The two-microphone method for the acoustic impedance tube

Abstract

The acoustic impedance tube is an apparatus that allows acoustic properties of a material to be determined. The two-microphone method is an improved technique for operation of the impedance tube compared with the traditional travelling microphone method. An existing impedance tube at the University of Queensland operates on the traveling microphone method, but is capable of conversion to the two-microphone method. The goal of this thesis was to set up a procedure for the measurement of data resulting from the two-microphone method, and to create an automated computational method to analyse this data. The impedance tube consists of a tubular cavity with a noise source at one end and a test sample mounted within the tube at the other end. The two-microphone method uses a random noise source, coupled with a pair of microphones positioned at two fixed locations along the tube. The sound pressure level at these locations is measured by the microphones, and analysed using a digital frequency analysis system. The transfer function of the two signals can be found, and then used to calculate the absorption coefficient and surface impedance of the test sample. [1] The two-microphone method has a number of advantages over the room absorption method. First, the apparatus itself is much smaller and therefore more practical. Secondly, only a small sized sample is required for the tests, and third, the acoustic impedance tube, in addition to the absorption coefficient, allows the surface impedance to be determined. [1] Ultimately a system was set up which was capable of determining the absorption coefficient and surface impedance of a test sample. The operational range of this system and the accuracy that it can achieve was examined and discussed.  A procedure for the operation of the system was created, allowing future practical use thereof. The results obtained in this project confirmed the correct operation of the two-microphone method for the impedance tube. Materials tested performed as expected in all cases, with slight variation in accuracy and precision. A number of possible sources of error were identified, and possible future improvements suggested. The MATLAB program written allowed for automated analysis of data and plotting of acoustic properties. While operating within the calculated frequency limits of the impedance tube, the two-microphone method proved to be an accurate and extremely fast method of determining acoustic properties of sample materials.