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Technical and Industrial
Page 60
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What do the Noise Proposals Mean?
'THE draft regulations on motor vehicle I noise reported in last week's issue could have a profound effect on commercial vehicle manufacturers and operators. Briefly, it is planned to carry out tests of new vehicles at full throttle under special test conditions and roadside tests of vehicles in normal use. The new vehicle test proposals are substantially as laid down in British Standard B.S.3425 : 1961; which in turn are based on 1.5.0. recommendations. They require a site with considerable open space and low background noise.
As indicated in a book by C. H. Bradbury entitled "Engine Noise", reviewed on page 63 of this issue, the relationship of sound level as measured by an instrument to its degree of undesirability as experienced by a human being is extremely complex. Not only do sound intensities vary over an exceptionally wide range, but the decibel used as a sound-level unit, is measured on a logarithmic scale. '
The instruments are designed to match the variation of the ear's sensitivity but cannot directly measure how objectionable a noise is. Mr. Bradbury quotes an example of the way in which this can mislead, in which machinery which produced a comparatively high decibel reading was in fact less objectionable than other machinery which, according to the meter, was less noisy. The reason was that most of the former's noise was at a very low frequency.
Some indication of the standards can be derived from tests carried out by the Motor Industry Research Association using methods akin to the proposed "new vehicle" tests. In these the vehicle approached the test area at approximately 31 m.p.h. in whatever gear corresponded to about threequarters of maximum engine revs, full throttle being applied as the test area was crossed. Makes and models are not specified, but a light van with 950 c.c. four-cylinder petrol engine produced 77 decibels, the lowest of any commercial vehicle tested and comfortably below the proposed ultimate limit of 80 decibels for vehicles of this class. At the other end of the scale three lorries with 326litre three cylinder two-stroke diesel engines produced between 85 and 91 decibels, and would thus contravene the proposed 1965-67 requirement for a maximum of 85 decibels as well as the figure of 83 decibels planned for 1968 onwards for heavy commercials. However, a coach with a similar engine gave a figure of only 82 decibels.
A double-decker bus with 9-8-litre sixcylinder four-stroke diesel engine produced 83 decibels, just on the proposed limit for 1968 onwards,
A varied selection of goods vehicles with both diesel and petrol engines produced between 85 and 87 decibels, but a lorry chassis with 7-7-litre sixcylinder four-stroke diesel engine produced a reading of 80 decibels, the lowest of any " heavy " commercial vehicle.
The roadside test requirements allow 3 decibels above the corresponding new vehicle requirements, and it is understood that it is not proposed to require the vehicles to stop, so that some of the responsibility so far as a potentially noisy vehicle is concerned will rest on the driver and his methods. A.A.T.