Sound and Vibration Basics
Reverberation Time Meters and Measurements
Reverberation is the sound that persists in an enclosed space due to multiple reflections, even after the source of the sound has stopped.
Reverberation is an important parameter for describing speech intelligibility and the perception of music and is used to correct or normalise sound insulation and sound power measurements.
Reverberation Time is the time taken for a sound level to decay by 60 dB after the excitation stops. However it is not practical to measure 60 dB in 'real life' rooms so the decay is usually measured over the first 20 dB and then extrapolated to the full 60 dB range, see the diagram to the right. This procedure is in accordance with BS EN ISO 3382 and the results are labelled RT60 (T20).
EDT - Early Decay Time is measured over the first 10 dB decay by acousticians, interested in clarity and the direct sound field. Early reflections that reach the listener within 50 ms integrate with the direct sound and can improve speech clarity - see the Haas Effect.
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 B&K 2250 or a NT-XL2 reverberation time meter, a tripod and a balloon (or other impulsive source, such as a starting pistol). After you pop the balloon, meters start measuring, analyse the decay and present the reverberation time as well as the decay curves for all frequency bands. They will also display the average reverberation time for the bands you select.
Interrupted Noise Method - When using a power amplifier and loudspeaker sound source both the B&K 2250 and the NTi-XL2 will 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.
Standardised Reverberation Times are used in Sound Insulation Rating measurements.
The graphic is by courtesy of Bruel & Kjaer