The Effect of Active Noise Control on the Effec- Tive Material Properties

Active noise control has been used in a variety of applications where it is impractical to achieve significant levels of noise control using traditional passive noise control treatments, for example, at low frequencies. In recent years, a similar performance benefit has been gained through the use of acoustic metamaterials, which achieve levels of noise control performance that are not achievable with naturally occurring materials. The high levels of noise control performance are generally attributable to the dispersive properties of these metamaterials and, as such, their behaviour has generally been evaluated in terms of their effective material properties, such as the effective density and bulk modulus. It is well known that the performance of active noise control systems is also frequency dependent, however, the links between these two advanced noise control strategies have not been extensively investigated. Therefore, this paper presents an investigation into how active noise control systems implemented in a duct modify the effective material properties. This work, therefore, begins to make links between the behaviour of acoustic metamaterials and active noise control systems.