An Electronically Tunable Resonator for Noise Control

Many engineering systems create unwanted noise that can be reduced by the careful application of engineering noise controls. When this noise travels down tubes and pipes, a tuned resonator can be used to muffle noise escaping from the tube. The classical examples are automobile exhaust and ventilation system noise. In these cases where a narrow frequency band of noise exists, a traditional engineering control consists of adding a tuned Helmholtz resonator to reduce unwanted tonal noise by reflecting it back to the source (Temkin, 1981). As long as the frequency of the unwanted noise falls within the tuned resonator frequency range, the device is effective. However, if the frequency of the unwanted sound changes to a frequency that does not match the tuned resonator frequency, the device is no longer effective. Conventional resonators have fixed tuning and cannot effectively muffle tonal noise with time-varying frequency. The electronically tunable Semi-active Helmholtz Resonator (SHR) is a novel noise control device that includes a combination of conventional acoustic components combined with a hybrid control system. This device can be fully designed externally and attached to a primary acoustic system, such as a duct or pipe to reduce the transmission of tonal noise. It is adaptively tuned on-line to track a disturbance signal with slow time varying frequency. The paper presents a physical analytical model of the SHR, a power flow model that answers the question "Where does the power go?" and finally demonstrates the effectiveness of the device with experimental test results.