Active acoustic metamaterials for high performance noise control

Acoustic metamaterials have been shown to have the ability to achieve high levels of low frequency noise attenuation through the use of engineered structures, which consist of arrays of subwavelength unit cells. These metamaterials exhibit effective material properties that are not available in naturally occurring media. The bandwidth over which these metamaterials can achieve negative effective material properties is generally relatively-narrow due to the use of resonant unit cells. Although the bandwidth of negativity has been extended by integrating active control technologies into metamaterials, the performance of these hybrid systems is still limited by the use of the resonant inclusions. Thus, this thesis presents an investigation into how conventional active control systems influence the effective material properties and whether active acoustic sources can be optimally-driven to directly control the effective material properties and also realise nonreciprocal behaviour. An effective material property optimisation procedure is proposed and it is shown that the optimised single monopole and single dipole control sources achieve broadband negative effective bulk modulus and density respectively, and by combining the two optimised acoustic sources, broadband double negativity is also achieved. Nonreciprocal acoustic devices, which are based on the design of acoustic metamaterials, have shown the ability to achieve nonreciprocal sound transmission. Although existing nonreciprocal acoustic devices can achieve broadband nonreciprocal sound transmission in a one dimensional space, it has not yet been shown that nonreciprocal sound absorption can also be achieved by nonreciprocal acoustic devices in one or two dimensional spaces. Therefore, this thesis also presents an investigation into how feedforward wave-based active control systems can be used to achieve broadband nonreciprocal sound transmission and absorption in both one and two dimensional environments. This active nonreciprocal control strategies are demonstrated through both simulations and experiments in one and two dimensional spaces and demonstrate an effective and adaptable means of realising nonreciprocal control