What Is Microphone Feedback And How To Eliminate It For Good

Written by Arthur in Microphones

We've all heard it—the unpleasant squeal of microphone feedback being projected by the loudspeakers during a live event. Let's talk about microphone feedback, what causes it, and strategies to eliminate it for good!

What is microphone feedback? Microphone feedback is a positive gain loop between a microphone and a loudspeaker. Speakers amplify the mic signal, and then the mic picks up the sound from the speakers. This positive gain loop continues and the system overloads, resulting in the terrible screech of microphone feedback.

Understanding microphone feedback and how to eliminate it comes with experience. This article will help simplify microphone feedback and offer strategies to help performers and audio technicians eliminate it.

What Is Microphone Feedback?

As discussed, microphone feedback is a positive feedback loop that causes the microphone and loudspeaker to rapidly feed each other more and more signal until the system overloads. This loop causes the microphone signal to distort and the loudspeaker to project a gross screeching, humming, or whistling noise.

Feedback only happens when the microphone picks up a critical level of sound from the loudspeaker that projects the microphone's signal.

  • The microphone captures sound and turns it into an audio signal
  • The audio signal is sent to the loudspeaker and is amplified as sound
  • The microphone picks up the sound from the loudspeaker
  • A greater signal is sent to the loudspeaker and it gets louder
  • This creates even more sound to be picked up from the microphone
  • The cycle continues and turns into feedback
A simple diagram showing the basic feedback loop

So then why doesn't microphone feedback happen all the time? Well, there's a sort of feedback threshold. Turn the loudspeakers up loud enough or amplify the mic signal too much, and you're bound to create a feedback loop. This threshold is often referred to as gain-before-feedback.

Unfortunately, in many situations, the “ideal” loudness of the performance is above the feedback threshold. So what do we do? Too often, we ride the levels just below the threshold and hope the levels don't surpass this threshold. This is dangerous since an increase in level or even a repositioning of the microphone can send the signal into feedback territory.

The Influencing Factors Of Feedback

So we know what feedback is and the basics of what causes it, but other indirect factors influence microphone feedback.

The factors that influence microphone feedback are:

Let's touch on each of these points in more detail:

Amplification Of The Microphone Signal

Increasing the amplification (gain) of the microphone preamp sends more signal level to the loudspeakers while also raising the microphone's sensitivity and noise floor. The risk of feedback goes up.

To learn more about microphone gain/amplification, check out my article What Is Microphone Gain And How Does It Affect Mic Signals?

Volume Of The Microphone Channel

After gain is applied, live audio mixers also have channel volumes that adjust the amount of signal sent to the loudspeakers and monitors. Sliding the microphone volume too high will also result in microphone feedback if we're not careful.

Volume Of The Loudspeakers

Turning up the volume of the loudspeakers creates greater sound intensity in the air. This, in turn, causes the microphone to produce more signal. Boost the loudspeaker volume too much, and the microphone pickup will surpass the feedback threshold and cause feedback.

Distance Between The Microphone And Loudspeaker

The Inverse Square Law states that (in free fields) sound level intensity diminishes by a whopping 75% for each doubling of distance. Of course, conditions are never ideal, but this law is useful in estimating sound intensity as it propagates through air. Therefore, the further the microphone is from a loudspeaker, the smaller the risk of feedback.

As the distance between the mic and speaker grows, both the amplification of the mic and the volume of the loudspeakers can be increased!

For more info on microphones and loudspeakers, check out my article Do Microphones Need Loudspeakers Or Headphones To Work?

Directionality And Sensitivity Of The Microphone

The directionality or “directional sensitivity” of a microphone plays a crucial role in managing feedback. Typical sound reinforcement microphones have a cardioid polar pattern. They are the most sensitive to where they point, less sensitive to the sides, and barely sensitive to the rear.

