Page 753 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

20 log (Sc / Sp)
Xc SPL 1 - SPL 2 0
Sp 1 2 dB
Sc

0 1 2 3
2 Xc f/c
2 Xp f/c
1 2 3
SPL 1 - SPL 2
Xp 0
dB
Sc 1 Sp 2
20 log (Sp/Sc)
0
Xs
M = 0 M = 0 M = 0
SPL 1 - SPL 2 0
1 2 dB

0 M > 0 1 M > 0 2 M > 0 3
2 Xc f/c

Fig. 22.1-21 The performance of silencer elements in the absence of flow or temperature gradients.

volume of the silencer in a rational way as illustrated in
Fig. 22.1-23.

22.1.3.12.3 Design and development strategies
There is a spectrum of choice for design and de-
velopment strategies as illustrated in Fig. 22.1-24.
Empirical methods are at one extreme of the spectrum,
relying on know-how and on the extensive testing of
prototype silencers. The experienced silencer designer
will develop their own ‘favourite’ silencers. However,
acoustic theory allows them to tune such silencers to
a particular engine or car without too much prototype
testing. The theoretical tuning of the complex silencers
shown in Fig. 22.1-22 defies simple hand calculations,
and a computer model is required.

22.1.3.12.4 Traditional intake and exhaust
system design and development techniques
Cut-and-try methods have been very successful in
the past. They allow a designer, time and opportunity to
balance the need for low noise emission against
the desire for a particular quality of noise. However, there
are a number of disadvantages to this approach namely:
these are often time consuming;
they may be costly due to the need for a large number
of prototypes for testing;
it is often difficult to be sure that an optimum
solution has been obtained;
the quality of the final product is highly dependent
Fig. 22.1-22 Simple and complex silencing units used for intake on the talents and experience of the development
and exhaust systems. engineer.

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