Building Acoustics

340 Building acoustics

The constant ε is equal to 1.0 in the case of totally absorbing sidewalls and equal to 2.0
for totally reflecting ones. The corresponding sound reduction index for the transmission
path through the suspended ceiling, assuming minor attenuation in the plenum, is then
given by
2
R
cl S R

SS R

10 lg ,

4

where 10 lg and 10 lg.

L

RRR

h

RR

ε

τ τR

⎡⎤

=+−⋅⎢⎥

⎣⎦

=− ⋅ =− ⋅

(9.20)

The transmission factors τS and τR are calculated from Equation (9.14).

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Sound reduction index (dB)

Via ceiling

ex/ attenuation

Plasterboard

40 mm abs.

Figure 9.12 Sound reduction index Rcl for transmission by way of a suspended ceiling and plenum. Suspending
ceiling of 9.5 mm plasterboard with 40 mm thick porous absorber. Calculated sound reduction index of
plasterboard and absorber is presented by separate curves. The sound reduction index Rcl is also shown for the
case of no attenuation in the plenum, (Equation (9.20)).