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Sound absorption is the loss of sound energy when sound waves comes into contact with an absorbent material such as ceilings, walls, floors and other objects, as a result of which, the sound is not reflected back into the space.
Sound absorbent materials can be used to create a suitable acoustic environment within a space by reducing the ‘reverberation time’. Reverberation affects the way a space 'sounds'. A long reverberation time can make a room sound loud and noisy and causes speech to sound muffled and muddy. Rooms designed for speech therefore typically have a short reverberation time of less than 1 second. Conversely, a longer reverberation time can enhance a music hall by adding richness, depth and warmth to music.
Sound absorption can be a particularly important factor for spaces such as:
- Sports halls.
- Recording studios.
- Lecture theatres.
- Concert venues, cinemas and theatres.
Generally, sound absorption is applied in the form of treatment to floors, walls, ceilings, partition surfaces and objects such as chairs or bookshelves. The use of sound absorbing screens is also becoming more common.
Sound absorbers can be divided into three main categories:
- Porous absorbents.
- Resonance absorbents.
- Single absorbents.
Porous absorbents conventionally take two forms; fibrous materials or open-celled foam. Fibrous materials absorb sound as sound waves force the fibres to bend and this bending of the fibres generates heat. The conversion of acoustic energy into heat energy results in the sound effectively being absorbed. In the case of open-celled foam, the air movement resulting from sound waves pushes air particles through the narrow passages which in turn generate a viscous loss along with heat.
Usually a materials thickness has the greatest impact on its sound absorbing qualities. The thickness of materials can be compensated for with air space behind a wall panel or acoustic ceiling to improve performance at lower frequencies.
It is generally better to not include an airtight layer on the surface, such as a vapour barrier or paint layer, as this may reduce the sound absorbing qualities. However, architecturally, fibrous materials and open celled foams are not always considered attractive or robust. It is common therefore to cover these materials with an acoustically transparent finish such as a tissue, cloth or slatted wood, or with perforated materials such as wood, metal, plasterboard and so on.
Resonance absorbents consist of a mechanical or acoustic oscillation system, such as membrane absorbers, where there is a solid plate with a tight air space behind. Absorption reaches its maximum at the resonance frequency. The cavity can be filled with a porous material, to broaden the absorption over the range of frequency.
Single absorbers can be tables, chairs or other objects.
The sound absorbing characteristics of acoustical materials varies significantly with frequency. Low frequency sounds, below 500 Hz, tend to be more difficult to absorb whereas high frequencies sounds, above 500 Hz, are easier to absorb.
A material's sound absorbing properties can be expressed by the sound absorption coefficient, alpha, as a function of frequency. Alpha ranges from 0 (total reflection) to 1.00 (total absorption).
NB Sound absorption is not the same as sound insulation which is used to prevent sound travelling between separate spaces across a partition such as a wall, ceiling or floor. Sound absorbing materials can convert some of the absorbed sound energy into heat, rather than transmitting it, which improve sound insulation, but it should not be seen as a substitute for adequate sound insulation.
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