Page 680 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Interior noise: Assessment and control C HAPTER 21.1

Plug all gaps in the firewall caused by ill-fitting grom- 21.1.2.2 Coherence methods for noise
mets, etc. path analysis
Add sound-absorption treatment to the underside of
the hood (bonnet). In the first instance, readers will need to understand the
Structure-borne noise from the road significance of the terms coherence and frequency
Change tyres. response function. To do this, they are directed to:
Change suspension bushes. a review of some background materials on systems in
Change subframe bushes (if an isolated subframe is Appendix 21.1A;
used). an explanation of the convolution integral in Appen-
Add damping treatments to resonant portions of the dix 21.1B;
firewall and floor. explanations of the covariance function, correlation
Airborne noise from the tyres and coherence given in Appendix 21.1C;
Change tyres. the derivation of the frequency response function
Improve the TL of the firewall and/or floor by adding given in Appendix 21.1D;
a barrier layer along with a decoupling layer. Sinha (1991), Fahy and Walker (1998) and texts
Improve the door seals if necessary. similar to Weltner et al. (1986) for further
reading.
Structure-borne noise from the exhaust
This section studies one noise source identification
Improve the vibration isolation afforded by the
mounts by using more compliant mounts, fixed to method (or noise path analysis method) based on mea-
high impedance points on the chassis and nodal sured coherence – that proposed by Halvorsen and
Bendat (1975).
points on the exhaust system. The analysis starts with their linear single input, single
Improve the TL of the trunk and/or rear floor by output (two pole – Appendix 21.1A) problem as illus-
adding a barrier layer along with a decoupling layer.
trated in Fig. 21.1-2.
Add damping treatments to resonant portions of the If data were acquired at point A in Fig. 21.1-2, then
trunk and floor.
Add a flexible coupling between the exit of the cata- xðtÞ¼ uðtÞþ nðtÞ (21.1.2)
lyst and the remainder of the system.
would be recorded, where u is the wanted input and n is
Airborne noise from the exhaust
some unwanted but unavoidable noise.
Improve the TL of the trunk and/or rear floor by Equally, if data were acquired at point B in Fig. 21.1-2,
adding a barrier layer along with a decoupling layer. then
Improve the door seals if necessary.
Structure-borne noise from the intake yðtÞ¼ vðtÞþ mðtÞ (21.1.3)
Mount the body-side elements of the intake system would be recorded, where v is the wanted input and m is
(filter box and snorkel usually) on resilient mounts. some unwanted but unavoidable noise.
Airborne noise from the intake Now one can write down the following relationships
Improve the TL of the firewall and/or floor by adding (where G( f ) is a one-sided spectrum):
a barrier layer along with a decoupling layer.
Improve the door seals if necessary. G xx ðf Þ¼ G uu ðf Þþ G nn ðf Þ (21.1.4)
Add sound-absorption treatment to the underside of G yy ðf Þ¼ G vv ðf Þþ G mm ðf Þ (21.1.5)
the hood.
G xy ¼ G uv ðf Þ (21.1.6)
Aerodynamic noise
Re-contour wing mirrors, aerials, door-handles, etc.
Improve the door seals if necessary.
Engine component noise n(t) m(t)
Reduce at source by adopting a quieter component. x(t) u(t) v (t) y (t)
Improve the TL of the firewall and/or floor by adding H(f)
a barrier layer along with a decoupling layer. A B
Add sound-absorption treatment to the underside of Fig. 21.1-2 Halvorsen and Bendat’s single input/single output
the hood. problem (1975).

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