Study on howling in an active noise control (Two-microphone method)

A 1-microphone system is usually used to detect the signal of a noise source in an active noise control system. In this case, unstable oscillation, called howling, will occur according to the gain of the transfer function. In a long duct, where few reflected waves exist, a 2-microphone system has been used to eliminate the signal from the secondary source to detecting microphones. However, in a short duct, the reflected waves must be considered. The authors studied the acoustic conditions in the 2-microphone system theoretically with the existence of the reflected waves. The elimination effect from the secondary source was confirmed by simulation and experiment. The 2-microphone system is valid when sound absorption material is installed at the opposite side of the opening end.