Building Acoustics

Room acoustics 131

(^) r^2 ()
0

1.94

1.31

T N ,

fTM

σ

+

= (4.59)

where f 0 is the centre frequency in the one-third-octave band. This expression is also
used in the report by Olesen (1992) comparing with measurement results obtained in a
small laboratory room of volume 65 m^3 , having an almost frequency independent
reverberation time of two seconds. The numbers N and M of source and microphone
positions were two and six, respectively. The result is shown in Figure 4.12, given by the
reverberation time standard deviation, i.e. by the expression

s()TTT=⋅⋅σr( ) M,

and as seen, the fit between measured and predicted results is quite good.

63 125 250 500 1000 2000 4000

Frequency (Hz)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Standa

rd deviation (s)

Volume 65 m^3

Figure 4.12 Reverberation time standard deviation in a laboratory room of volume 65 m^3. The reverberation
time is approximately frequency independent (2 seconds). Solid curve – measured. Dashed curve – predicted.
After Olesen (1992).

4.5.2.3 Procedures for measurements in stationary sound fields

As is apparent from the discussions above, a number of the standard measurement tasks
in building acoustics; e.g. sound insulation, sound absorption and noise measurements,
are based on determination of the spatial averaged sound pressure squared and the
reverberation time. In the following, we shall use the sound pressure as an example.
We shall further assume that measurements are performed on band-limited
stochastic noise. This may comprise measurements on a broadband source of unknown
sound power where we apply filtering in octave or one-third-octave bands for the