A COMPARISON OF IMPEDANCE MEASUREMENTS USING ONE AND TWO MICROPHONES

Measurements of acoustic input impedance of wind instruments using two different approaches are presented. In the first approach, c ommonly referred to as the two-microphone transfer function method, a tube is connected to the instrument and excited with broad-band noise. Signals recorded at microphone pairs placed along the tube are then analyzed to estimate the instrument input impedance. A calibration step is described, wherein the position of each microphone pair is determined from the measurement of a rigid termination. The second technique, a novel variant of pulse reflectometry, ma kes use of a long tube with a single microphone located at its midpoint. Using a long-duration broad-band stimulus, the impulse response is measured for the tube, first with a rigid termination, and then with the system to be characterized attached. The system reflectance, and therefore its impedance, is found by comparing the first reflection from the tube end for both measurements. The d esign of the impedance probes and the data sampling and analysis procedures are presented. Measurements obtained using the two techniques are compared for various acoustic systems, including an alto saxophone neck and fabricated conical objects. The results show good agreement between the methods. Advantages of the one-microphone technique include ease of use and robustness to noise, while the two-microphone approach can provide a better high-frequency response for long objects.

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