Page 712 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Interior noise: Assessment and control C HAPTER 21.1

12 dB/octave

Sound reduction index (dB) 18 dB/octave 6 dB/octave

frequency
log 10
Fig. 21.1-20 Generalised TL curves: after Fahy (1985).

enclosure might be considered large if a complete which is the fraction of sound power transmitted
wavelength of sound at a low frequency separated the by the direct field added to the fraction of sound
nearest wall of the enclosure from the noise source. power transmitted by the reverberant field. In this
As already discussed, the installation of an enclosure equation
around a noise source will produce a reverberant field a i ¼ average internal Sabine absorptivity
within the enclosure. The distribution of sound pressure S i ¼ internal surface area including the surface area of
level within the enclosure can be ascertained, given its the noise source
absorption characteristics and a noise source of known
power output. S E ¼ external surface area of the enclosure
The sound field outside the enclosure may be as- This equation can be re-written as:
sumed to comprise a contribution from the direct
field of thesourcereduced in amplitudebythe p 2 1
normal TL (TL N ) of the boundary of the enclosure
S E ¼ WT E (21.1.162)
and a contribution from the reverberant field within rc
the enclosure reduced in amplitude by the field TL of
where
the enclosure.
Now
p 2
1
TL n ¼ 10 log s n dB (21.1.157) ¼ sound intensity ðBies and Hansen; 1996Þ
10
rc
TL ¼ 10 log s dB (21.1.158) (21.1.163)
10
where s n and s are transmission coefficients. Also T E ¼ s½0:3 þ S E ð1 a i =S i a i ފ (21.1.164)
TL N TL ¼ 5 dB (21.1.159) The sound pressure directly outside the enclosure can
therefore be obtained from (Bies and Hansen, 1996)
therefore
¼ L w TL 10 log S E þ C (21.1.165)
L p 1 10
s n ¼ 0:3s (21.1.160) where

Now, the total sound power radiated by the enclosure
is given by (Bies and Hansen, 1996): C ¼ 10 log 10 0:3 þ S E ð1 a i Þ=ðS i a i Þ dB
(21.1.166)
p 2 1
S E ¼ W s n þ Wð1 a i Þ S E s (21.1.161) It should be noted that this equation gives very
rc S i a i approximate results only.

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