The Measurement of Low Frequency Impedance Using an Impedance Tube

An accurate and reliable prediction of car interior noise would be a significant step towards improving the design of car body panels and trim for a quieter vehicle. Automobile noise is dominated by low frequencies, from 50 to 300 Hz and therefore a technique is necessary to measure acoustic impedance of the trim materials at low frequencies. This paper presents the use of a two-microphone technique to measure impedance at low frequencies. A purpose built impedance tube has been developed based on theoretical analysis of the influence of microphone location on the estimate of impedance. It has been found that the location of the two microphones is crucial to the accuracy of measurement. Analytical results have shown that the ideal microphone location is with both the microphone spacing and the distance from the tested material to the nearest microphone taking the value of a quarter wavelength. Microphone spacing of a half wavelength should be avoided.

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