Measurements of normal incidence sound absorption coefficient for non-standard sized samples in impedance tubes

A measurement method is proposed for the normal incidence sound absorption coefficient of non-standard sized samples in impedance tubes, and the corresponding parameters are systematically addressed as well. At first, a type of parallel-arranged acoustic material (PAM) is applied to fill the non-standard sized sample to be a flat inhomogeneous acoustic impedance sample (IAIS) with the standard size in impedance tubes. Secondly, the surface acoustic impedance of IAIS is obtained in impedance tubes according to GB/T 18696.2—2002, and then the surface acoustic impedance and absorption coefficient of the non-standard sample is evaluated with the proposed method based on the equivalent circuit method. A parameter study is conducted in numerical simulations and experimental measurements. Results show that, the accuracy of the proposed method improves with the increase of the area ratio of the sample to impedance tube cross section, and improves with the decrease of the length of the scattering edge. It is also found that, for porous material samples, a non-rigid PAM with surface impedance close to that of the sample is beneficial to the measurement accuracy; while for resonant absorbing samples, the measurement accuracy can stay reasonable even if a rigid PAM is applied.