Page 705 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

Fig. 21.1-13 Measuring TL. Fig. 21.1-14 Measuring insertion loss.

index (SRI) for a panel is described in terms of the ratio
The TL across sections of enclosures may be obtained
of sound power incident on the panel to the sound power
from measurements made in the field using an intensity
transmitted through the panel. Then,
meter, or the TL of single panels can be obtained from
measurements made in a pair of special interconnected
laboratory rooms known as transmission suite. TL ¼ 10 log 10 P i dB (21.1.113)
A third and intuitive method of assessment uses in- P t
sertion loss as a parameter. Insertion loss is obtained by Sound TL is a quantity that depends only on the fre-
comparing sound pressure levels at a point in space with quency of the sound and the properties of the panel.
and without the enclosure in place (see Fig. 21.1-14). Consider a panel, with fixed sound TL mounted in
Therefore,
a transmission suite (Fig. 21.1-15).
A given spatially averaged sound pressure level in the
IL ¼ SPL 1 SPL 2 dB (21.1.112)
source room (L i ) gives rise to a spatially averaged velocity
Only TL is a general and reliable performance in- on the surface of the panel (u) and hence a spatially av-
dicator for comparing the acoustic transmission through eraged sound pressure level in the receiving room (L t )
individual panels with the performance of an entire en- according to the relationship
closure. The main reason for this is that both the alter-
native NR and insertion loss methods can produce results L t fsSu 2 (21.1.114)
with negative values in the cases where the sound pres-
sure levels outside the enclosure actually increase at where
certain frequencies once the enclosure is installed. This S ¼ panel area (m)
seemingly unexpected situation will occur when resonant s ¼ acoustic radiation efficiency
acoustic modes within the enclosure result in standing
wave ratios that are greater than the TL of the enclosure
boundary. This phenomenon can catch the novice re-
finement engineer out.

21.1.10.3 Interpreting the acoustic
performance data supplied by enclosure
and panel manufacturers

Manufacturers of acoustic enclosures and high TL panels
will often publish details of the acoustic performance of
individual panels in terms of:
TL;
absorption coefficient;
sound transmission class.
It is important to ascertain the conditions under which
these data were obtained. They will generally be repre-
sentative yet conservative values for results obtained
from a number of laboratory tests made according to
relevant material or international standards.
TL data may be obtained from tests performed in
a transmission suite in accordance with ISO 140/1: 1990,
ISO 140/3: 1990. The sound TL or sound reduction Fig. 21.1-15 The transmission suite.

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