Measurements of the normal incidence sound absorption coefficient for Non-standard sized absorbers in impedance tubes

A measurement method is proposed for the normal incidence sound absorption coefficient of absorbers whose sizes are non-standard and distinctly smaller than inner dimensions of the impedance tube cross section, and the accuracy of the method is also evaluated. In this method, a type of parallel-arranged acoustic material (PAM) is to fill the non-standard sized absorber to be a flat inhomogeneous acoustic impedance boundary (IAIB) with the standard size. Then acoustic impedance of IAIB is measured in impedance tubes according to ISO 10534-2, and the acoustic impedance and absorption coefficient of the non-standard absorber is evaluated with the proposed method. Numerical simulations and experimental measurements were conducted to compare absorption co-efficients of non-standard absorbers evaluated by the proposed method with those of the standard sized absorbers of same materials, where the influence from the IAIB parameters on accuracy of the proposed method are investigated as well. Numerical results show that, absorption coefficients measured by the proposed method for non-standard absorbers differ within 10% from the actual values if the area ratio of the absorber to the impedance tube cross section is larger than 43%, and acoustic impedance of PAM and the area ratio of the absorber mainly influence the evaluated results. Experimental results provided further validations on the pro-posed method.