There is a growing interest in the automotive industry on micro-perforated acoustic elements for reducing noise radiation from engine systems. The traditional means of acoustic attenuation through porous materials has some serious drawbacks (poor efficiency at low frequencies, low capability to withstand flow and thermal conditions commonly encountered in engine systems). Novel elements consisting of holes/slits of sizes of about 10-1 mm are currently investigated. An indicator of paramount importance when designing such elements is the acoustic flow impedance (pressure drop across the sample/acoustic particle velocity). A detailed description of the impedance of these elements is necessary for evaluating the efficiency of reflection of acoustic waves. This is required for reducing the propagation of noise further downstream. A concise fundamental theoretical investigation for optimal design is not possible due to highly complex geometries attributed to micro-perforated patterns. Moreover, mean flow, thermal and non-linear effects come into play for vehicle applications. This paper describes preliminary results from a project investigating the above effects. Perforated samples have been used in building acoustics for many years. Its new use for the automotive industry requires a more detailed knowledge of the flow impedance. Specifically, it is necessary to estimate the influence of parameters solely encountered in an automotive environment such as: flow Mach number up to 0.3, high sound level that might cause non-linearity effects, chemical and thermal effects due to combustion mechanisms. The ultimate goal is to derive semi-empirical laws including these parameters to assist muffler design procedures. This study has only laid the foundations of a vast investigation by presenting the basis of the experimental technique to obtain the absorption coefficient of perforated samples of well-defined geometries as a benchmark test. The aim is to refine such technique to include transmission loss, two-port measurement to perforated samples with more complex geometries.
Development of micro-perforated acoustic elements for vehicle applications
Entwicklung von mikroperforierten Schallelementen für Fahrzeuge
2005
8 Seiten, 5 Bilder, 6 Quellen
Conference paper
Storage medium
English
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