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Absorbing comments are not really sound


I was concerned about the technical article in AJ Focus January on acoustics, which included the statement: 'We can summarise that there are two concerns: a) the sound is slower to decay, so any disturbance to residents is extended?'.

This suggests that longer reverberation times cause longer-duration sounds which are, therefore, noisier. The actual mechanism is for reverberant interiors to have higher reverberant sound pressure levels, resulting from activity sound energy build-up (for example, each doubling of the amount of absorption in a space reduces its reverberant SPL by 3dB). The other benefit of adding absorption in otherwise acoustically 'hard' spaces is to localise a specific noise source by enhanced attenuation with distance.

The good news is that the first absorption added into a 'hard as nails' reverberant space is very effective, as it is not 'competing' with other absorbent surfaces to catch incident sound energy (you have to keep doubling the absorption units to halve the reverberation time, the classic Sabine hyperbola).

Duncan Templeton, director, Building Design Partnership:

BDP Acoustics, Manchester

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