The drive-past noise of diesel-engined commercial vehicles is controlled by engine speed rather than road speed. The principal noise source related to the power unit is the noise radiated by the engine structure, which has a level of 85–91 dBA during the ISO test. The combined intensity of all the other such sources is, at the most, equal to the noise radiated by the structure. It has been found that the parameters controlling engine noise are the bore, speed, and combustion systems. This has led to an empirical formula which predicts engine noise to within ±2 dBA.
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