Page 734 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Exterior noise: Assessment and control C HAPTER 22.1

Fit a large and effective additional silencer(s) to the Remove the exhaust system lagging and test for
exhaust tailpipe (often called an infinite silencer). exhaust shell noise contribution.
A series of absorptive silencers may be sufficient. The Remove the engine bay encapsulation and re-test for
infinite silencer should be of low backpressure design engine noise contribution.
so as not to restrict the performance of the vehicle. Remove the infinite intake silencer and re-test
Care must be taken to have the outlet from the for intake noise contribution.
additional silencers as near to the original exhaust Remove the infinite exhaust silencer and re-test
tailpipe position as possible. for exhaust noise contribution.
Lag the standard exhaust system silencers with
The apparent sound pressure level due to each noise
a layer of glass fibre, and then cover in sheet lead.
source in isolation from the others is derived from the
This is to reduce any contribution from exhaust shell
increase in overall sound pressure level as each additional
noise (noise radiating from the structure of the
source is uncovered. Therefore,
exhaust system).
Fit a large and effective silencer to the intake system. L Si ¼ 10 log 10 10 10 10 dB (22.1.1)
L S
Lu
This silencer will often be strapped to the hood 10
(bonnet) of the car, and can be constructed from
perspex to allow for driver’s visibility (large plastic where
drinking-water bottles are commonly adopted for L Si is the apparent sound pressure level due to source i
use). The silencer system must be of low pressure loss in isolation.
design in order not to restrict the performance of the L u is the total sound level recorded once source i
vehicle. It is vital that the outlet from the silencer is as is uncovered.
near to the original snorkel position as possible. L S is the sound level recorded before source i is
Encapsulate the engine bay witha thick plywood under uncovered.
tray, ensuringthat the edges are wellsealed.The engine
encapsulation must be performed carefully if engine This noise source ranking procedure is relatively
breathing and cooling problems are to be avoided. simple, but involves a large number of tests. The results
Encapsulate the drive train (if appropriate) using should be viewed only as being approximate, due to the
plywood, or lag with lead. potential for significant test-to-test variations.
Fit treadless tyres (or tyres known to be relatively It is important to reduce the number of variables
quiet) to reduce tyre noise. during the tests, so it is wise to use the same test routine
Perform identical interior and exterior noise tests for all tests and try to perform the tests on or near the
on the fully treated vehicle. same day. Once the relative noise contribution from each
Fit conventional tyres and re-test for tyre noise noise source is determined, the degree of additional
contribution. Once tested, re-fit the treadless tyres. silencing required is assessed. Only when there is a
Remove the transmission lagging and re-test potential for significant decrease in overall noise levels is
for drive-line noise contribution. For speed, it is the costly process of component development carried
customary not to re-fit any acoustic treatments after out. Some sample vehicle noise source ranking results are
testing. shown in Fig. 22.1-2.

Other Intake
Intake Other

Transmis'n.
Transmis'n.
Engine
Engine

Exhaust Exhaust
(a) (b)

Fig. 22.1-2 Noise source ranking results adapted from Balcombe and Crowther (1993). (a) Results at the start of the pass-by test.
(b) Results at the end of the pass-by test.

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