Thin Acoustic-Composite Structures With Metamaterial Inclusions for Enhanced Low-Frequency Sound Absorption

Abstract Thin acoustic composites made of a conventional porous material with subwavelength metamaterial inclusions are investigated in this work. The labyrinthine inclusions improve sound absorption by introducing a tuned multiresonant behavior. The aim of their design is high performance at lower frequencies, which conventional materials lack. The resonance frequencies of metamaterial inclusions can be tuned to known noise conditions and/or to compensate for particularly poor performance of the conventional porous matrix. To avoid unnecessary constraints in composite design, the thickness of the porous material can be different (smaller) than the thickness of the inclusions. The result is a two-layer matrix in the form of a porous material with an air gap underneath. The gap can be used to reduce the added thickness of the composite panel by integrating the lower parts of the inclusions into the supporting wall. The developed modeling and design procedure is illustrated with two examples of hybrid composites with labyrinthine inclusions. Acoustic tests performed on two manufactured samples confirmed the predicted multiresonant behavior and overall sound absorption of the composites. This approach can also be used to test new acoustic metamaterials whose samples, due to their shape or size, cannot fit tightly into an impedance tube.