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Reverberation Time (RT) is crucial for describing the acoustic quality of a room or space. It is the most important parameter for describing sound levels, speech intelligibility and the perception of music and is used to correct or normalise building acoustics and sound power measurements.

Reverberation Time is the time for a 60 dB drop in the sound level after the excitation stops. This decay is usually measured over the first 10, 20 or 30 dB and then extrapolated to the full 60 dB range, see the diagram to the right.

Reverberation Times may be labelled EDT, T20 and T30 respectively for those three evaluation ranges. EDT is used in room acoustics only, while T20 and T30 may be used in all applications. Reverberation Time is measured in 1/1- or 1/3-octave frequency bands, some of which may be averaged to provide a single-number result for the most significant bands.

Reverberation Time may range from 0.1 seconds (or less) in anechoic chambers, to 10 or more seconds in large public spaces.

Reverberation Times vary between positions in a room, so it is usually measured at several positions. The average for all positions gives an overall assessment, and the position results may be used to indicate the acoustic quality as a function of location.

Reverberation Time can be measured using either Impulsive Excitation (Schroeder Method), such as from a pistol or balloon burst or by using Interrupted Noise, with the built-in noise generator.

Impulse Excitation Method - all you need, for this method, is a 2250, a tripod and a balloon (or other impulsive source, such as a starting pistol). After you pop the balloon, the 2250 will start measuring, analyse the decay and present the reverberation time as well as the decay curves for all frequency bands. It will also display the average reverberation time for the bands you select. The single input range means that trial measurements are not necessary when using the impulsive excitation method.

Interrupted Noise Method - When using a power amplifier and loudspeaker sound source the 2250 with BZ 7227 software will turn it's noise generator on and off, then measure and display the reverberation time spectrum and decays.

The measurement can be in octaves or 1/3-octaves in parallel over a selectable frequency range, allowing you to focus sound power on the relevant range. In each frequency band, the decay is sampled 200 times each second, for reverberation times as long as 20 seconds.


The above extract is courtesy of Bruel & Kjaer - see also the B&K 2250 Reverberation Time Meter