Here is a polar graph of the cardioid pattern with a standard live vocal mic for reference:

Shure SM58 Overlaid On A Cardioid Polar Response Graph

For more information on directional cardioid microphones, check out my article What Is A Cardioid Microphone? (Polar Pattern + Mic Examples).

Pointing a directional microphone at a loudspeaker is a surefire way to get microphone feedback. The directional microphone is most sensitive where it points, and the loudspeaker passes the greatest amount of sound where it “points.”

For this reason, foldback monitors can be placed in front of singers. The directional microphone should point away from the monitor speakers. In this scenario, the feedback threshold is quite high.

However, if the singer points the top or front of their microphone at the foldback monitor, feedback would ensue!

For in-depth reading on directional microphones and mic sensitivity, check out the following My New Microphone articles:
• A Complete Guide To Directional Microphones (With Pictures)
• What Is Microphone Sensitivity? An In-Depth Description
• What Is A Good Microphone Sensitivity Rating?

Frequency Response Or “Tone” Of The Microphone

Frequency response and tone basically mean the frequency-specific sensitivity of a microphone.

  • A microphone with poor high-end frequency response is less sensitive to high-end sounds and will effectively handle more high-frequency sound before feeding back.
  • A microphone with poor low-end frequency response is less sensitive to low-end sounds will effectively handle more low-frequency sound before feeding back.

Dynamic microphones typically have high-frequency roll-offs well in the audible range of human hearing. Perhaps this is another reason why dynamics are the go-to's of live sound reinforcement!

For a detailed read on dynamic microphones and frequency response, check out my articles The Complete Guide To Moving-Coil Dynamic Microphones and Complete Guide To Microphone Frequency Response (With Mic Examples), respectively.

Size And Shape Of The Physical Space

The size and shape of the physical space produce particular acoustic characteristics. Standing waves and reflections within an acoustic space have the potential to create microphone feedback.

Standing waves are wavelengths of sound that fit perfectly between two parallel surfaces because the wavelength fits perfectly. When perpendicular to the surface, its reflections will be in phase and get boosted in intensity via constructive interference.

Each wavelength of sound has a corresponding frequency, and the frequencies of the standing waves are amplified naturally. These are”problem frequencies and will typically cause feedback at lower levels (before other frequencies in the sound spectrum).

Reflections may also cause problems. They are essentially sound waves bouncing off surfaces in an acoustic space. Small spaces have stronger reflections since the initial sound wave will not travel as far (and lose as much intensity) before reaching a surface. Therefore, smaller spaces offer a greater potential for these reflections to enter the microphone at levels that could cause feedback.

How To Eliminate Microphone Feedback For Good

There is always a risk of feedback when microphones and loudspeakers are used together in the same space. We discussed the factors that influence feedback, but what are the methods of eliminating feedback?

Both the audio technician and the performer play critical roles in keeping microphone feedback out of the loudspeakers. Each strategy for eliminating feedback involves one or both of these parties.

The top strategies for eliminating microphone feedback are as follows:

Let's discuss each of these strategies in a bit more detail, shall we?

Turn It Down

Turning the system down is always the first line of defence. If feedback ever occurs, be quick to turn the microphone down (typically via the channel volume). Note that you could also reduce the mic's gain or, alternatively, turn the loudspeaker down.

Feedback happens, and this is the quickest and most effective way to eliminate it.

Point Directional Microphones Away From Loudspeakers And Monitors

Cardioid microphones are the most common mics in sound reinforcement for a reason. They are sensitive to sound in the direction they point and reject sound from behind them.

Pointing directional microphones away from loudspeakers reduces the likelihood of positive gain loops, allowing more gain before feedback!

Place Microphones Close To Their Dedicated Sound Sources

Placing directional microphones close to the sounds they are tasked with picking up helps immensely in controlling feedback.

The closer the sound source is to the mic, the cleaner the microphone signal will be. Less gain, then, is needed for the source/vocal/instrument to be heard. This means a better signal-to-noise ratio and has the added benefit of reducing the sound from the loudspeaker in the microphone!

