Acoustic studies on small engine silencer elements

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. Acoustic 2-port determination procedure has been followed by implementing a dedicated hot-flow test facility set-up in Tallinn University of Technology. The complete 2-port analysis has been performed on a formula SAE straight-flow racing muffler and on a custom designed small series exclusive motorcycle muffler. Additionally, the acoustic characteristics of wools commonly used in small engine mufflers are studied by testing a number of prepared wool samples. Results are provided to exhibit a vibroacoustic effect of silencer outer shells on the sound transmission through the unit, which was found to significantly affect the passive acoustic data. Experimental results measured for a complete multi-element commercial silencer are compared to the ones obtained by 1D and 3D simulations. Copyright © SAE Japan and Copyright © 2011 SAE International.