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Algorithm to suppress self-excitation?

Dear Sir

You know there exists self-excitation in Microphone-Speaker-Microphone application.

I add a 5-band EQ, the center fre are 50, 150, 400, 1K, 5K. I try to lower down some center fre to suppress the excitation. It helps a little, but not solve the problem completely. Do you have a algorithm to solve this self-excitation problem, or any suggestion?