Insert loss experiment and acoustic impedance theoretical study on the noise barrier with MPP-QRD top structure.

• The insertion loss of noise barrier with MPP-QRD top structure are measured on site. • The impedance of MPP and QRD structure was calculated respectively to analyze the matching degree. • The acoustic impedance matching degree is significantly correlated with the improvement in noise barrier performance. • The MPP-QRD top structure with acoustic impedance matching can significantly enhance the insertion loss of noise barrier. The effective edge processing is the key to the performance of noise barrier. To prevent the sound diffraction within the low-medium frequency range, the quadratic residue diffuser (QRD) was combined with microperforated panel (MPP) in this study. The impedance of MPP with different parameters and QRD structure with N = 7 was calculated respectively and the matching degree of the composite structure was obtained. The insertion loss of noise barriers with different MPP-QRD top structures was field measured and compared to the equi-height vertical noise barrier. The results show that the degree of impedance matching between microperforated panels and quadratic residue diffusers is significantly correlated with their performance as the top structure of noise barrier. The acoustic impedance matched MPP-QRD top structure can significantly improve the insertion loss of the vertical noise barrierwith the same height. When the microperforated panel perforation is 3 %, the thickness is 0.8 mm, and the aperture is 0.6 mm, performance of the MPP-QRD structure is the best, the relative insertion loss can reach 4.07 dB. [ABSTRACT FROM AUTHOR]