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The engine noise accounts for a relatively large percentage among the noises generated by a motorcycle. Among the parts of the engine, the cover is important in design as well as a source of the engine noise, being at the end of the vibration transfer path. This paper clarifies that the natural frequency of the cover with a flat surface clearly affects its vibration and noise radiation and that its vibration depends on this surface, by investigation of the effect of surface shape of the cover on its vibration and noise radiation and by a modal analysis of its vibration characteristics. In addition, we confirmed that the calculated value of the radiated noise from the cover agrees well with the measured value.

With an increasing number of vehicles, the reduction of traffic noise emissions becomes a greater social requirement. On the other hand, as users' tastes for motorcycles have diversified, it becomes important to develop and supply products to meet customers' requests within a short period. Therefore, it is a key factor to efficiently develop motorcycles which conform to the noise regulations. This paper describes concrete examples of currently utilized noise-source proving methods and noise reduction methods for motorcycles.

The quality of exhaust sound has become one of the important factors in the motorcycle market. Therefore both an efficient sound quality evaluation method and technology to achieve ideal sound quality have become necessary. Sound qualify evaluation has generally been performed by trial and error through repeated modification of exhaust silencer construction until desired quality was obtained. But it usually took painstaking work and long hours. In order to solve such problems, we established an objective auditory evaluation method. We also applied Principal component analysis to analize the result of the “Semantic Differential Method (SD method)” so as to determine the affecting elements. Through this analysis system, “powerful sound” caused by relatively higher content of the low frequency range and “crispy sound” caused by a cyclic sound pattern were determined to be desirable sounds for “American type” motorcycles.