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?
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
- 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:
- 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 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.
A microphone’s feedback is a very annoying and unpleasanthigh pitch noise that can happen in any situation where there are a speaker anda mic. Understanding what causes feedback is the first step in knowing how to stopit.
Audio feedback happens when a sound coming out of aloudspeaker is picked up by the microphone reentering the sound system againand again in an always increasing loop. The result is a loud rumble or a highpitch ringing noise.
In this post, I will explain several factors that can cause amicrophone to feedback so that you can avoid these situations and significantlyreduce the occurrence of feedback in your gigs and events. Plus I have includedsome tips and solutions that will help you stop microphone feedback
Table of Contents
- Main Causes of Microphone Feedback
- Mic Placement in the Stage
- Loudspeaker and Monitor Placement
- The Type of Microphone Used
- Polar Pattern of the Mic
- Equalization of the Microphone
- Number of Open Microphones
- The Acoustics of the Room
Main Causes of Microphone Feedback
Mic Placement in the Stage
One of the main reasons for audio feedback is becausemicrophones are placed in front or too close to a loudspeaker. If loudspeakersare placed behind the microphone, feedback is almost guaranteed because whateversound comes out of the speaker will be captured by the mic, and the loop will causefeedback.
Placing a mic to the side of a loudspeaker is anotherpotential reason for feedback because sound can bounce back and forth until itreenters the sound system through a microphone.
The Solution
To help reduce the occurrence of feedback, place all openmicrophones behind the loudspeakers, making sure that there is enough distancebetween the mic and the speakers.
A rule of thumb commonly used by sound engineers is, if youdouble the distance between a microphone and the speaker, the volume for thatchannel can be increased by 6dB, providing you more volume headroom beforefeedback on that channel.
When setting up your mics, make sure you allow enough timeto try different positions until you get the best results. With time and practice,it will become easier because you will understand the behavior of your soundequipment.
Loudspeaker and Monitor Placement
Another reason that causes feedback is the placement ofspeakers and monitors. The dispersion angles for a loudspeaker varies, buttypical angles of dispersion are 90 degrees horizontally and 60 degreesvertically.
Any microphone located in that dispersion area has thepotential for feedback. When setting up a sound system, you want to cover asmuch space as possible so that your audience can hear everything.
This is where it is crucial to place loudspeakers correctlyso that they can cover as much area as possible, but staying clear of thepickup pattern of the mic. When setting up microphones on stage, make sure tokeep clear of the dispersion area of the speakers.
The Solution
Place the main loudspeakers as close to the audience aspossible. That way, you can decrease the volume in the mixer.
Place the monitors exactly behind the mics on stage,especially if you are using cardioid or super-cardioid mics.
A rule of thumb when placing speakers is if the distancebetween the audience and the main speakers is halved, by keeping the volumeconstant, the sensed volume by the audience will increase by 6dB. In otherwords, halving the distance between speakers and the audience increases thesound volume by 6dB.
If you are in a situation where mics can feedback but stilldon’t have the volume you need, try moving the speaker towards the audience.That way, the sensed sound volume increases without actually increasing it onyour mixer.
Another solution is to use in-ear monitoring systems; thatway, stage monitors can be removed entirely, significantly reducing theoccurrence of feedback. The only disadvantage is that in-ear monitoring systemstend to be expensive, but if you can afford them, I highly recommend it.
Allow enough time when setting up everything so that eachmic is tested with the main speakers and monitors as well.
Simulate a situation so that you know what is your volume headroombefore feedback. In other words, slowly increase the volume of all microphonesand generate a sound to see how the equipment behaves at higher volumes.
If you still see that you don’t have enough volume for yourmix, try moving the speakers to the left, right, and front until you find asweet spot.
The Type of Microphone Used
Feedback can be caused by the type of microphone you use. There are different types of microphones for live performance, namely dynamic, condenser, ribbon, etc. Each one has its sonic characteristics, and some are more sensitive than others.
For example, using condenser microphones for live vocal performance is the right choice because they provide studio-quality sound, but are way more sensitive to sound. Placement with this type of mic is a crucial factor in reducing feedback due to their sensitivity.
The Solution
Keep to a minimum the number of sensitive microphones suchas condensers on stage, use them only for lead vocals and backup singers. Forinstruments, try to use dynamic mics if possible.
If acoustic guitars need to be miked, place the microphoneas close as possible to the guitar. That way, it picks up more of the soundfrom the guitar and less from the environment.
If possible, use electro-acoustic guitars that come alreadywith installed piezoelectric microphones. That way, you use less open to airmicrophones for acoustic instruments.
