Active control of wall acoustic impedance

A novel kind of acoustic liner with controllable impedance is proposed. A set of active control systems has been designed for this kind of liner to control the sound absorption coefficient or the impedance of the liner. The control is achieved by adjusting the liner cavity depth and bias flow rate through the perforated plate of the liner. The theory needed to assess the details of this idea is described. Results also are provided of control experiments in which the sound absorption coefficient of the liner is controlled to remain optimal for any external sound source condition or the impedance is controlled to satisfy a specific impedance condition. When compared with existing methods for the active control of sound propagation with loudspeakers as the actuators, active control of acoustic impedance might lead to a new concept for the control of sound propagation and unsteady flow in turbomachinery. The present experiments show the possibility of using this kind of liner to realize these objectives.