Development of multi-microphone acoustic impedance tubes to measure at frequencies higher than 10 kHz

Multi-microphone high frequency extension methods have gained attention for measuring the sound absorption coefficient and transmission loss using impedance tubes. The usual upper-frequency limit can be extended by approximately three times beyond the traditional two-microphone tube of the same diameter by suppressing the contributions of high frequency cross-sectional modes. Initially, a 3-D-printed, a 75-mm square-section tube showed promising results, enabling measurements up to 6 kHz. Additionally, the larger sample size significantly reduces edge-constraint effects compared to the traditional, smaller diameter circular tube. Next, the performance of an aluminum tube was compared with that of a 3-D-printed plastic tube. More noise was observed in the measurements taken from the aluminum tube compared to the 3-D printed tube, which was hypothesized to be due to the structural responses of the tube walls. A multi-physics model was developed to simulate the interaction between tube vibrations and the surrounding air, and the results supported the idea that wall vibrations contribute to distorted results in aluminum tubes. Finally, a smaller tube with dimensions of 40 mm x 40 mm was built to enable measurements at frequencies exceeding 10 kHz, but in that case managing minimal microphone-spacing during the fabrication process poses challenges.