Effect of Howling Canceller in Active Noise Control

Howling is one of the most important problems to be solved in active noise control. The authors have shown that a 2-microphone system utilizing the time delay corresponding to the distance between the two microphones should have an acoustical condition wherein no reflection wave should exist from the opposite side to the targeted opening end. In this paper, another solution called 2-microphone transfer function ratio system is introduced to eliminate the signal from the secondary source. An echo canceller technique is also studied in this paper. The signal convoluted by the transfer function from the secondary source to the detection microphone is subtracted from the detection microphone signal. The acoustical condition is studied theoretically, taking into consideration the reflection wave from both sides. The effect of eliminating the signal from the secondary source was confirmed by the experiment. The coherence function between the source signal and the howling canceller output was measured. The correlation was recovered by using these systems.