Enhancing sound absorption towards low frequency in graded anechoic coatings: from inclusion shape and orientation to triple-graded design

Research on the absorption performance of anechoic coatings with graded structures has been insufficient to date. In this study, we examine the influence of inclusion shape and orientation on the absorption characteristics of dual-graded anechoic coatings (DGACs) and subsequently propose a triple-graded anechoic coating (TGAC) to further enhance the sound absorption performance towards low frequency. The results reveal that the inclusion shape plays an important role in shifting the absorption peak positions. For DGACs with rectangular inclusions, aligning the heavy cores parallel and the cavities perpendicular to the incident direction results in the lowest second absorption peak frequency. Leveraging this configuration, the TGAC incorporates trapezoidal inclusions as an additional gradient, resulting in downward shifts of absorption peaks and improved low-frequency absorption. Specifically, the TGAC achieves a first absorption peak at 700 Hz, corresponding to a 21.3% downward shift, and an absorption coefficient of 0.86, corresponding to a 6.2% increase compared with the corresponding DGAC. These findings demonstrate that tailoring inclusion shape and orientation, together with multi-graded structural design, provides an effective strategy for tuning the sound absorption performance of anechoic coatings.