The active control of acoustic impedance
The application of an active control force on a thin-walled acoustic boundary can modify the motional dynamics, and so influence the impedance presented to incident waves. This impedance determines transmission of acoustic energy, reflection of acoustic waves from the boundary and absorption of incident energy. This thesis studies control systems that generate control forces for the active control of surface acoustic impedance. The proposed systems rely on measurement of the acoustic pressure and surface velocity of the boundary. The systems can use adaptive digital signal processing, which offers significant advantages over non-adaptive techniques. The active control of the specific acoustic impedance of a loudspeaker that terminates a waveguide for axially propagating plane waves provides a motivating problem. Theoretical analysis establishes the control of specific acoustic impedance of a simple compliantly-suspended piston by a control force. Operational constraints of a physical piston define theoretical operating limits for controlled specific acoustic impedances. The control systems use either feedback or feed-forward techniques for which theoretical treatment reveals restrictions on the range of controlled specific acoustic impedance. A novel result is that conventional implementations of the control systems can be unstable for certain desired impedances unless feedback cancellation is used. Digital feedback techniques are less effective for broader frequency bandwidth where feed-forward techniques may work. Theoretical analysis produces solutions that confirm the feasibility of these control techniques for the active control of specific acoustic impedance. Potential errors in the implementation of the systems have predictable effects on the controlled specific acoustic impedance. Experimental results support the theoretical work presented in this thesis, demonstrating active control of specific acoustic impedance for normally incident acoustic plane waves. An adaptive digital feed-forward control system creates desired specific acoustic impedances for band-limited noise and transient signals.