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Dedicated testrig for acoustic characterization of automotive silencers
Abstract
Acoustic silencers are widely used in applications where suppression or attenuation of sound is required and they have become a common part of IC engine exhaust system. In practice the performance of a silencer is usually characterized by the transmission loss and the pressure loss coefficient. Despite of being designed to silence the noise in high temperature exhaust gas flow it is common to determine their performance in room temperature air flow environment. This naturally leads to a questionable validity of the experimental data. In this paper an overview of unique experimental facility established in TUT Dept. of Automotive Engineering for characterization of silencers is given. In this test facility the temperature and the velocity of the flow through the silencers can be varied to simulate the exact operating conditions of interest. The acoustic transmission loss is measured by following the classical two-port determination procedure. The results presented for a passenger car exhaust silencers together with the pressure loss data are measured in hot mean flow conditions. Based on the results the effect of the flow temperature and velocity on the sound transmission is critically analyzed.
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Sound reflection from hot flow duct openings is a classical problem in acoustics. In practice this is important for effective modelling and prediction off noise radiation from engine exhaust systems, burner pipelines, exhaust nozzles etc. Despite several experimental and theoretical investigations in the field, there is still limited experimental data available to validate the existing theory. In the present study, experimental investigations of plane acoustic wave reflections at duct openings where a hot jet flows into relatively cold surrounding media have been carried out. Heated air with well determined and homogenous chemical consistency along the duct axis was used as a testing media inside the duct during the experiments. The studied jet temperatures exhausting from the pipe ranged from room temperature up to 500 degrees C. The standard two-microphone technique was applied to determine the reflection properties at the duct opening. The experimental results for the reflection coefficient magnitude and phase have been compared with Munt's theory and good correlation was found. This result is a first experimental validation of the theory for hot flow conditions. (C) 2008 Institute of Noise Control Engineering.