Svantek SVAN 979, User Manual

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SVAN 979 User Manual - Appendixes
APPENDIX E. REVERBERATION TIME CALCULATIONS
E.1. I
NTRODUCTION
If an impulsive sound is generated in a room with reflecting boundaries, repeated reflections at the boundaries
result in the rapid establishment of a more or less uniform sound field. This field then decays as the sound
energy is absorbed by the bounding materials. The rate at which the sound energy decays is determined by
the absorptive properties of the reflecting surfaces and the distances between them. The time taken for the
sound intensity or the sound pressure level to decay by 60 dB is called the
reverberation time (RT). The values
of RT may range from fractions of a second to a few seconds and depend upon the size of the room and the
nature of the materials used in its construction.
The graphs below present the reverberation time nature (in the case when only one frequency is emitted):
Fig 1. The acoustic pressure versus time (a) and the value of the sound pressure level versus time, so-called decay
curve (b)
The marker t
1
indicates the moment when the sound source was switched off. From this moment the acoustic
sound pressure / acoustic power (reflected waves propagate in the room) decreases till the moment indicated
by the marker t
2
. The lower graph presents so-called the decay curve . The reverberation time value is equal
to t
2
– t
1
when the difference between sound pressure levels L
1
and L
2
is 60 dB. The 60 dB dynamic condition
is impractical in real measurements (very difficult to fulfil) hence the reverberation time (RT 60) is obtained
t [s]
p
[Pa]
L
[dB]
t
2
t
1
Reverberation time
Sound
source
emits
Background
noise
L
1
L
2
a)
b)