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.

What is acoustic feedback?
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.

And 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

How to avoid it
Here are some suggestions on how to interrupt the feedback loop:
• 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.

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.

And finally …

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 about EQ, see this Shure Notes blog post.

One of the most commonly asked questions in professional audio is “What microphone can I use that doesn’t cause feedback?” The answer is: no such microphone exists. Feedback results from a combination of many factors, including loudspeaker placement, microphone placement, and the frequency response of both devices and room acoustics.

It’s a complicated process. Entire books have been written on the subject and the mathematical formulas that model feedback are quite involved. So let’s summarize the problem.

What is it?

Feedback is an oscillation at a resonant frequency of your sound system and the room. Take a soda bottle and blow over the top. It will resonate at a certain frequency depending on the volume inside. Change the inside volume by adding liquid and the resonant frequency changes. Feedback is analogous to blowing over the bottle top.

Each mic is slightly different; therefore each one will resonate at different frequencies. Each loudspeaker is slightly different and will resonate at different frequencies. Each location in the room has its own resonant frequency. As the gain of your sound system is increased, eventually a common resonant frequency is found and the system (mic – loudspeaker – room) goes into feedback. Change one component, or location, and the feedback will also change. The resulting noise is a sustained, ringing tone, which can vary from a low rumble to a piercing screech.

What causes it?

The root causes of feedback are irregularities in the frequency response and polar patterns of the microphones, the loudspeakers, and the room acoustics. Consider this example: You have three Shure SM58® mics, but the manufacturing tolerance of those mics over the complete frequency range is +/- 3dB. At any certain frequency, two of your SM58s might have an output variation of 6dB. Loudspeakers are even worse with many having variations of +/- 6dB, which equals a range of +12dB. And room acoustics can boost certain frequencies due to room resonance as much as +/- 12dB! This is a range of 24dB. With this much variation, it’s no surprise that mics in a slightly different locations will feed back differently.

Also note that each time the number of open mics in a sound system is doubled (1 to 2, 2 to 4, 4 to 8, etc.), the overall gain of the sound system drops by 3 dB. This means that more open mics equals less gain in the PA system.

Simply stated, feedback occurs whenever the sound entering a microphone is reproduced by a loudspeaker, picked up by the microphone, and re-amplified again and again. The familiar howl of feedback is an oscillation that is triggered by sound entering the microphone. The easiest way to create feedback is to point a microphone directly into a loudspeaker. (We don’t recommend you try this!) Placing the microphone too close to the loudspeaker, too far from the sound source, or simply turning the microphone up too loud exacerbates feedback problems.

What can I do about feedback?

You can:

• Change the acoustics by adding sound absorbing material walls.
• Change to different mics that have different resonance frequencies.
• Use in-ear monitors to eliminate the feedback path from loudspeaker monitors to mics.
• Employ precise parametric equalization that might add 4 to 6 dB of gain. Change to loudspeakers with a very smooth frequency response at all frequencies and all angles (big money!).

The single easiest way to reduce feedback is to:

• Move the microphone closer to the desired sound source
• Increase the amount of gain before feedback by using a directional microphone (cardioid, supercardioid, etc.)
• Reduce the number of open microphones
• Try to keep microphones and loudspeakers as far away from each other as possible.
• Acoustically treat the room (if possible) to eliminate hard, reflective surfaces such as glass, marble, and wood

But wait – there’s more!

When all of these solutions have been exhausted, the next step is to look toward equalizers and automatic feedback reducers.

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 feed back first. After the techniques described in the above section have been applied, 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). 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.

After locating the first feedback frequency, begin turning up the system again until the next frequency begins ringing. Repeat these steps until the desired level is reached, but don’t over-equalize. Keep in mind the equalizers can only provide a maximum level increase of 3 to 9 dB.

Parametric equalizers, though more confusing to the novice user, allow for more precise control of feedback frequencies. A graphic EQ allows the user to cut fixed frequencies with a fixed filter width. A parametric EQ allows the user to isolate specific frequencies and adjust the width and depth of the filter.

Automatic feedback reducers will accomplish the same results. They find and cut the frequencies that are feeding back automatically. The same precautions listed above apply to feedback reducers as well as equalizers. Automatic feedback reducers are very helpful in wireless microphone applications.

Back to basics

Remember that microphone placement is crucial to eliminating feedback, and the temptation to wander away from the ideal microphone position when using a wireless mic 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 human operator.

Proper implementation of these techniques will go a long way toward eliminating feedback in your sound system. Don’t rely solely on equalizers or feedback reducers, and remember that feedback results from more than just the microphone.

Understanding Sound System Design and Feedback Using (Ugh!) Math
By Rich Frank

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