Sound Attenuation Using MEMS Fabricated Acoustic Metamaterials

Abstract

Unlike traditional rotational gyroscopes, MEMS gyroscopes use a vibrating proof mass rather than a rotational mass to sense changes in angular rate. They are also smaller and less expensive than traditional gyroscopes. MEMS gyroscopes are known to be susceptible to the effects of acoustic noise, in particular high frequency and high power acoustic noise. Most notably this has been proven true in aerospace applications where the noise can reach levels in excess of 120 dB and the noise frequency can reach levels in excess of 20 kHz. The typical resonant frequency for the proof mass of a MEMS gyroscope is between 3 and 20 kHz. High power, high frequency acoustic noise can disrupt the output signal of the gyroscope to the point that the output becomes unreliable. In recent years, a large amount of research has focused on the fascinating properties found in metamaterials. A metamaterial is an artificially fabricated device or structure that is built to produce desired material responses that can either mimic known behaviors or produce responses that do not occur naturally in materials found in nature. Acoustic metamaterials in particular have shown great promise in the field of sound attenuation. This thesis proposes a method to limit the signal degradation of the MEMS gyroscope in the presence of high power, high frequency acoustic noise by using a new acoustic metamaterial in the form of a one-dimensional array of Helmholtz resonators. The Helmholtz resonators are fabricated on a silicon wafer using standard MEMS manufacturing techniques and are designed to attenuate sound at the resonant frequency of the gyroscope proof mass. The resonator arrays were diced from the silicon wafer in one inch squares and assembled into a box open on one end in a manner to attenuate sound on all sides of the gyroscope, and to seal the gyroscope inside the box. The acoustic metamaterial was tested in acoustically harsh environments and was found to successfully attenuate sound as much as 18 dB. This attenuation is in the form of a notch filter at and around 14.5 kHz, which was the target frequency of attenuation. The notch filter attenuation occurred over a 700 Hz frequency band with 18 dB being the largest attenuation in the band.