Least-squares approach for improving the accuracy of bulk acoustic property measurements using an impedance tube

The bulk acoustic property (characteristic impedance, propagation wave number, or propagation constant) of porous acoustic material with an impedance tube is generally calculated by the two-cavity method, which is calculated from the specific acoustic impedance obtained by the two-microphone transfer function method (ASTM E1050, ISO 10534-2) or from the transfer matrix measured by the four-microphone transfer matrix method (ASTM E2611). These methods can obtain the bulk acoustic property from the results of up to two measurements with different termination conditions. However, the bulk acoustic property may not be obtained correctly due to calculation singularities caused by the test specimen and/or measurement conditions, and calculation errors caused by setting errors regarding the dimensions or environmental conditions of the tube nd/or test specimen. As a result, to reduce the errors in the bulk acoustic property, workarounds such as measuring the specific acoustic impedance or transfer matrix under multiple termination conditions (n conditions) and averaging the bulk acoustic properties obtained for all combinations of two termination conditions have sometimes been used. Therefore, to improve the error in the bulk acoustic properties, a new calculation method based on the least squares method using all the data measured under multiple termination conditions is proposed.