but good insulators are generally heavy and rigid.
Like absorption, the insulating property of materials is frequency dependent. In general, it is harder to attenuate low frequencies.
Successive layers of insulation are not directly additive.
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, 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.
Absorption vs Insulation
Good absorbers are generally poor insulators and vice versa. Insulation is generally a more effective means of noise control than absorption however.
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 spaces, and is an 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, and 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 is generally taken on a 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.
Sound
Sound consists 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 ear's 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 0 dB (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 scales 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) do not add up to 100dB(A). In fact, the result is 53 dB(A). A difference of 3 dB is only just perceptible. In terms of a subjective impression when we compare loudness, an increase of 10dB is roughly equivalent to a doubling in loudness.
