Low Flow-Noise Microphone for Active Noise Control Applications

A method that couples output from a hot-wire anemometer with that of a microphone to reduce flow-induced pseudonoise from the microphone signal was developed. In these experiments, a microphone and a hot-wire sensor were placed in a well-defined low-speed turbulent flow in a rectangular duct. Controlled acoustic noise, both random and time harmonic, was superimposed on the flow noise by placing a speaker source close to the entrance of the duct. Detailed studies of the coherence between the hot-wire and microphone signals in the presence of flow and acoustic noise indicated that the proper combination of the two signals could reduce the turbulence noise contamination in the microphone signal. Subsequent tests demonstrated that using an adaptive least-mean-square algorithm to filter the hot-wire signal before subtracting it from the microphone signal produced broadband flow noise attenuation on the order of 20 dB at frequencies below 100 Hz and spectra that approached those of the uncontaminated microphone signal. Moreover, the resulting 'hot-mic' signal retains the acoustic pressure of interest, making it an ideal sensor for use in active noise control applications where the sensing or error microphone must be placed in a flowfield. (Author)