Copyright SFA - InterNoise 2000 1 inter.noise 2000 The 29th International Congress and Exhibition on Noise Control Engineering 27-30 August 2000, Nice, FRANCE I-INCE Classification: 2.6 IDENTIFICATION OF THE LINK BETWEEN DISTURBING ELEMENTS OF AUTOMOTIVE EXHAUST NOISE AND THE CONSTRUCTION OF THE EXHAUST SYSTEM L. Dedene*, R. Valgaeren**, M. Van Overmeire*, P. Guillaume* * Dept. of Mechanical Engineering, Univ. of Brussels, Pleinlaan 2, B-1050, Brussels, Belgium ** Bosal Research NV, Dellestraat 20, B-3560, Lummen, Belgium Tel.: +32-2-629.28.06 / Fax: +32-2-629.28.65 / Email: ldedene@vub.ac.be Keywords: SOUND QUALITY, EXHAUST SYSTEM, WHISTLE ABSTRACT In this contribution a sound quality investigation concerning automotive exhaust noise will be presented. The investigation deals with the disturbing elements of exhaust noise (e.g. booming noise, hollow sound, whistle, ...). The investigation consists of three parts. The first part consists in determining a definition for each disturbing element (establish a sound quality catalogue). The next step is to establish a metric for each defined element. The final step is than to find the link between the established metrics and the geometry of the exhaust system. This is a difficult step due to the large number of components (tubes, baffles, perforations, ...). The exhaust noise is also influenced by the flow-induced noise. This paper will be focused on determining the link between the metrics and the exhaust system’s geometry. For the disturbing element whistle the link between metric and geometry will be discussed in detail. 1 - INTRODUCTION The sound quality of an exhaust system has become very important. Up to a few years ago, experts performed the sound quality assessment by listening. This is a subjective approach based on experience. To improve the design process there is a need for the objectivation of the sound quality. Once the sound quality can be determined objectively, one can set up demands and ”measure” to what extend these are met, which allows a direct optimisation. Thus the sound quality is converted in a set of objective design parameters (metrics). The next step is then to find the link between the construction of the exhaust system and the metrics. The sound quality of an exhaust system consists of two parts: the disturbing sound elements and the sound quality character. The disturbing sound elements are sounds that make the exhaust system sound deviant or strange. Generally, a disturbing element is only noticeable in a certain rpm range and/or in a specific engine operation. The sound quality character describes the sound perception of the exhaust system, such as sporty, cheap, ... First the disturbing elements have to be removed or at least reduced and afterwards the sound quality character can be evaluated. This article is focused on the disturbing elements. The research discussed in this paper is performed on exterior tailpipe noise. The artificial head is positioned behind the car at 1.5 m from the tailpipe. This position combined with the use of insulating shields quasi eliminates all other noise sources and only exhaust noise is measured. 2 - APPROACH The final goal of a sound quality research should be to determine the link between the sound perception and the construction of the product. Once this link is known each sound quality problem can be solved directly. This goal can be achieved in three steps: the sound quality assessment, analysis and identification. Each of these steps requires knowledge of a specific (and thus different) domain. The approach is schematically represented in figure 1. This approach is applicable to any product and for