Study On Numerical Prediction And Experimental Measurement Of Acoustic Performance Of Silencers In The Medium Frequency Range

Transmission loss is an important index to evaluate the acoustic performance of a silencer. The calculation and measurements of transmission loss is based on the assumption of plane wave propagation in inlet/outlet tubes previously, which limits to evaluate correctly the acoustic performance of large size silencers in the medium to high frequency range. According to the basic theory of excitation and propagation of higher-order acoustic modes, the computational method and experimental measurement of transmission loss of silencers with consideration of the first high order mode in inlet and outlet tubes are investigated in detail.The finite element method module in SYSNOISE software was used to simulate and calculate the internal sound field of the silencer. Sound field model including (0, 0) and (0, 1) modes was built. The relationship between different mode waves and the sound pressure distribution in the tube was established through 3-D acoustic theory, and the different modes are decomposed. Finally, the transmission loss was determined by the total incident sound power in the upstream tube and the total transmitted sound power in downstream tube.Based on the same theory for the computational method. The relation between internal sound field and the mode waves was built. The sound field is complex in medium frequency range, so an improvement was made based on traditional wave decomposition method, several sensors are used, and the number of testing points was increased through changing the position of sensors. In the final, the mode waves was decomposed by multi-sensor method,and the transmission loss was determined.A code was compiled with FORTRAN language to calculate the transmission loss. Below the plane wave cut-off frequency, same results are obtained by using the plane wave approach and 3-D wave approach in the inlet and outlet tubes. Beyond the plane wave cut-off frequency, a correct transmission loss prediction may be achieved only when the 3-D approach was used. The experimental measurements agree well with the numerical computations in the frequency range of interest. The reasons for the measurement errors are analyzed.