Low-frequency impedance measurement using the two-microphone tube technique

Abstract

As progress is made toward environmental protection and developing lightweight vehicles, noise control in automobiles is becoming increasingly significant. For designing a quieter vehicle, the knowledge of acoustic impedance of the trim materials is crucial to the prediction of the interior noise level in the passenger compartment. Since automobile noise is usually dominated by low frequencies, 50 to 300 Hz, the knowledge of low-frequency impedance of trim materials is particularly important. This paper presents the use of the two-microphone tube technique to measure low-frequency impedance. A purpose-built impedance tube was developed based on theoretical analysis of the evaluation accuracy of impedance. The measurement results are reported for the tests on some typical trim materials, carpet, underlay, seat foam, and acoustic foam, over the frequency range of 100 to 600 Hz. It has been found that the location of microphones is crucial to the accuracy of measurement. Analytical and experimental results showed that the ideal microphone location is that both microphone spacing and the distance from the tested material to the nearest microphone takes the value of a quarter wavelength. Care should be taken to avoid a microphone spacing of a half-wavelength of the frequency of interest.