A Method for Experimental Determination of Source Impedance in Helium-Air Mixture

摘要

Acoustic properties of the hot in-duct noise sources (for instance IC engine exhaust) are most precisely determined under the real working conditions. However, the hot pulsating exhaust gas flow causes technical problems: need for transducer cooling, vibration etc. Therefore from practical point of view in many cases the cold environment (room temperature airflow) is used for measurements. Due to the different sound speeds of the environments this usually leads to the incorrect measurement results. In the paper an improved measurement method to characterize the hot in-duct noise system has been developed and tested. In order to simulate the acoustical conditions of the hot gases, helium is used as a testing environment. Since the speed of sound in helium is close to that in the hot gases, the cold exhaust system filled with helium simulates the hot system. The measurement method used is based on the one-port presentation of the source and the standard two-microphone technique is used to determine the acoustic properties of the source. As a test of the method the reflection coefficient of the exhaust port of the automotive IC engine was measured. The measured reflection coefficients are compared with the source data determined using the classical cold airflow technique.