Polar Pattern of the Mic
Choosing a microphone with the wrong polar pattern could be another reason for feedback. The polar pattern of a microphone is a visual representation of how well your microphone picks up sound from different angles. To learn about polar patterns, read this post.
For live performance, the recommended polar patterns are cardioid,super-cardioid, and hyper-cardioid. The reason is that these mics pick up mostof the sound at the front and less in the back and sides, significantlyreducing the sensitivity to feedback.
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The solution
Make sure that whatever microphones you use on stage are oneof the mentioned polar patterns. I recommend using cardioid and super-cardioid,especially for condenser microphones.
Equalization of the Microphone
Feedback happens at different frequencies, depending on thevolume of the microphone. Pandora sims login password. If the feedback is a low rumble or a high pith noise,you can easily detect the offending frequency. Usually, the offendingfrequencies can be found between 250Hz and 2kHz and between 2kHz – 8kHz. It’s rarelyfound above 8kHz.
This is where correctly equalizing each microphone comeshandy. By doing a few sound tests, you can ring out offending frequencies. Soundengineers use the term “ringing out” as the process of detecting resonantfrequencies that can cause feedback.
The Solution
To maximize volume before feedback occurs, ring outoffending frequencies. To do this process, you need a graphic equalizer. Theprocess is simple. After mics are placed in the stage, slowly start bringingthe volume up until you start hearing feedback.
If the feedback noise you hear is a low rumble, try reducingfrequencies between 250kHz and 1kHz. If the feedback sounds like a high pitchscreech, try changing frequencies between 2kHz and 8kHz.
You may want to run this test several times until you canbring all your system up to the volume you want without any feedback.
This process requires that your ears are trained to identifywhich frequency is creating the problem. There is equipment you can get thatwould do this automatically without the need of a graphic equalizer. They arecalled automatic feedback reducers.
These systems use a microprocessor that can identifyoffending frequencies and automatically reduce them before feedback evenhappens. They are convenient if you run many microphones on stage, and youdon’t want to deal with checking offending frequencies, or if your ears are nottrained to identify problematic frequencies quickly.
Even though an automatic feedback reducer can be very helpful in getting rid of feedback, you can still get it if you use the wrong type of microphone, have speakers in the wrong place, and use an event hall with bad acoustics.
No matter how good of an audio engineer you are, even if you have feedback reducers, there are some places where the acoustics are just terrible, and there is not much you can do about it.
In such cases where the acoustics are awful, you need to be like a hawk watching your mixer to catch feedback while the event is happening, this is not ideal, it’s stressful, and is just frustrating.
Number of Open Microphones
As more microphones are open (On), the possibility offeedback increases exponentially. This is because more sound can be picked upand reenter the system, creating a feedback loop.
The Solution
Maxqda keygen. Try to keep only the required mics ON; just use the mute button in your mixer, but make sure you are paying attention so that when someone needs to use the mic, you turn it on.
The Acoustics of the Room
Another factor that affects feedback from microphones is theacoustics of the room or event hall. Sound from the loudspeaker can bounce backand hit the microphone’s membrane even if you have followed all the suggestionsin this post.
The acoustics of the room is the only thing you have nocontrol over, so you would have to pay lots of attention to your EQ and volumelevels.
The Solution
Mic Sound Effect Download
In situations like this, an automatic feedback reducer canbe very helpful because the system can be faster in detecting offendingfrequencies before they get out of control.
In case you don’t have a feedback reducer, try placing themain speakers closer to the audience and all microphones as far as possiblefrom the speakers. Keep the volume in check and be on the watch to make sureyou can catch possible feedback.
Turn off any mic that is not needed and make sure to useeither cardioid or super-cardioid mics.
Perform several tests before the even until you have a clearunderstanding of what is your maximum volume headroom and don’t go over it.
Stopping Feedback – Summary
Conclusion
In this post, I have presented what causes microphonefeedback and how to stop it. I have included solutions for each case, butremember, you must practice putting yourself in different feedback situationsso that your ear is trained until you can automatically adjust. Plus, exploredifferent setup configurations until you get comfortable.
If you are a sound tech person that does it for a living by renting your equipment for parties and events, or if you are the sound tech in a church or any other venue, training yourself in avoiding and controlling feedback is a must. Remember, there is no substitute for practice when it comes to sound.
I hope this post helped you in some way to understand what causes microphone feedback and how to stop it. If you have questions, please leave it in the comments sections, I will respond within a few days. Thank you for reading my blog.