Page 710 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Interior noise: Assessment and control C HAPTER 21.1

(a) 9 dB/octave

6 dB/octave

TL (dB) Mass law Coincidence region

First panel resonance

Frequency

(b)

TL (dB) Coincidence region

Mass law

First panel resonance

Frequency
Fig. 21.1-19 Typical panel TL (after Bies and Hansen, 1996). (a) Isotropic panel and (b) orthotropic panel.

where f is the third octave centre frequency and for the The TL characteristics of a single panel has been
case when fm > 1. shown to be influenced by two frequency bands, the first
rc
Sharp (reported in Bies and Hansen (1996)) also centred on the lowest-order panel resonance and the
suggests the following relationship for the field incidence second on the critical frequency. In the double leaf case,
TL of an isotropic panel above the critical frequency: the influence of the lowest-order resonance of a single
panel is replaced with the lowest-order acoustic reso-

pfm 2hf nance within the cavity at f 2 where (Sharp reported in
TL ¼ 20 log 10 þ 10 log 10 dB Bies and Hansen (1996))
rc pf c
(21.1.137) c
f 2 ¼ (21.1.138)
2L
where h is the panel loss factor and f is the frequency
(Hz) which this time is independent of bandwidth. L ¼ longest cavity dimension (m)
Higher levels of TL may be achieved if a double- The critical frequencies f c1 and f c2 are still important
skinned enclosure is used. This construction is usually and added consideration must be given to the mass-air-
more cost-effective than constructing a very massive mass resonance at f 0 of the two panels on the compliance
of the cavity and a limiting frequency f 1 related to the
single-walled enclosure. For best results the two skins width of the air gap (d ) between panels. Sharp (reported
should be mechanically and acoustically isolated from in Bies and Hansen [1996]) gives the following re-
each other. Mechanical isolation can be achieved by lationships for panels which are totally mechanically and
mounting the two skins on separate beams or by using acoustically isolated from each other:
neoprene rubber between the skins and the common
studs. Acoustic isolation can be achieved by filling the air 2 1=2
gap with an absorptive material. The material should be f 0 ¼ 1 1:8rc ðm 1 þ m 2 Þ Hz (21.1.139)
2p dm 1 m 2
at least 15/f metres thick with an impedance of 3–5 r c
(too high an impedance might result in a mechanical path f 1 ¼ c Hz (21.1.140)
through the material). 2pd

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