ACOUSTICAL MODELING OF AUTOMOTIVE EXHAUST SYSTEMS

MAREHALLI GOPALAN PRASAD, Purdue University

Abstract

The performance of automotive exhaust muffler systems has been studied using an acoustical model based on electrical analogue. The various types of performance descriptions studied are transmission loss, noise reduction, insertion loss and radiated sound pressure level for both without and with a mean flow. The prediction schemes developed are verified on both a model exhaust system and a multi-cylinder engine exhaust system.^ Among the studies conducted on the model exhaust system, transmission loss and noise reduction are predicted and measured using both the standing wave technique and the random excitation technique. The source impedance of an electro-acoustic driver was measured using the two microphone random excitation technique. Insertion loss and radiated sound pressure level were predicted by using the (measured) source impedance and compared with experimental results for various model exhaust system configurations. It was observed that provided the measured source impedance is used, the performance can be predicted quite well.^ The investigations on the multi-cylinder engine exhaust system included measurement of source impedance, prediction of insertion loss and radiated sound pressure level. The engine impedance was measured using a transfer function (random excitation) technique. It was observed that speed and load did not have any significant effect on the impedance measured. Insertion loss and radiated sound pressure level were predicted and measured for various operating conditions of the engine. Both mean flow and temperature gradient effects were included in the development of prediction schemes. Also, four-pole parameters for a straight pipe element in the presence of a mean flow and a linear temperature gradient have been evaluated.^ The main observations from the studies on the multi-cylinder engine tested are that the measured engine impedance tends towards the characteristic impedance of the fluid medium for all the operating conditions considered. However, the assumption that the engine impedance is zero or infinity gives poor predictions. The argument between predicted and measured acoustic performance is good, provided that both mean flow and temperature gradient effects are included in analysis. ^

Degree

Ph.D.

Subject Area

Engineering, Mechanical|Energy

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