Q In the description of the Scania quiet
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bus (CM. May 7 1971), it is stated that a noise level of 77 dBA is approximately haff an 86 dBA level. Is this correct and what is the explanation? And were the noise levels quoted measured inside or outside the vehicle?
Is there any reason why the same engine fitted in a lorry should make more noise? Does the noise that can be heard by a bystander entirely depend on the amount of money spent on noise suppression?
ASound level in clEIA is derived from a formula that includes log,„. Thus in a
particular case, dBA= D N/3300 + 50 log,„ 8+53.8 where D is the slope of the cylinder pressure spectrum in dB/decade: N is the rotational speed in rpm and B is the bore in cm. A -=A weighting, which is determined by the response to noise of a typical group of people. The formula shows that an increment of 10 dB is required to double noise. Noise is "unwanted sound" and in the case of a vehicle is measured at a predetermined distance from the vehicle. BS 3425:1966 gives the test procedure used in the UK.
The noise of the Scania engine in the quiet bus was reduced from around 86 dBA to 77 dBA by insulating the compartment.
In another application, the noise recorded might be increased to more than 86 dBA by the resonance at a critical speed of bodywork panels. The noise of the Scania 110 truck engine is of the order of 86 dBA, Insulation can be applied to the engine compartment or to sections of the engine, such as the tappet cover, timing gear cover and sump, as indicated by experiments of the Institute of Sound and Vibration Research (CM, December 25 1970). A report on the work of the Institute was published in the March 19 1971 issue of CM. As mentioned in this article the noise produced by a bare engine can be reduced by providing an appropriate structure and (as also shown by the formula) noise is proportional to engine speed and cylinder bore size.