A comparison of the two and four microphone methods for deriving the characteristic acoustic properties of porous materials

Abstract

The characteristic acoustic impedance and wave number of a porous material can be derived using either a two microphone or a four microphone impedance tube setup. For the four microphone method, these complex properties are calculated directly using the transfer matrix obtained from a single sample of any available thickness. The two microphone method relies on surface impedance measurements obtained from two different sample thicknesses. The thickness ratio dictates the suitability of the characteristic property eduction technique-if the ratio is exactly 2, then one can use the two-thickness method to calculate the characteristic acoustic properties; if the ratio is not 2, then one must use the arbitrary-thickness method. In this study, we compare the results obtained from the four microphone and two microphone methods for commonly used porous materials. The obtained results are compared for various sample thicknesses within different frequency ranges. The suitability of each method for different types and thicknesses of porous materials is studied. Our results indicate that the four microphone method provides the most robust measurements and that the results educed from the two microphone methods can show significant deviations from expected values for certain sample thicknesses and frequency ranges.