A COMPARISON OF lWO ACTUATORS FOR A SEMI-ACTIVE HELMHOLTZ RESONATOR

Two electro-mechanical actuators are examined for a semi­ active Helmholtz resonator acoustic device. The device is used to reflect narrowband noise back to the source in an acoustic duct. The controller and actuator are used to tune the system on-line allowing optimum performance over a range of operating conditions. Actuator. dynamics play an important role in the controller design and the operation of the device. Two variations of an electro-mechanical actuator are considered here. The first uses a dual voice coil speaker with local feedback compensation and the second uses the same speaker without the compensation. It is shown that both arrangements are effective but with competing advantages. The compensated actuator provides more control authority but adds considerable background noise while the uncompensated actuator provides less control authority but adds no background noise. The choice of actuator depends on the noise control objectives of the particular application. INTRODUCTION The semi-active Helmholtz resonator (Birdsong, 1999) is an acoustic device with behavior that can be used to selectively quiet narrow band noise in acoustic systems. It consists of a . static Belmholtz resonator with a sensor, controIler, and actuator added to the interior of the resonator cavity (Figure 1). The nominal resonant frequency and damping of the device is determined by the dimensions of the resonator neck and cavity (Temkin, 1981) but can be modified by the closed loop feedback system. When driven bya pressure from a primary acoustic system, such as an acoustic duct, the resonator responds with a large magnitude volume velocity through the resonator neck, which is in phase with the pressure. This creates a "pressure release" boundary condition, which inverts and reflects the incident pressure wave back up the duct, thus reducing the transmitted pressure wave and reducing the transmitted sound