Building Services Engineering 5th Edition Handbook

336 Room acoustics
The target room may be adjacent to, or close to, the plant room, or it may not be influenced
by the plant room other than by the transfer of noise through the interconnected air-ductwork
system. Analysis of the ductwork route for noise transfer is calculated separately and is not
covered in this book.
EXAMPLE 14.7
The showroom in Example 14.6 has floor dimensions of 25 m × 10 m and a height of
3.6 m to a suspended tile ceiling. The reverberation time of the showroom is 0.5 s. The
air-conditioning plant room generates a sound pressure level of 39 dB in the roof space.
The acoustic tile ceiling has a sound reduction index of 12 dB. Calculate the sound pressure
level that is produced within the showroom by the air-conditioning plant.
SPL 3 = 39 dB
SRI = 12 dB
S 5 = 250 m 2
T 3 = 0.5 s
V 3 = 25 × 10 × 3.6 m 2
= 900 m 3
SPL 4 = SPL 3 − SRI + 10 log(S 5 ) + 10 × log(T 3 ) − 10 × log(0.16 × V 3 ) dB
= 50 − 12 + 10 × log(250) + 10 × log(0.5) − 10 × log(0.16 × 900) dB
= 50 − 12 + 23 − 3 − 21 dB
= 37 dB
Noise rating
The human ear has a different response to each frequency within the audible range of
20–20 000 Hz. It has been found that a low-frequency noise can be tolerated at a greater sound
pressure level than a high-frequency noise. Noise rating (NR) curves are used to specify the loudness
of sounds. Each curve is a representation of the response of the human ear in the range of
audible frequencies.
The design engineer makes a comparison between the sound pressure level produced in the
room at each frequency and the noise rating curve data at the same frequency. When all the
noise levels within the room fall on or below a noise rating curve, that noise rating is attributed
to the room. Noise rating curves for NR 25 to NR 50 are shown on Fig. 14.3. The values are
plotted from,
SPL = NR f × B f + A f dB
SPL = sound pressure level at frequency f and noise rating NR
NR f = noise rating at frequency f Hz
B f and A f = physical constants
f = frequency
dB
dimensionless
dB
Hz