Method for measuring normal incidence sound absorption coefficient of non-standard sized samples

A method for measuring a normal incidence sound absorption coefficient of a non-standard sized sample, comprising the following steps:

• arranging, next to the non-standard sized sample, a parallel-arranged acoustic material (PAM) having a specific acoustic impedance and a same thickness to form a flat-surface inhomogeneous acoustic impedance sample (IAIS) having a same cross section as an impedance tube;
• obtaining a relational expression between a surface acoustic impedance of the IAIS and the non-standard sized sample and the PAM by using a parallel relationship between the non-standard sized samples and the PAM;
• measuring the surface acoustic impedance of the IAIS and a surface acoustic impedance of the PAM;
• deriving a surface acoustic impedance of the non-standard sized sample according to the relational expression between the surface acoustic impedance of the IAIS and the non-standard sized sample and the PAM and by using the measured surface acoustic impedances of the IAIS and the PAM, and obtaining the normal incidence sound absorption coefficient of the non-standard sized sample by using real and imaginary parts of the surface acoustic impedance;
• wherein a cross-sectional size of the non-standard sized sample is smaller than a cross-sectional size of the impedance tube;
• and
• wherein the non-standard sized sample is arranged at an end of the impedance tube, and a gap between the non-standard sized sample and an inner wall of the impedance tube is filled with the PAM having the same thickness, to form the IAIS having a uniform thickness and a flat surface without a back cavity from the end of the impedance tube;
• and a plane wave is incident from a left side of the impedance tube.