By Shure Notes Editors. Contributors: John Chevalier, Bill Gibson, Frank Gilbert, June Millington, Dan Murphy
'John had a semi-acoustic Gibson guitar. It had a pickup on it so it could be amplified. We were just about to walk away and listen to a take when John leaned his guitar against the amp. He really should have turned the electric off. It was only on a tiny bit and John just leaned it against the amp when it went 'Nnnnnwahhhh!' And we went, 'What's that? Voodoo?'. 'No, it's feedback.' 'Wow, it's a great sound!' George Martin was there so we said, 'Can we have that on the record?' It was a found object, an accident caused by leaning the guitar against the amp.'
– Paul McCartney (Source: Many Years From Now, Barry Mile)
It's pretty much common knowledge among students of pop music that The Beatles' 1964 recording of 'I Feel Fine' was one of the first known examples of feedback as a recording effect, even though The Kinks and The Who reportedly (and intentionally) used it in live performances. For most musicians and engineers, though, audio feedback is something to avoid.
In this post, we'll cover some of the fundamentals – what causes feedback and how to avoid it - along with tips from some of our favorite audio pros.

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What is acoustic feedback?

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Acoustic feedback occurs when the amplified sound from any loudspeaker re-enters the sound system through any open microphone and is amplified again and again and again. We've all heard it – it's that sustained, ringing tone, varying from a low rumble to a piercing screech.

What causes it

The simplest PA system consists of a microphone, an amplifier and one or more speakers. Whenever you have those three components, you have the potential for feedback. Feedback happens when the sound from the speakers makes it back into the microphone and is re-amplified and sent through the speakers again, like this:

Here's an example: Let's say that that you place the microphone in front of the speaker as shown here. If you tap the microphone, the sound of the tap goes through the amplifier, comes out the speaker and re-enters the mic. This feedback loop happens so quickly that it creates its own frequency, and that produces the howling sound — an oscillation triggered by sound entering the microphone. Placing the microphone too close to the loudspeaker, too far from the sound source, or simply turning the microphone up too high all raise the likelihood of feedback problems.

Pro Tip #1

'The worst is vocalists who cup the mic capsule (e.g. rappers who put their hand around the grill of the mic because they think it looks cool). This invariably makes the mic sound horrible and very susceptible to feedback. More importantly, it changes the directional nature of the microphone, changing it to essentially an omnidirectional microphone. One trick is to cut everything from 800 Hz to 2 kHz, compress it, and hopefully the horrible howling sound will go away and the vocals will still be intelligible. But don't forget, the best thing to do to control feedback is turn everything down.'
– Frank Gilbert, FOH Engineer Park West, The Vic Theater, and The Mayne Stage - all in Chicago

Suggestions on how to interrupt the feedback loop

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• Move the microphone closer to the desired sound source.
• Use a directional microphone to increase the amount of gain before feedback.
• Reduce the number of open microphones – turn off microphones that are not in use.
• Don't boost tone controls indiscriminately.
• Try to keep microphones and loudspeakers as far away from each other as possible.
• Lower the speaker output. Move the loudspeaker farther away from the microphone. Each time this distance is doubled, the sound system output can be increased by 6dB.
• Move the loudspeaker closer to the listener. Each time this distance is halved, the sound system output will increase by 6dB.
• Use in-ear monitoring systems in place of floor monitors.
• Acoustically treat the room (if possible) to eliminate hard, reflective surfaces like glass, marble and wood.

Pro Tip #2

'In a well-designed system, the irritating high-pitched brand of feedback isn't much of a problem unless someone points a mic into a monitor. So long as the performers are careful to always keep their mics pointed away from the monitors, or specifically to point that tail end of the mic at the monitor at all times, that shouldn't be an issue.'
– Bill Gibson, author of over 30 books, producer, performer and Berklee School of Music faculty member
When these solutions have been exhausted, the next step is to look toward equalizers and automatic feedback reducers.

Ringing Out

A common technique used by sound engineers is 'ringing out' a sound system by using a graphic equalizer to reduce the level of the frequencies that feedback:

1. Slowly bring up the system level until you begin to hear feedback. Now go to the equalizer and pull down the offending frequency roughly 3dB.
2. If the feedback is a 'hoot' or 'howl', try cutting in the 250 to 500 Hz range. A 'singing' tone may be around 1 kHz. 'Whistles' and 'screeches' tend to be above 2 kHz. Very rarely does feedback occur below 80 Hz or above 8 kHz. It takes practice to develop an ear for equalizing a sound system, so be patient.
3. After locating the first feedback frequency, begin turning up the system again until the next frequency begins ringing.
4. Repeat the above steps until the desired level is reached, but do not over-equalize. Keep in mind the equalizers can only provide a maximum level increase of 3 to 9 dB.

Pro Tip #3

'The last time I experienced feedback was in a small venue where I was onstage. As a musician and an audio tech, I'm a sound guy's worst nightmare. During rehearsal, my headset mic was feeding back and the audio tech kept turning my volume down and telling me that I couldn't move around. I knew the problem was midrange feedback, so I explained to him that if he just lowered the midrange on the EQ, the problem would go away. He 'passionately and firmly' explained to me that the only way to get rid of feedback was for him to lower the volume and for me to stand still.
After enduring the first song, I walked back to the board, reached over his shoulder and dropped the midrange. I sang a couple notes, looked at him, smiled and walked back onstage. (Did I mention I was wireless, too?) The problem was solved and we didn't talk after the set, but I know he learned something that night.'
– John Chevalier, pro audio/video expert, writer and speaker at InfoComm, NAB and other industry events

Pro Tip #4

'If there's one thing I've learned in all my years of playing, it's that the sound engineer has to be extraordinarily vigilant even about protecting the performers' hearing.
My last bad feedback incident was caused by gain stage being manipulated by the engineer without telling us - after we'd gotten to a good place. The resulting, shrieking feedback changed everything - there was nothing but pain filling up space between our ears. Many people forget that EQ'ing something can cause a volume change - right in that frequency.
Of course, EQ can remedy volume problems quite easily. Just take a moment to ferret out the offending frequency or cluster of frequencies - band members protecting their ears, of course - and 'forensically' attenuate, which will immediately solve the problem. A hall of mirrors, isn't it?'
– June Millington, FANNY frontwoman, musician and songwriter, co-founder of IMA
Automatic feedback reducers are very helpful in wireless microphone applications. Remember that microphone placement is crucial to eliminating feedback, and the temptation to wander away from the ideal microphone position when using a wireless is great. If the performer gets too close to a loudspeaker, feedback will result; a good feedback reducer will be able to catch and eliminate the feedback faster than a sound engineer.

Pro Tip #5

'The best 'gear' a sound person has is his or her ears. Learn to identify the ringing frequency by doing blind 'what is that frequency?' tests using a sine wave generator or test tone generator. Have someone dial up a tone and see if you can identify what frequency it is. This is great training to identify the problem frequency during feedback howl and how I learned how to tame feedback.'
– Dan Murphy, Sound Tech Director, Lakeside Church
NOTE: Don't rely on an equalizer/feedback reducer alone to provide sufficient additional output in a sound system where the microphones and loudspeakers are too close together. You probably won't get the results you need. For more information, read our post EQ IQ: A Quick Primer.