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A modern exhaust silencer system designed for an internal combustion engine typically incorporates a number of acoustic elements, which all contribute in the overall acoustic performance of the system and determine the sound radiation into the surroundings. The characteristics of individual elements in acoustic silencers affecting sound propagation are referred to as the passive acoustic effect treated in this paper. An acoustic transmission loss is a parameter often used in engineering to describe the passive acoustic performance of exhaust system elements. However, in order to provide a complete acoustical characterization of silencers and silencer components the acoustic 2-port elements (the scattering matrix or alternatively the transfer matrix) should be additionally analyzed. In this paper the scattering matrixes are studied systematically for several small engine silencer elements in a variety of operating conditions.

A critically high noise level inside protective helmet is a prevalent concern for motorcyclists. Especially at highway speeds where the noise level, regardless of helmet type can exceed 100 dB(A) and approaches threshold of discomfort, often resulting in temporary hearing loss. Despite of large share of persons exposed to such noise disturbance around the world, the in helmet noise levels have not significantly decreased over the last decades. Only few scientific publications can be found to systematically address this issue. Furthermore, in respect of driving safety even moderate noise levels are reported to impair reaction times and reduce attention of motorcyclists. At higher speeds the dominant helmet noise source is linked to aerodynamic turbulence around the helmet shell. The loudness and spectral contents mainly depend on the driving speed, windscreen configuration, riding position and helmet geometry.