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Reverberation in buildings

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[edit] Introduction

Sound is caused by vibrations which transmit through a medium such as air and reach the ear or some other form of detecting device. Sound intensity is measured in Decibels (dB). This is a logarithmic scale in which an increase of 10 dB gives an apparent doubling of loudness.

Approved document E, Resistance to the passage of sound defines 'Reverberation' as the persistence of sound in a space after a sound source has been stopped. Reverberation time is the time, in seconds, taken for the sound to decay by 60dB after a sound source has been stopped.

The reverberation time of a room is linked to the the surfaces that enclose it and the volume of the room by the Sabine equation:

RT = Volume x 0.161 / Total Acoustic Absorption

Reverberation.jpg

Image: To control reverberation time, acoustic absorption is used.

Room acoustics / reverberation affects the way a space sounds. A high reverberation time can make a room sound loud and noisy. Speech intelligibility is also a function of reverberation, a high reverberation time causes speech to sound muffled and muddy. Rooms designed for speech therefore typically have a low reverberation time: ≤1 second. A high reverberation time can enhance a music hall by adding richness, depth and warmth to music. A higher level of reverberation within a concert hall is therefore critical.

The illustration below provides indicative reverberation times for a range of building types and room volumes.

Indicative reverberation times.jpg

[edit] Acoustic properties of materials

To control reverberation time, acoustic absorption is used. Absorbent materials 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.

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.

The thickness of a given material along with properties such as its fibrousity governs its acoustic performance. Finishes within a space are defined in terms of their absorption coefficient. This is a number between 0.0 (100% reflective) for example stone, tiles or concrete and 1.0 (100% absorbent), for example high performance acoustic ceiling tiles, slabs of mineral wool, etc. Products such as carpets typically have an absorption coefficient between 0.1 and 0.3 depending on their thickness. Perforated plasterboard generally provides around 0.6 to 0.7.

It is also common to classify absorbent materials in categories, A to E, where A is highly absorbent and E is almost fully reflective.


This article was created by --MACH Acoustics 11:04, 28 November 2013 (UTC)

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