A square cross-section impedance tube for extending the effective frequency range

The most widely used method for measuring the acoustic properties of materials is the two-microphone method (TMM). However, the TMM is limited by the plane wave assumption, which constrains its effective frequency range. While simultaneously utilizing an impedance tube with smaller diameter can expand this range, it also intensifies edge constraint effects and increases measurement inconvenience. To address these issues, this paper constructs a square cross-section impedance tube and applies the modal cancellation method (MCM) to extend the effective frequency range. The proposed method can extract plane waves within a cut-off frequency of order (2, 2) by summing sound pressures measured by four microphones positioned at the midpoint of each side of a cross section of a square tube. As a result, it is capable of measuring acoustic properties across a frequency range three times wider than that achievable with conventional TMM, with a more straightforward implementation and without the need to reduce tube size. Finally, simulations and experiments are conducted to verify the validity of proposed method.