Page 713 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

The sound pressure level at a point in free space some The problem is not so simple when the direct field is
distance r away from the enclosure can be determined dominant at one or more walls of the enclosure. In this
from (Bies and Hansen, 1996): case, the enclosure should be treated as a barrier.

L WE x L p1 þ 10 log S E dB (21.1.167)
10
21.1.10.7 Sound inside and outside
D q
¼ L WE þ 10 log dB (21.1.168) close fitting enclosures
4pr
L p 2 10 2
When the enclosure is positioned on a hard floor: Jackson produced two useful papers on the performance
of close fitting enclosures (Jackson, 1962; 1966). He
D q ¼ 2 (21.1.169) developed a 1-D model of such an enclosure by treating
the noise source and the enclosure as a pair of concentric
The sound pressure level at a point in a reverberant pulsating boxes. The potentially complex three-di-
space can equally be found using (see Section 21.1.3.5) mensional problem was reduced to that of a pair of flat,
infinite parallel plates separated by a distance l and im-
D q 4ð1 aÞ mersed in air as shown in Fig. 21.1-21.
¼ L WE ¼ 10 log þ dB
L p 2 10 2
4pr Sa Jackson made the following assumptions:
(21.1.170) 1. That generally large radiating areas of machinery are
involved in cases using close fitting enclosures which
By performing this calculation twice, once with the
enclosure in place and once without, it can be shown that encourage the propagation of acoustic waves normal
to their surface.
NR ¼ TL C dB (21.1.171) 2. The enclosure does not touch any part of the body it
encloses.
A similar calculation may be performed to estimate
the sound pressure field within an enclosure sited in 3. The presence of the enclosure does not affect the
magnitude of vibration of the enclosed surfaces.
a reverberant field. The power flow into the enclosure is
equal to 4. Direct transmission of vibration through the support
of the sound source does not occur.
p 2 Jackson developed an equation describing the attenua-
1
W i ¼ S E s (21.1.172) tion produced by such an enclosure, which is:
4rc

so that Y 1

¼ A
Y 0
L Wi ¼ L p1 þ 10 log S E TL 6 dB (21.1.173) 2sin qðX cos q R sin qÞ
10
¼ 1
so rc
2 2 2 1=2
1 4ð1 a i Þ sin qðX þ R Þ
L pi ¼ L Wi þ 10 log 10 þ dB þ 2 2 (21.1.175)
S E S i a i r c
(21.1.174)
where
It can be shown that
S
NR ¼ TL C dB (21.1.171) X ¼ uM (21.1.176)
u

Driving plate Attenuating plate

l
Sound Attenuating box
source
Resilient mounts
Rigid base

Fig. 21.1-21 Jackson’s models (Jackson, 1962).

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