Design and experimental investigation of broadband quasi-perfect composite loaded sound absorber at low frequencies

We designed a composite loaded sound absorber (CLSA) composed of a sub-wavelength Helmholtz resonator and porous material, which was loaded on the side branches of a rectangular tube. Based on the transfer matrix method, an equivalent acoustic model was established to calculate the acoustic performance of the absorber. According to the equivalent acoustic model and critical coupling theory, a loaded absorber with thickness of 51 mm and perfect absorption at 156 Hz was achieved using the complex frequency plane analysis method (CFPM). The finite element simulation and acoustic experiment results verified the perfect sound absorption effect of the absorber at low frequencies. To overcome the defects of single frequency and narrow band of the perfect absorber, we designed nine different porous perfect absorber units that could achieve perfect sound absorption between 160 Hz and 312 Hz. Different numbers of porous perfect absorbers were placed on the side branch of the rectangular tube, and a broadband quasi-perfect sound absorption of more than 0.9 between 160 Hz and 285 Hz was realized by optimization. The experimental results further verified the quasi-perfect sound absorption effect of the CLSA. The theoretical model of low-frequency broadband quasi-perfect sound absorber and the design of a CLSA in this study provide a new possibility for the reduction of low or medium frequency noise in the tube.