Novel fractal acoustic metamaterials (FAMs) for multiple narrow-band near-perfect absorption

In this work, we introduce fractal acoustic metamaterials (FAMs), in thicknesses ranging from 5 (λ/69) to 25 mm (λ/18), which are observed to provide multiple narrow-band low-frequency absorptions of acoustic signals. The fractal structures used in this work are carefully designed and fabricated using a side branch Helmholtz resonator design, making these structures easily tunable to multiple frequencies. Using different sizes of the side branches distributed in a fractally oriented configuration onto a plane rigid baseplate, the propagation velocity of acoustic waves is slowed down considerably. There is also a shifting resonating response of the structures toward lower frequencies (<1600 Hz). These FAM structures exhibit no dependence on the acoustic traverse length, as is otherwise commonly seen in coiled meta-structures and others. In order to achieve a near-perfect sound absorption behavior, the geometry of the structure is theoretically ascertained and validated numerically and experimentally. Significant emphasis has been placed on the associated physical mechanism modulating the loss of intensity of the incident acoustic signals. Moreover, with regression analysis performed on a response surface-based optimization scheme (using Design Expert 11 software), the geometric parameters are determined in a way that the absorption demonstrates a narrow-band characteristic at a frequency of 1 K Hz. We have shown in this work the tunability aspect of the various absorption frequency bands through appropriate designs of the FAM. It opens up wide application possibilities of multiple frequency sound absorptions (acoustic cloaking).