Active Helmholtz Resonator With Positive Real Impedance

The impedance of a passive noise control device is strictly positive real, if the device is installed in noise fields with weak mean flows. Passive noise control devices are, therefore, more reliable than active ones. Active control may be applied to a Helmholtz resonator to introduce electronic resonance. It will affect the impedance $Zact$ of the resonator. A controller may be designed such that (a) $Zact$ is small and resistive at some tunable frequencies; and (b) $Re{Zact}⩾0$ in the entire frequency range of interest. If criterion (a) is satisfied, the active resonator can suppress duct noise at tunable frequencies. It is difficult to design a controller to satisfy criterion (b) because parameters of the controller depend on acoustic parameters of the noise field. A new method is proposed here to design an active controller to meet both criteria simultaneously. The satisfaction of criterion (b) implies a positive real $Zact$ and a robust active resonator with respect to parameter variation in the noise field. Experimental results are presented to verify the performance of the active resonator.