Development of acoustic metamaterial vibrational shields for application at cryogenic temperatures

Abstract

Phononic crystals and acoustic metamaterials are prominent areas of research in the fields of acoustics and solid mechanics. These materials allow for the manipulation and suppression of elastic waves, for example sound waves in air or vibrational waves in solids. In particular, the mitigation of vibrational waves is of interest, with even small vibrations potentially causing noise in some low temperature experiments. In this thesis phononic crystals are presented as a method of vibration isolation at cryogenic temperatures. The design and development of the acoustic metamaterials was guided by numerical simulations performed using the finite element method (FEM), where band structures were optimized and designed for a suitable geometry and material composition. Two of the simulated designs were fabricated with acrylic plastic and characterized using optical interferometry. By comparing the motion of the metamaterials with a reference bulk material the attenuation caused by it could be determined, resulting in an upper bound of 15 to 20 dB in attenuation across a frequency interval of roughly 7.0–11.0 kHz. Simulated metamaterials show encouraging results with band gap generation at frequencies below 4 kHz through careful considerations of the utilised materials. Further work is required to realise the experimental implementation, with reducing the noise floor of the interferometry measurements of particular importance, along with producing and testing more samples.