BRUEL & KJAER 4204 - Sound Absorption Measurements

Reference Sound Source Type 4204 – User Manual12
2.2.6 Background Noise Corrections
When measurements are recorded in the presence of background noise, it may be necessary to
apply correction factors to the measured band sound pressure levels both for the device under
test and for Type 4204. For each frequency band, the background noise level should be
measured with neither the machine nor Type 4204 operating. Measurements should then be
carried out with the machine in operation, and finally with Type 4204 in operation.
Most standards and codes of practice give tables where the correction factor may be found as a
function of the difference between the levels measured with and without the machine in
operation. The minimum difference below which no data should be reported is normally also
specified. (In the case of ISO 3741, this is 6 dB.) The correction factors K (to be subtracted
from the measured sound pressure level) and minimum differences specified by ISO 3741 and
ASHRAE 36-72 are given as an example in Table 2.1.
2.3 Sound Absorption Measurements
The Reference Sound Source Type 4204 may be used for sound absorption measurements. The
reference sound source is activated in the test room and the sound pressure level Lp is
measured in the central part of the room. This is performed for each frequency band of interest.
The absorption of the room is given by:
(2.3)
where:
ρ is the density of air
c is the sound velocity in air
W
R
is the sound power emitted from the reference sound source
is the mean of the sound pressure squared (averaged over the entire room)
Table 2.1.
Background noise
corrections as specified
by ISO 3741 and
ASHRAE 36-72.
Correction factor K to be
subtracted from
measured sound
pressure levels
ISO 3741 ASHRAE 36-72
Diff. (dB) K (dB) Diff. (dB) K (dB)
61.3
7 1.0 4 – 4.5 2
8 0.8 5 – 5.5 1.5
9 0.6 6 – 7.5 0.5
10 0.4 8–12 0.5
Minimum Difference: 6 dB Minimum Difference: 4 dB
A
4ρcW
R
p
2
------------------
=
p
2