An Improved Study On The Sound Insulation Test Method For Standing Wave Tubes

The development of automobile industry brings a serious problem of noise pollution. How to effectively reduce the noise has been the concern of researchers at home and abroad. The acoustic properties of material has an important influence on reducing noise, and acoustic characteristics is usually used to measure sound insulation of material, sound insulation performance will enhance with the increase of amount of sound insulation. A quick and accurate measurement of sound insulation is beneficial to choose suitable material, so as to reduce pollution and protect the environment.The method of standing-wave pipe is used widely in laboratory measurement of sound transmission loss, wherein the four microphone method is adopted to measure the sound transmission loss, but the sound absorbing end must be installed to the transmission pipe. It is useful that to install a wedge absorber at the end of transmitting pipe to reduce the acoustic reflection, and the reflection coefficient must be less than 0.01, otherwise multiple reflection of sound waves will existence at the end of transmission pipe. But it is difficult to completely avoid the multiple reflections at the end of transmission pipe. The four-microphone method ignores the multiple reflections, which leads to the test error.In this thesis, the problem of multiple reflection at the end of the acoustic was studied. At first, a modified formula considering all the multiple reflections was obtained, based on the theory of acoustic. Secondly on the four microphone were collected through the experiments in the standing-wave pipe. Then the values of sound pressure were imported into the program of Matlab, the sound transmission losses of sound sample before and after the modifying were calculated, and these results were compared with the theoretical values.The modified formula was applied to the measurement of sound transmission loss for composite board, the results show that the measurement result varies greatly without considering multiple reflections, while it is relatively stable with modified formula, which prove that the modified formula is suitable for the measurement of sound transmission loss for composite board. The sound transmission losses of single board and double layer aluminum plate containing middle air layer were calculated through the Virtual. Lab software, and the simulation results were compared with the theoretical values. It is found if the end of transmission pipe has an ideal acoustic, the end can absorb sound completely, wherein the threemicrophone method could be used to measure the sound transmission loss.