Active control of viscoelastic metamaterials

Abstract

Metamaterials have been the subject of significant interest over the past decade due to their ability to produce novel acoustic behaviour beyond that seen in naturally occurring media. As well as their potential in acoustic cloaks and lenses, of particular interest is the appearance of band gaps which lead to very high levels of attenuation across the material within narrow frequency ranges. Unlike traditional periodic materials which have been employed at high frequencies, the resonant elements within metamaterials allow band gaps to form within the long wavelength limit; at low frequencies where it is most difficult to design satisfactory passive isolation solutions. Hence metamaterials may provide a useful path to high performance, low frequency isolation. Passively these band gaps occur over a narrow bandwidth, however the inclusion of active elements provide a method for enhancing this behaviour and producing attenuation over a broad band. Two active metamaterials are investigated in this thesis, first a novel viscoelastic metamaterial is developed that achieves double negativity and could be employed as a high performance vibration isolator at low frequencies. A prototype is produced, the band gap confirmed in the laboratory, and active control is applied to create a wideband region of attenuation. Next an acoustic metamaterial consisting of an array of Helmholtz resonators is presented and it is shown that such a metamaterial has a resonant band gap and dispersive density and bulk modulus. The acoustic metamaterial is produced using 3D printing techniques and the predicted behaviour confirmed in the laboratory. Finally an active Helmholtz resonator is introduced as a pathway to creating an active acoustic metamaterial and the potential for band gap and material parameter manipulation is investigated before a prototype resonator is produced and feedback controllers applied, enhancing the band gap attenuation.

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