For more info on signal-to-noise ratios, check out my article What Is A Good Signal-To-Noise Ratio For A Microphone?

Placing microphones close to their dedicated instruments sometimes has the added benefit of furthering the distance between the microphone as its closest loudspeaker, which also reduces the amount of loudspeaker pick up in the microphone.

To learn more about microphone placement, check out my article Top 23 Tips For Better Microphone Placement.

Stay Shy Of The Feedback Threshold

For some reason, humans like to believe that louder is better. And so, when mixing live, we tend to push the system as loud as possible, just below the feedback threshold.

This is a dangerous strategy since a sudden increase in the loudness of performance could potentially push the system above the feedback threshold. No one wants the powerful climax of a song to sound like a screeching sonic nightmare of feedback.

For this reason, it's often best to mix well under the feedback threshold. Use your discretion when mixing live sound. We don't want the performance to be too quiet either!

Tune Out The Room's Resonant Frequencies

This is live sound 101 for audio technicians, although it may seem counterintuitive to creating a loud live mix. I remember learning this in college and being very impressed that cutting frequencies could actually increase the potential volume of a live performance.

As mentioned earlier, the shape and size of the room will cause some standing waves. Standing waves, or better yet, “room resonant frequencies,” are naturally amplified by the room's acoustic properties. Finding and cutting these frequencies will effectively “tune the room,” helping to improve the mix and reduce the risk of feedback in those sensitive frequencies.

Graphic equalizers are often used to tune the room. Though with digital audio mixing consoles, audio technicians have the ability to tune the room without a graphic EQ unit.

For more information on graphic EQ, check out my article The Complete Guide To Graphic Equalization/EQ.
To learn more about EQ in general, check out my article The Complete Guide To Audio Equalization & EQ Hardware/Software.

Automatic feedback suppressors are now on the market. These effectively target the problem frequencies and respond by lowering the gain/volume of these specific frequencies in real-time. These units basically tune the room for you. The main complaint with automatic feedback suppressors is their tendency to overreact and produce a thin-sounding mix.

What are the best microphones for on-stage live performance? The best microphones for live performance depend largely on which instruments are being miked up. Directional microphones are ideal for minimizing feedback and capturing clean signals of individual instruments. Dynamic microphones with cardioid polar patterns have become the go-to microphone for on-stage live sound reinforcement.

To learn about my microphone recommendations for all practical applications, check out my page Recommended Microphones And Accessories.

What are the most important instruments to mic up when options are limited? When determining a sound reinforcement strategy, some instruments take precedence over others. This is apparent when working with small mixers or limited microphone selections. Thought should be put into miking the instruments that most need miking. Of course, the “most important” instruments depend on the genre of music and the group of performers.

Here are some general questions to ask when determining which instruments to mic up.

  • Vocals are always first to be considered.
  • Think about miking soft acoustic instruments next.
  • Then look at miking hard acoustic instruments such as percussion instruments.
  • For added clarity in the monitor and front-of-house mixes, mic up the guitar/bass amplifiers.

If there's a big ensemble to reinforce, contemplate the following:

  • Think about miking each section of musicians.
  • Try spot miking soloists, but not too close as to create an uncanny capture of the “soloed” instrument.

To learn more about spot-miking, check out my article What Is Spot/Accent Miking? (Why And How It Is Done).

For further reading on microphones and microphone feedback, please check out my article 12 Methods To Prevent & Eliminate Microphone/Audio Feedback.

Choosing the right microphone(s) for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Microphone Buyer's Guide. Check it out for help in determining your next microphone purchase.

This article has been approved in accordance with the My New Microphone Editorial Policy.

Arthur

Arthur is the owner of Fox Media Tech and the author of My New Microphone. He's an audio engineer by trade and works on contract in his home country of Canada. When not blogging on MNM, he's likely hiking outdoors and blogging at Hikers' Movement (hikersmovement.com) or producing music. For more info, please check out his YouTube channel and his music.

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