Low flow-noise pressure measurements using a “HOT-MIC”

Real-time attenuation of flow-induced pseudo noise from the pressure signal of a microphone placed in a flowfield was performed by coupling an uncalibrated hot-wire signal with that of the microphone (the combination is called a “hot-mic”). A pinhole microphone and a hot-wire sensor located just upstream of the pinhole opening were placed in a welldefined, low-speed turbulent flow in a rectangular duct. Controlled acoustic noise, both random and timeharmonic, was superimposed on the flow noise by a speaker source located near the entrance of the duct. Experiments indicated that significant flow-noise attenuation is achievable in real time by adaptively filtering the hot-wire signal using a least-mean-square algorithm before subtracting it from the microphone signal. The resulting hot-mic signal retains the acoustic pressure information of interest. Moreover, it was shown that the measured coherence between two hotmics in the presence of flow is higher than the measured coherence between two identically placed microphones. These results indicate that the hot-mics would make better acoustic pressure sensors for use in active noise control applications where the sensing and/or error microphone must be placed in a low-speed, turbulent flowfield.