BRUEL & KJAER 4204 - Sound Power Measurements

Reference Sound Source Type 4204 – User Manual6
2.2 Sound Power Measurements
2.2.1 Principle
The use of a reference sound source is described as an alternative to direct sound power
measurements in several standards such as ISO 3741 and ASHRAE 36-72.
The method applies to reverberant or semi-reverberant sound fields and allows sound power
measurements to be performed without measuring the reverberation time, which is needed in
the direct method.
The reference sound source method only requires that the sound pressure levels produced by
the device under test and the reference sound source be measured. The sound power level of
the device under test is then determined from:
L
w
= L
wr
+ L
p
– L
pr
(2.1)
where:
L
w
= sound power level of the device under test (dB re 1 pW)
L
wr
= sound power level of Type 4204 (dB re 1 pW)
L
p
= sound pressure level of the device under test (dB re 20 µPa)
L
pr
= sound pressure level of Type 4204 (dB re 20 µPa)
Measurements are normally carried out in 1/3-octave or octave bands. The corresponding
sound power level for the reference sound source is read from the calibration certificate which
is delivered with the instrument. A typical calibration certificate is shown in Fig.2.1 and details
on the calibration procedure are given in section 3.2. Care should be taken to select the column
corresponding to the appropriate mains frequency (50 Hz or 60 Hz). Actual field sound power
levels are determined from the calibration certificate with appropriate correction factors as
described in Chapter 3.
The standards and codes of practice normally require that measurements are made for several
microphone positions. As the sound field should be diffuse, measurements may be made along
various paths (a line, an arc, a circle, etc.) when using a traversed microphone mounted for
example on Rotating Microphone Boom Type 3923. When fixed microphones are used, a
minimum of three microphone positions is required, as specified by ISO 3741. The
microphone outputs should be scanned automatically and/or averaged on a mean square basis
before they are entered in eq. (2.1). This is done according to the formula:
(2.2)
where:
L = average sound pressure level (dB re 20 µPa)
L
i
= sound pressure level measured at point i (dB re 20 µPa)
n = number of measuring points
L 10log
10
1
n
---
10
L
i
10⁄
i 1=
n
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=