Characterization of rasping noise in automotive engine exhaust ducts

This paper presents an experimental study of a non-linear acoustic phenomen on resulting in the generation of a large high component in the automotive engine exhaust system. This phenomenon may occur when an engine is submitte d to an abrupt acceleration. The perceived noise, known as "rasping noise" has a metallic and strident tone. The results of frequency and wavelet anal ysis of the tailpipe noise are presented. Both of them show that a high-fre quency component is instantaneously generated at a critical engine speed. A n efficient criterion based on multiresolution wavelet analysis is proposed in order to detect the rasping occurrence and quantify the rate of high-fr equency component in the tailpipe noise. The obtained results are well corr elated with subjective analysis. The analysis of the internal pressure alon g the exhaust pipe shows that the rasping noise is generated in the interme diate tube and that the acoustic pressure measured at the exit of the manif old does not contain any high-frequency component. The authors favour the h ypothesis of non-linear coupling between resonance modes which explains the sensitivity of the rasping noise generation to the slope of the accelerati on. (C) 2001 Academic Press.