Page 731 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

Walker (1994) reports on an experimental in- dominate even the low-speed acceleration test. Once
vestigation to test the effectiveness of these three goals in that becomes the case (and it already is with many pas-
practice. Noise measurements were made on an ISO senger cars fitted with wide tyres), the homologation
10844 test track and on another track surfaced with high- level cannot be reduced further without reducing tyre
drainage asphalt. An omni-directional noise source was noise first.
positioned on the centre line of the track at heights be- The EC have responded to this problem by pro-
tween 100 mm and 400 mm above the surface. A micro- posing noise homologation levels for tyres (C30/8, 28/
phone was positioned 7.5 m away from this at a height of 1/98). These have not yet been adopted. A new higher
1.2 m from the surface. The comparison showed that: speed variant of the acceleration test is proposed,
The ISO 10844 surface did indeed behave as whereby a vehicle coasts through the 20-m test area at
a close approximation to a perfectly reflective (low a constant approach speed but with the engine turned
absorption) surface at most frequencies of interest. off. The approach speeds would vary in the range of
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The sound absorption characteristics of the ISO 60–90 km hr according to the type of tyre. The noise
10844 track are more uniform across the frequency limits would vary according to the type of tyre and tyre
spectrum than those of the high-drainage asphalt width.
surface. This suggests consistent surface character-
istics over the 7.5 m acoustic path length and should 22.1.1.5 Noise homologation in the US
result in results being more repeatable and
reproducible. and otherwise outside the EC
Because of the relative smoothness of the ISO 10844 Most countries outside the EC have their own systems of
surface, low-frequency tyre noise was reduced vehicle type approval that include restrictions on noise
compared with the high-drainage surface, but
levels. Most tests and limiting levels are based on those of
high-frequency tyre noise was increased.
the EC or of the United States.
Walker concluded that with all these taken into account, There are two US noise homologation tests – SAE
the ISO 10844 would produce drive-pass noise levels J986 AUG94 which has the vehicle entering a test area
some 2 dB greater than those expected for a given car with predetermined vehicle speed and SAE J1030
tested on the high-drainage asphalt surface. FEB87 which has the vehicle leaving the test area with
In drafting Directive 92/97/EEC, Dunne and Yarnold predetermined engine speed. Both tests feature vehicle
(1993) report that the European Commission recognised deceleration as well as acceleration.
that other factors such as meteorological conditions may The acceleration part of SAE J986 AUG94 is broadly
influence the drive-pass test result. similar to that in the EC test except that the microphone
is positioned at 15 m from the vehicle pathline (rather
than 7.5 m) and the test area is much longer (53 m) with
22.1.1.4 Future developments in noise the aim of allowing the vehicle to reach its rated engine
homologation in the EC speed during the test. In the EC test, the vehicle will
seldom reach its rated engine speed.
Although noise homologation limits have been reduced Generally, a vehicle that achieves noise homologation
significantly since 1970, comprehensive noise testing in the EC will achieve US Federal homologation with
alongside roads in Germany (Steven, 1995) has shown comparative ease. Japan and Switzerland have tradi-
that noise levels produced by vehicles in normal use have tionally had homologation restrictions that are more
reduced only slightly over the first twelve years of that onerous than those of the EC or the US.
period.
The main reason for this seems to be that the homol-
ogation process has forced vehicle manufacturers to 22.1.1.6 The consequences of meeting
reduce engine and intake/exhaust noise which are strongly homologation noise limits
engine-load-dependent and thus dominate the low-speed,
high-load acceleration test. However, the process has The current homologation noise limits according to 92/
had less impact on tyre noise and on the noise caused by 97/EEC are very demanding. A great deal of engineer-
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vehicles at speeds much in excess of 50 km hr . ing effort was devoted in the early 1990s to reduce
The higher speed noise is most important in the envi- noise levels from accelerating passenger cars by the
ronmental impact of vehicles in normal use outside the 3 dBA required to meet a level of 74 dBA. Vehicle
urban setting. manufacturers are achieving type approval for their
It is intuitively obvious that once engine, intake and current vehicles, but it is difficult for those with higher
exhaust noise are reduced further, tyre noise will powered engines (Sports Utilities, GTi models and the

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