Multifunctional acoustic metamaterial composites

Abstract

Composite structures used in aerospace applications are designed to optimize high bending stiffness to weight ratio. Acoustical performance of composite structures, however, is notoriously poor due to decreased mass law performance and decreased coincidence frequency. Efforts to improve the acoustic properties of composite structures have included optimization of material selection as well as addition of acoustic treatments. Additionally, development of composite structures to include multifunctional properties has increased. In this study, two types of multifunctional approaches were examined using acoustic metamaterials. Locally resonant acoustic metamaterials (LRAM), which have sound insulation performance of 500% increase over the mass law prediction, were integrated into a high-strength sandwich structure array to provide increased acoustic performance without increase in mass. LRAM were constructed of thin, tensioned membranes with centrally located masses in the form of small magnets. Varying the geometry of the metamaterial to tune effective frequencies optimized acoustic performance. Acoustic excitation of the central magnet was used to harvest energy by positioning a wire coil around each cell such that the displacement of the magnet induced a voltage. Combination of energy harvesting and array configurations produced multifunctional materials with three applications: increased stiffness, optimal acoustic performance, and harvesting of acoustic energy.