Abstract: A speaker 20 has a cavity 28 that has dynamic pressure actively controlled by a pressure control device 42. The control circuit 40 may be analog or digital, open loop, closed loop, or a hybrid open closed loop design. In the closed loop and hybrid versions one or more sensors generate signals for a control circuit 40. In the open loop version predetermined signals independent of cavity pressures drive device 42 and sensors are not used. The pressure control device can alter changes in the dynamic pressure in the cavity 28 to reduce distortion of the sound transducer T1; or produce a multitude of effects on the output of T1 such as maximums and minimums over arbitrary bands, a flat response, simulated passive cavity effects, or other effects not possible with passive cavity designs. The cavity is actively driven to produce arbitrary cavity waves or pressure effects. In this way any arbitrary passive cavity design may be simulated.