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The common thing to do when ringing out microphones is to use a paramertic-eq with a narrow q and notch out the first few problematic frequency's. There are also some feedback-killer products that attempt to do so live.

I think that an all-pass filter would be a better option, leaving the frequency response flat and just altering the phase of problematic frequency's. Offcourse this will only work for static mics that don't move during a performance.

The only reason they are not commonly used, that I can think of, may be that they are just not commonly available. However these days with almost everything digital that could easily change with some firmware updates.

Is there any other reason why they are not used for this purpose?

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  • The ideal feedback remedy would surely be to measure the exact impulse response from the speakers into the mic, with all reflections, and subtract that from the signal. That would not only eliminate feedback, but all consequences of speaker-mic backfeed. Actually doable nowadays, but probably not that useful precisely because mics aren't immovable.
    – leftaroundabout
    Mar 5, 2015 at 0:34

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Well if your mic is static, point it away from the speakers and you should be good.....not meaning to dismiss your question but most real world situations call for vocal mics that are in constant motion as among your hottest items onstage. All the movement means a phase-based solution is impossible.

I'm also not at all convinced that when feedback occurs, it is always or even usually in the same phase....with a lot of copies of a given signal are flying around in space the process by which they build into a piercing tone is somewhat random, even if the mic is static tiny variations in the sound of the room and the initial signal will make the feedback sound slightly different each time.

That second part is just me musing - I honestly don't know if this is possible in the hypothetical static-mic situation that you are discussing. Can you please try this and tell us what you discover?

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    I'm not trying to solve any specific problem, but I'm just wondering why I've never come across this solution. Especially for the automated gadgets this would be a far more preferable way to kill feedback (if a flute is mistaken for feedback it will not be completely killed, in worst case a brief phaser effect may occur). You're right, I will try it out with a small test setup in my home when I happen to have all the gear at hand (not brave enough to experiment live :-). I was hoping to get answers from folk that had already tried this, but it feels like I'm the only one with such crazy ideas.
    – Louis Somers
    Mar 1, 2015 at 19:11

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