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To obtain good estimates of reverberation time, the minimum source-receiver distance should
be used in order to avoid strong influence from the direct sound. The minimum source–
receiver distance according to ISO 3382-1 is:
dmin 2
V
c*
T
(m)
where :
V is the volume of the room (m 3 ).
c is the speed of sound (m/s).
is an estimate of the expected reverberation time (s).
T
Thus for a typical concert hall a source-receiver distance less than 10 metres should be
avoided in order to get good predictions (measurements) of the reverberation time.
9.1.8 Minimum distance from the receiver to the closest surface
If a receiver is placed very close to a surface then results will be sensitive to the actual
position of the secondary sources generated by ODEON’s late ray method. If such a secondary
source happens to be very close to the receiver, e.g. 1 to 10 centimetres, this may produce a
spurious spike on the decay curve, resulting in unreliable predictions of the reverberation time
– indeed if the distance is zero then in principle a contribution being infinitely large would be
generated. To avoid this problem it is recommended that distances to surfaces are kept
greater than say 0.3 to 0.5 metres. Anyway for measurements it is, for other reasons,
recommended to keep distances greater than a quarter of a wavelength, i.e. 1.3 metres at 63
Hz. A distance of 1 metre is required by ISO 3382.
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