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Noise > Building Acoustics > Reverberation Time

In acoustics the reverberation time at a particular frequency is defined as the time taken for sound to decay by 60 dB; this is often abbreviated to RT60. The decay used to be measured by firing a starting pistol, and then measuring the decay; nowadays a constant noise source is used, which is then shut-off.  However it is often difficult or even impossible to measure a 60 dB decay, so that the RT60 is calculated from the first 20 dB or 30 dB of decay [T20 or T30].

If the overall RT60 is short (say less than 0.3 seconds) the room acoustic will be "dead"; for example a heavily furnished room with thick carpets, curtains and upholstered furniture may have such an acoustic.

If the overall RT60 is long (say more than 1.5 seconds) the room acoustic will be "live" and echoy; for example a large empty room with painted plaster walls and a tiled floor may have such an acoustic.

In a given room, the reverberation time can be altered by adding (or removing) absorbent materials; for example in a school hall, a long RT60 can be reduced by the introduction of a heavy carpet; however, heavy curtains may be preferred so that the RT60 can be altered (see below).

The desirable reverberation time will vary depending on the use to which the room is put. Too little reverberation and sound does not travel well throughout the room, equally the acoustic is dead and rather unpleasant. More reverberation aids the sound to carry and adds character to the acoustic; too much reverberation and clarity is lost in a confusing "mish-mash" of echos. The balance between sound carrying, acoustic quality and clarity is different for speech or music.


Graph of Reverb Time vs Volume

DES Building Bulletin 51 "Acoustics in educational buildings", was published in the UK by HMSO in 1975, but internationally recommendations are similar. Reverberation times are considered at page 29 et seq. The figure above is a copy of Figure 50 of DES 51. At any given room volume optimum RTs are quoted for both speech (teaching) and music; allowable limits on the optimum values are also shown.

It is also stated that "The optimum reverberation time is given at 500 Hz but the balance of the reverberation over the whole frequency range is very important.... For speech, the reverberation time should be the same at all frequencies, but for music the reverberation at low frequencies should be increased so that the time at 125 Hz is up to 1.5 times the value at 500 Hz...In rooms which are used for both speech and music, the reverberation time must be a compromise between those required for the two uses. The direction which this compromise takes will depend on the major use of the room, but it is probably an advantage to err towards that required for music if this is a significant use."

Some of these problems can be heard on the BKLA website.  If you need to predict reverberation time it can be done from a knowledge of the absorption coefficients of the room finishes and their surface areas.  The absorption coefficients of some common finishes can be seen elsewhere.

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