Building Services Engineering 5th Edition Handbook

Room acoustics 333
The human ear is most sensitive to sounds around 1000 Hz, making this the critical frequency
for the acoustic design engineer.
Sound pressure level in a plant room
The sound source space is normally the mechanical services plant room. The reverberant sound
pressure level in a plant room can be taken as
SPL 1 = SWL + 10 × log(T 1 ) − 10 × log(V 1 ) + 14 dB
(Sound Research Laboratories Limited; see also, Smith et al. (1985)), where
SPL 1 = sound pressure level in plant room
dB
SWL = sound power level of source mechanical plant dB
T 1 = reverberation time of plant room
s
V 1 = volume of plant room m 3
The reverberant sound pressure level is independent of the measurement location within the
room. When a sound pressure level is required at a known location, the earlier equation is used
with the radius from the source, r m,
(
Q
SPL = SWL + 10 × log 10
4 × π × r 2 + 4 )
dB
R
EXAMPLE 14.4
A refrigeration compressor has an overall sound power level of 86 dB on the ’A’ scale. The
plant room has a reverberation time of 2 s and a volume of 70 m 3 . Calculate the plant
room reverberant sound pressure level.
SWL = 86 dBA
T 1 = 2s
V 1 = 70 m 3
SPL 1 = SWL + 10 × log(T 1 ) − 10 × log(V 1 ) + 14 dB
= 86 + 10 × log(2) − 10 × log(70) + 14 dB
= 83 + 3 − 18 + 14 dBA ignoring decimal places
= 85 dBA
Outdoor sound pressure level
The sound pressure level in the outdoor environment immediately external to the plant room can
be taken as:
SPL 2 = SPL 1 − B + 10 × log(S 2 ) − 20 × log(d) + DI − 17 dB