The active control of acoustic impedance

Nicholson, GC

Abstract

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.

Citation

Nicholson, G. The active control of acoustic impedance. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Sep 16, 2011
• Publicly Available Date: Sep 16, 2011
• Award Date: Jan 1, 1994