Acoustic Pitfalls of Impedance Tube Measurements
The impedance tube is commonly used to characterize sound absorbing material. In order to obtain reliable results, the sample must fit perfectly into the cross section of the tube. In practice, this often results in a minimal gap between the sample and the wall of the tube. A narrow gap causes thermoviscous damping. We show by FEM simulations and by experiments that a small gap produces an absorption peak at low frequencies. The sample and the tube behave as a slit absorber. The numerical results were generated with the COMSOL Multiphysics® Pressure Acoustics, Frequency Domain interface and the Narrow Region domain as well as with the Thermoviscous Acoustics Frequency Domain interface. The experiments were performed in an impedance tube of square cross-section and an opening at the front. Cutting and adjusting the size of a square sample is easy. However, to avoid a possible gap and a low-frequency absorption peak, it is necessary to seal the sample with plasticine at all four edges. In addition for the seal to be effective, the sample must be terminated by an airtight frame. Without the frame, air can penetrate through the sample into the gap and produce thermoviscous damping. In this case again the impedance tube becomes a Helmholtz resonator at low frequencies. We can demonstrate by calculations, that complete absorption is achievable for any frequency between 100 and 500 Hz by varying the slit geometry.