Calibration of the two microphone transfer function method by determining the hard wall impedance at shifted reference sections

In many acoustic simulations, particularly when using lumped parameter models or electrical analog circuits, the acoustic impedance of a component needs to be determined accurately. A widely used acoustic impedance measurement method is the ”two microphone transfer function method”, which is standardized in ISO-10534-2. When the acoustic impedance is needed over a wide frequency band with a high impedance magnitude range, this method faces some limitations. In this paper, a calibration method is proposed which uses hard wall impedance measurements at different positions of the reference section. The measured hard wall impedance is used to calibrate the microphone positions, to compensate the microphone mismatch and to estimate the wave guide damping. Also, the measured hard wall impedance can be used as performance criterion. It can be used to select frequency bands from different load impedance measurements where the accuracy is maximum and to assemble them in a load impedance measurement. As result, impedances with a high ratio with respect to the characteristic duct impedance can be accurately measured. The capability of the presented calibration method is illustrated by the impedance measurement of an open duct end and a closed tube.