Active acoustic metamaterial based on Helmholtz resonators to absorb broadband low frequency noise

Abstract

Metamaterials are a new class of materials that have properties that cannot be found in nature. Though these artificially engineered structures have not been prominently used in many applications, they have been theoretically studied to have a variety of applications in many sectors. Acoustic metamaterials have the capability of manipulating sound waves in order to achieve the required unique properties.
The aim of the project is to design an active acoustic metamaterial consisting of an array of Helmholtz resonators to assist in reduction of noise levels in aerospace applications using the method of 3D printing.
This project presents an active Helmholtz resonator based design that can attenuate a broadband range of targeted frequencies in the low-frequency regime. To this end, a passive metamaterial consisting of an array of Helmholtz resonator unit cells, with a single varying design variable, is designed and tested to establish the effectiveness and region of performance. The selected design variable for change is identified through the frequency response for each parameter of the Helmholtz resonance equation, to achieve a broadband frequency range of the metamaterial. An active model of this design is then fabricated and tested. Two actuation mechanisms are presented for this design. The resulting acoustic systems are capable of providing an attenuation of up to 20 dB, for an open system, and up to 35 dB, for a closed system, at frequencies between 150 Hz and 500 Hz. Unlike most other studies conducted, this active acoustic design is capable of attenuating isolated frequencies as well as multiple frequencies simultaneously. The added control that is achieved through the incorporation of the electric linear motor based actuation allows for the advantage of accurate frequency targeting along with the base attenuation levels of the passive resonant acoustic metamaterial.