Methods of Correcting Errors caused by Reflection from the Measuring Microphone in the Measurement of Acoustic Impedance by the Standing Wave Method

Abstract

When acoustic impedances are measured by the standing wave method, scattered waves generated at the movable microphone in the acoustic tube cause errors in measured values of acoustic impedances. Since these errors are complicatedly affected by reflections from the both ends of the tube, it is very difficult to correct these errors directly and to acquire the true value of the acoustic impedance. The correction will be made much more easily if another fixed microphone is added at the test piece end of the standing wave tube. The standing wave is then measured as the ratio of output of the movable microphone to that of the fixed microphone. It could be proved theoretically that the ratio of the true value to the measured value of reflection coefficients depends solely upon the four-terminals coefficients of the movable microphone. Following the method described above, the error caused by the scattered waves can easily be corrected by comparison with the measured value of a standard sample, or by calculation involving four-terminals coefficients of the movable microphone.