An Eigenvalue Based Acoustic Impedance Measurement Technique

A method is developed for measuring acoustic impedance. The method employs a one-dimensional tube or duct with excitation at one end and an unknown acoustic impedance at the termination end. Microphones placed in the tube are then employed to measure the frequency response of the system from which acoustic impedance of the end is calculated. This method uses fixed instrumentation and takes advantage of modern Fast Fourier Transform analyzers. Conventional impedance tube methods have errors resulting from movement of microphones to locate the maxima and minima of the wave pattern in the impedance tube or require phase matched microphones with specific microphone spacing. This technique avoids these problems by calculating the acoustic impedance from measured duct eigenvalues. Laboratory tests of the method are presented to demonstrate its accuracy.