Kabral, R., Rämmal, H., and Lavrentjev, J., "Acoustic Studies of Micro-Perforates for Small Engine Silencers," SAE Technical Paper 2012-32-0107, 2012, doi:10.4271/2012-32-0107.
To respond growingly strict environmental regulations the acousticians are challenging to develop novel types of silencing elements. There are different types of flow duct elements designed for silencing the pulsating gas flows into and out of fluid machines. The silencing effect is typically achieved by introducing acoustic reflection and absorption.In order to achieve a good absorption in a wide frequency band, various fibrous materials e.g. wools are typically implemented. However, the physical properties of such materials do not often remain constant during the lifetime of a silencer. As the fibers tend to relocate and can partly be blown out to surroundings, acoustical performance may deteriorate. Therefore, it is in great interest to avoid fibrous materials in the design of the flow duct silencing elements.The present work is focused on the modern type of absorptive acoustic element - a micro-perforated element. The absorption in micro perforated element is dominantly originating from the viscous effect inside the openings, non-linear acoustic vortex shedding and grazing flow iteration. To exert the advantages of this type of perforation, the opening dimensions have to be smaller than the thickness of the acoustic boundary layer i.e. in the sub millimeter range. Although different micro-perforated panels have been studied by several authors mainly for room acoustic applications, only a relatively few publications concerning the flow duct elements are currently available.In this paper a number of different perforated elements including micro-perforates have been studied by varying the perforation shape and porosity. The studies have resulted in the development of micro perforated tubes with high absorptive properties. The micro-perforated tubes have successfully been implemented in the design of a Formula SAE racing silencer.The scattering matrix elements, as well as the transmission loss are presented and analyzed for a variety of perforated tubes and the complete silencer.The results exhibit a good potential for the micro-perforated elements developed. Also it has been demonstrated that this type of absorptive element can effectively substitute the conventional designs incorporating fibrous materials.