Page 711 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

Below f c l,2 /2 Below u 0

pfM TL 0 ¼ TL M;0 dB (21.1.149)
TL M ¼ 20 log 10 5 dB (21.1.141)
rc
At u 0
Above f c l,2
0
TL 0 ¼ TLð0; M; u Þþ 20 log 10 h dB (21.1.150)
pfM 2hf
TL M ¼ 20 log 10 þ 10 log 10 dB 0
rc pf c for the case of m 1 ¼ m 2 , and u is the natural frequency
of the panels which Bies and Hansen (1996) suggest
(21.1.142)
to be:
where r ffiffiffiffiffi
0
B i 2 n 2 1
0
2
M ¼ m 1 þ m 2 (21.1.143) u i;n ¼ p 2 þ 2 ðrad s Þ i; n ¼ 1; 2; 3; .
m a b
Finally (21.1.151)
0
TL ¼ TL M f < f 0 (21.1.144) where a, b are panel width and length, B is the bending
stiffness per unit width and the natural frequency occurs
TL ¼ TL 1 þ TL 2 þ 20 log 10 fd 29 f 0 < f < f 1 when i ¼ n ¼1.
(21.1.145) Between u 0 and kd ¼ p/4 where
where TL 1 and TL 2 are found by using m 1 or m 2 ,
respectively in the mass law: k ¼ u (21.1.152)
c
TL ¼ TL 1 þ TL 2 þ 6 f < f 1 (21.1.146) TL 0 ¼ TL 1;0 þ TL 2;0 þ 20 log ð2kdÞ dB
10
These equations relate to the ideal case where the two (21.1.153)
leaves are mechanically isolated from one another and At frequencies corresponding to acoustic anti-
absorptive material is introduced between the leaves to resonances of the air gap the TL is maximum at:
eliminate the effect of acoustic resonances in the cavity.
Sharp presents some algorithms to predict the effect that
the mounting of the leaves (line, line-point or point- kd ¼ð2n 1Þ p n ¼ 1; 2; 3; .fc 1;2 (21.1.154)
point) has on the ideal TL, which will not be presented 2
here (Sharp reported in Bies and Hansen (1996)). TL x TL 1;0 þ TL 2;0 þ 6 dB (21.1.155)
Fahy presents an alternative model for the TL of
double-leaved panels in Fahy (1985). Here he presents At frequencies corresponding to resonances of the air
a 1-D model that assumes mechanical isolation between gap, the TL dips down to the combined mass law for the
the leaves, but takes account of the acoustic resonances two leaves:
in the air gap. It should be noted that Fahy’s method
produces predictions of normal incidence TL while TL 0 ¼ 20 log M þ 20 log f 20 log r c dB
0
Sharp’s method relates to field TL. Fahy’s equations 10 10 10 p
are: (21.1.156)
1=2
r c 2 m 1 þ m 3 when
0
u 0 ¼ (21.1.147)
d m 1 m 2
kd ¼ np
(note the omission of the factor 1.8 which appears in These equations can be used to form a generalised TL
Sharp’s equation);
curve as shown in Fig. 21.1-20.
TL M;0 ¼ 20 log 10 M þ 20 log 10 f

r c 21.1.10.6 Sound inside and outside large
0
20 log 10 dB (21.1.148)
p enclosures
where M ¼ m 1 þm 2 and TL 1,0 and TL 2,0 are obtained by Enclosures are deemed to be large if they are not
substitution of m 1 and m 2 respectively. designed to be close fitting around a noise source. An

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