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Reverberation time measurement
A little theory
The reverberation time is defined as the time it takes for the sound level to decay by 60dB after a sound source has been switched off.
Rooms with small amounts of absorbing materials will have longer reverberation times than rooms with more absorbers.
For normal rooms, with reasonable amounts of absorbers, the reverberation time is approximately given by:
T=0.16V/A
in which T is the reverberation time in seconds; V is the room volume in cubic metres; A is the absorption of the room, measured in equivalent square metres open window (i.e. if the absorber has half the absorbing property as an open window, it should be used with half its surface area in the formula) and finally 0.16 is an empirical constant determined by Wallace C. Sabine and published in 1898.
This model cannot be used in rooms with excessive amounts of absorbers, such as anechoic chambers.
Measuring the reverberation time
To measure the reverberation time you will need a sound source and instrumentation able to capture the sound decay.
Theoretically, you have two options with respect to the sound source; impulse excitation or noise excitation. However, the old ISO 140 Series of Standards as well as the new laboratory version called ISO 10140, require the use of noise excitation. It is important that the noise is broadbanded enough to cover the entire frequency range of interest.
Although the reverberation time is defined as the time it takes for the sound to decay 60dB, this is seldom possible to measure due to the unavoidable background noise. Further, the initial part of the reverberation decay is usually the most interesting part. The reverberation time is therefore normally based on the decay rate for a range of 20 or 30 dB starting 5dB below the stationary level. The value is afterwards extrapolated to 60dB assuming that the part of the decay that we used is representative for the entire decay. It is common practise to specify the range used as T20, T30, etc., all having the same numeric value if the decay is linear.
One way of checking the consistency is by comparing e.g. T20 and T30. Any discrepancies between the two will normally originate from a non-linear decay (when plotted as a graph with a logarithmic level scale). All the Norsonic building acoustics instrumentation currently available has this capability of presenting at least two ways of calculating the decay simultaneously.
The frequency range required for field measurements is 100-3150Hz, but many measurements are now made up to 5000Hz which is the requirement for laboratory measurements. Optionally, you may extend the frequency range downwards to 50Hz, which would make the requirement even tougher with respect to output levels.
How reverberation time is measured in Norsonic instruments
An ideal sound decay will form a straight line when drawn in a coordinate system as level (logarithmic) versus time. In reality, however, sound decays will always contain fluctuations. Two problems will then immediately arise; viz. how to have the analyser accurately determine the initial level and when to start the calculation.
Using noise as excitation, the calculation starts at 5dB below the mean level (the Leq that is) of the noise measured at the microphone position before the noise is switched off.
As long as the noise then stays below this -5dB threshold the time elapsed is counted. Once the level drops below a second, much lower (i.e. 20dB or 30dB below) the counting is discontinued. Should the level for any reason again exceed the second line, the counting will be resumed and go on until the level again drops below this second threshold.
Likewise, should the level exceed the first threshold any time after the counting was started, the counting will be discontinued until the level drops back below this threshold.
The first threshold is as said above positioned at -5dB, while the second threshold is positioned at -25dB, corresponding 20 dB below where the calculation started fro T20 calculations, or 35 dB below for T30 calculations and so on.
Norsonic normally apply one of two different methods for the reverberation time calculation: The triangular method and the least-square-fit method. The triangular method is a weighted time measurement where the weighting reduce the influence of fluctuations at the start and end of the time measurement. The name is adopted from the triangular shape of the weighting function. More emphasis is then applied to the middle part of the decay and correspondingly less to its extremes. The least-square-fit method is based on a time measurement on a straight line fitted to the decay my linear regression.
In order to minimise any possible influence from the background noise level, you may – in Norsonic instruments – specify a minimum distance to the noise floor. Should, for example the background noise level suddenly rise this will cause the reverberation time to seem longer than it actually is.
Some International Standards, such as the ISO354 require a minimum of 15dB distance to the noise floor. This is a tough requirement actually. From a signal theory point of view a distance of 5dB will suffice in most cases.
