The effect of an air flow on a single side branch helmholtz resonator in a circular duct

A single side branch Helmholtz resonator has been tested in a circular duct in which both sound and an air flow could be passed in the same direction. The sound input was broad band and the transfer function across the resonator was determined by using two microphones and a cross-correlation analysis technique in which the broad band sound input was cross-correlated with each microphone signal in turn. The fundamental resonant frequency, as obtained from the transfer function of the resonator, was found to increase with increasing flow velocities in the duct. It was possible for the entire mass end correction of the orifice of the resonator to be eliminated by the flow. The higher natural frequencies of the resonator were not affected as much by the air flow.