Page 691 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 2 1. 1 Interior noise: Assessment and control

P
5 log 10
2
c
0
10 log 10 (radiation index) 6 dB /octave S 2 c 1.8 dB /octave Perimeter P

Area
10 log
10
S

= c
f c

2 f Frequency (Hz)
c 2 f c c 2f c
– 1
2Sf c 8S 4
Fig. 21.1-5 Theoretical modal average radiation efficiency of a baffled rectangular panel: after Ve ´ r and Holmer (1971).

of measurement positions apply equally to intensity As long as background noise levels are not a problem, there
techniques as they do to sound pressure techniques. are occasions where a more meaningful estimate of sound
The determination of sound power using an intensity power can be obtained from a few quick far field sound
meter has the following advantages over sound pressure pressure measurements rather than a detailed and labo-
techniques to the same aim: rious survey in the nearer field with an intensity meter.
1. Sound pressure techniques must assume free-field
conditions where the sound is only travelling away 21.1.3.9 Standard methods for
from the source. The sound intensity meter deter- measuring sound power under different
mines the direction of propagation and therefore circumstances
sound power may be determined even in the geo-
metric near field. There are UK national (BS) and internationally (ISO)
2. Some areas of vibrating surfaces may act as radiators recognised standard methods for measuring sound
of sound while other areas may act as absorbers of power. They broadly follow the methods already
sound. A sound intensity meter scanning a surface discussed so far, and therefore will not be discussed
will detect such phenomena. further. The list of standards includes, but is not neces-
sarily limited to (source www.bsi.org.uk).
3. The output of an intensity meter gives magnitude
and direction which can usefully be used for source BS 4196-0:1981 (ISO 3740:1980)
location. The output from pressure measurements Sound power levels of noise sources. Guide for the use
over a surface is less informative. of basic standards and for the preparation of noise test
codes.
Although the intensity meter is a very flexible tool, there
are conditions under which it will not perform well. BS 4196-1:1991 (EN 23741:1991 ISO 3741:1988)
These are mostly where pressures are large and the Sound power levels of noise sources. Precision
pressure gradient between the two microphones is small methods for determination of sound power levels for
resulting in a poor estimate of particle velocity. Such broad-band sources in reverberation rooms.
a condition will occur near to highly reflective surfaces
BS 4196-2:1991 (EN 23742:1991 ISO 3742:1988)
where the incident power is approximately equal to the
Sound power levels of noise sources. Precision
reflected power. Therefore, the use of an intensity meter
near to highly reflective surfaces should be avoided. methods for determination of sound power levels for
The other, unexpected, difficulty with sound intensity discrete-frequency and narrow-band sources in re-
measurements is that due to the high detail of the verberation rooms.
output, a large number of measurements must be made BS 4196-5:1981 (ISO 3745:1977)
at different locations to ensure that a realistic spatial Sound power levels of noise sources. Precision
average of sound intensity has been achieved. This methods for determination of sound power levels for
problem gets worse the nearer to the source one gets. sources in anechoic and semi-anechoic rooms.

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