Reverberation Time
Work spaces, rooms or general commercial building designs can quickly be perceived as having poor quality acoustic environments if resonances and reverberations occur in them. Meeting rooms and performance spaces can suffer greatly from poor speech intelligibility levels through reflected sound, while reverberation in a factory can create a potential health hazard for workers.
Reverberation is the measure of time that an echo takes to decay and is key in a noise control strategy. The perception of sound is affected either compounding or reducing potentially both the perceived and actual noise level.
Reverberation in enclosed spaces
When a sound is generated in an enclosed space, it echoes from solid surfaces such as walls, floors, ceilings and hard furniture. When the noise source is switched off or taken away, the echo gradually gets absorbed by its surroundings until the noise decays and stops altogether.
Long reverberation times often make speech unintelligible and empty rooms impossible for musicians to perform in. On the other hand, a lack of reverb leads to a dead sound that does not carry, so concert halls, auditoria, classrooms, theatres and even recording studios sometimes prefer a degree of reflected sound, provided that it can be controlled and directed. This is a complete contrast to the requirements of an industrial setting which instead aims to reduce reverberation as much as possible
Getting it wrong can be a very expensive mistake. Arguments over the design of Sydney Opera House caused designer, Jørn Utzon, to walk out, leaving a hall that John Malkovich said “would do an aeroplane hangar a disservice”. It has since had a $150million refit.
Measurement and control
Measurement is the first step in professional noise control – an acoustic survey enables the best noise control measures to be identified in advance of any solutions being implemented. The usual measure of reverb time is called RT60. It is defined as the time taken for sound to decrease by 60 dB (in an acoustically clean environment). The standards for assessing RT60s are laid down in ISO 3382-1 for performance spaces, and ISO 3382-2 for regular rooms.
A project begins by generating clean test noises rather than the complex sounds likely to be generated in it subsequently. The usual source is an omni-directional speaker, although engineers have also been known to use starter pistols, clapper boards and balloons. Sensitive equipment then records the rates at which different frequencies decay in different locations.
Once you have definite measures of reverberation times and the behaviour of room furnishings and surfaces, you can take the optimum steps to rectify your sound problem. Guides for the optimum RT60 for different kinds of environment are provided by the ISO specifications and your acoustic consultant can provide a range of product solutions which compliment the space and RT requirements. These can range from acoustic absorption panels through to acoustic doors, windows and operable partitions to control noise movement within the space.
Jay Owen
Sales Director
+61 (0) 3 5521 1994
jay.owen@iac-australia.com.au