Boonen R., Sas P., Desmet W.

The acoustical impedance of a component (for example a silencer) is one of its most important characteristics. It is needed when acoustical simulations using lumped elements or electrical analog circuits are carried out. The calibration procedure for the two microphone transfer function method described in ISO 10534-2 is not sufficiently accurate in most cases. An improved calibration of the measurement setup is necessary for accurate measurements in the high frequency range. In this research, a new calibration method for the two microphone transfer function method is proposed such that an acoustical impedance can be measured with high accuracy over a wide frequency range. In this method, the estimation of the speed of sound has been eliminated. Exchanging microphone positions is not necessary. Also, the microphone amplitude and phase calibrations have been eliminated. The calibration procedure is reduced to the calibration of the microphone positions. This is established by measuring two transfer functions between the two microphones in the wave guide, whereby the wave guide is closed at two different positions. A first estimation of the sensor positions is made from the measured transfer functions between the microphones. Then, the obtained positions are refined using a recursive procedure until the required calibration accuracy has been obtained. As result, the acoustical impedance of a component can be accurately determined. Finally, the results obtained on an experimental setup, on which the new calibration method will be verified, will be presented.