A multi-microphone technique for extending the upper-frequency measurement range in an acoustic impedance tube

High frequency extension methods have recently been gaining attention for measuring the sound absorption coefficient and transmission loss when using acoustic impedance tubes. The usual upper-frequency limit can be extended by approximately four times beyond the traditional two-microphone tube of the same diameter by suppressing the contributions of high frequency cross-sectional modes within the tube. In this study, Finite Element Method (FEM) simulation models were developed to compare circular and square-section tubes and distinct advantages of the square-section configuration were identified. In square-section tubes, all cross-modes are symmetrical and so can be easily canceled by simply summing the signals from flush-mounted microphones. In contrast, circular tubes feature radial modes that cannot be so easily canceled. To further explore the extension method, the effects of microphone placement and spacing were studied with simulated porous foams and micro-perforated plates as the test articles. Subsequently, a square-section tube was constructed and used to measure actual samples; good agreement with the simulations was found. This study demonstrates that using multiple microphones allows for accurate absorption coefficient measurements up to 6 kHz without the significant edge constraint errors typically observed when using smaller tubes to cover the same frequency range.