Introduction to Acoustics

An intensity analyzer is more complex than a sound
level meter in the sense that the two channels required
for the measurements (Sect. 23.1.8) must be carefully
matched in phase. When using discrete microphone positions,
a very large number of microphone positions
may be required to obtain a specified accuracy in the
measurements. Mean square sound pressures on a measurement
surface are always positive, whereas sound
intensity may be outward from the measurement surface
(positive) or inward (negative). Thus, the summation
of sound intensities over the measurement surface may
produce a small number, and the accuracy of the measurement
may be difficult to determine.
Standards have been developed to overcome these
disadvantages. Instrument standards have been developed
[23.11, 12] and there are standards available for
the determination of sound power via sound intensity
both for fixed intensity-probe positions [23.20] andfor
scanning [23.21–23] with an intensity probe over the
measurement surface. The standards also describe field
indicators that can be used to estimate the accuracy of the
sound power determination, and technical information
on the use of field indicators is available [23.24, 25].
After substantial experience with the determination
of sound power via sound intensity has been achieved,
it is expected that these standards will be revised.
Determination of Sound Power in a Duct
The most common application of in-duct measurements
is to determine the sound power radiated by air-moving
devices. The sound power level of a source in a duct
can be determined according to ISO 5136 [23.26] from
sound pressure level measurements, provided that the
sound field in the duct is essentially a plane progressive
wave, using the equation
LW = Lp + 10 log S
, (23.33)
S0
where LW is the level of total sound power traveling
down duct, L p is the sound pressure level measured just
off centerline of duct, S is the cross-sectional area of
duct, and S0 = 1m 2 .
The above relation assumes not only a nonreflecting
termination for the end of the duct opposite the source
but also a uniform sound intensity across the duct. At
frequencies near and above the first cross resonance of
the duct the latter assumption is no longer satisfied.
Also, when following the measurement procedures of
ISO 5136, several correction factors are incorporated
into (23.33) to account for microphone response and
atmospheric conditions.
Noise 23.2 Noise Sources 977
Equation (23.33) can still be used provided Lp, is
replaced by a suitable space average 〈Lp〉 obtained by
averaging the mean square sound pressures obtained at
selected radial and circumferential positions in the duct,
or by using a traversing circumferential microphone. The
number of measurement positions across the cross sectionusedtodetermine〈Lp〉
will depend on the accuracy
desired and the frequency [23.27].
In practical situations, reflections occur at the open
end of the duct, especially at low frequencies. The effect
of branches and bends must be considered [23.28].
When there is flow in the duct, it is also necessary to surround
the microphone by a suitable windscreen. This is
necessary to reduce turbulent pressure fluctuations at the
microphone, which can cause an error in the measured
sound pressure level.
23.2.3 Emission Sound Pressure Level
Another measure of the noise emission of a source is
the emission sound pressure level. The sound pressure
level measured in the vicinity of a noise source is dependent
not only on its operating condition, but also the
distance from the source and the acoustical environment
– primarily reflections from the room surfaces
– but also on the presence of other nearby sources.
There are, however, cases where the noise emissions of
a source can be described using sound pressure levels.
These levels may be specified at an operator’s position
and at selected bystander positions, and in a controlled
acoustical environment. Peak C-weighted sound levels
(Sect. 23.1.4) may also be important when the levels are
high enough to cause hearing damage, and these levels
are not determined when using sound power level as
a measure of noise emissions.
A series of international standards has been developed
to ensure consistency in the measurement
process and to estimate emission sound pressure levels
when only sound power level information is available.
ISO 11200 [23.29] provides an introduction and
overview of the series. ISO 11201 [23.30] provides
guidance for measurements at the operating position
and bystander position when the acoustical environment
is a free field over a reflecting plane. If the measurements
cannot be made in a free field over a reflecting
plane, it may be necessary to use a semi-reverberant
environment. Using this method, A-weighted or Cweighted
peak sound levels may be measured using
ISO 11202 [23.31]. When only the sound power level of
the source is available, it may be necessary to make
estimates of the emission sound pressure level, and
Part G 23.2