How to Control Feedback in a Sound System
By Shure Notes Editors. Contributors: John Chevalier, Bill Gibson, Frank Gilbert, June Millington, Dan Murphy
Key Takeaways
- Audio feedback occurs when sound picked up by a microphone is amplified through a loudspeaker and then re-captured by the microphone, creating a feedback loop that produces a ringing, howling, or screeching sound.
- Proper microphone placement, loudspeaker placement, and gain structure are some of the most effective ways to prevent microphone feedback and speaker feedback.
- Directional microphones, fewer open microphones, and improved gain before feedback can help increase system stability and reduce feedback problems.
- Equalization, system tuning, and feedback suppression tools can help identify and control frequencies that are prone to feedback.
- Combining good microphone technique with proper sound system setup is the most reliable way to prevent feedback in live sound and installed audio systems.
| "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 Miles) |
One of the earliest and most famous examples of feedback in popular music can be heard on The Beatles' 1964 recording I Feel Fine. According to Paul McCartney, the effect was discovered accidentally when a guitar was left too close to an amplifier, creating a feedback loop that became part of the recording.
While feedback is sometimes used creatively in music production, most audio engineers and live sound professionals view it differently. In a sound system, feedback occurs when amplified sound from a loudspeaker re-enters a microphone and is repeatedly amplified, creating the familiar squeal, screech, or sustained tone.
In this article, we'll explain what causes microphone and speaker feedback, how feedback loops form, and the most effective techniques for preventing feedback in live sound and installed audio systems.
What Is Acoustic Feedback?
Acoustic feedback occurs when sound captured by a microphone is amplified and played through a loudspeaker that is part of the same sound system. When that amplified sound is then picked up again by the same microphone or another open microphone in the system and re-amplified, a continuous feedback loop is created.
This feedback loop produces the familiar ringing, squealing, screeching, or low-frequency rumble commonly known as microphone feedback or speaker feedback.
What Causes Feedback in a Sound System?
The simplest PA system consists of three main components:
- A microphone
- An amplifier
- One or more loudspeakers
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:
For example, placing a microphone directly in front of a loudspeaker significantly increases the likelihood of feedback. When sound from the speaker repeatedly re-enters the microphone, the system creates a feedback loop, producing the characteristic howling sound associated with audio feedback.
Several common factors increase the risk of feedback:
- Placing the microphone too close to the loudspeaker
- Positioning the microphone too far from the intended sound source
- Using excessive microphone gain or volume
- Operating multiple open microphones simultaneously
Understanding what causes microphone feedback is the first step toward preventing it and improving overall sound system performance.
Pro Tip #1: Avoid Cupping the Microphone Capsule
Why Cupping a Microphone Causes Feedback
Many vocal microphones use a directional pickup pattern to focus on the desired sound source while rejecting sound from nearby loudspeakers and stage monitors. Covering or "cupping" the microphone grille can alter that pickup pattern, reducing the microphone's ability to reject unwanted sound.
As a result, the microphone may pick up more amplified sound from nearby speakers, increasing the likelihood of a feedback loop and reducing overall sound quality.
"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." |
How to Interrupt a Feedback Loop
If you're experiencing microphone feedback, speaker feedback, or a feedback loop in a sound system, the following techniques can help:
- 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.
- Avoid excessive EQ boosts.
- Try to keep microphones and loudspeakers far away from each other, as best managed within the environment.
- Lower the speaker output or move loudspeakers farther away from the microphones (Each time this distance is doubled; the sound system output can be increased by 6dB).
- Move the loudspeaker closer to the listener so less overall system volume is required (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: Pay Attention to Microphone and Monitor Placement
| "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 |
Even the best microphones and sound systems can experience feedback if microphones are pointed directly at monitor speakers. Understanding a microphone's pickup pattern and positioning monitors correctly can significantly reduce the likelihood of microphone feedback and improve gain before feedback.
Using EQ and Feedback Reduction Tools
When these solutions have been exhausted, the next step is to look toward equalizers and automatic feedback reducers.
Ringing Out a Sound System
A common feedback-control technique used by sound engineers is "ringing out" a sound system. This process uses a graphic equalizer to identify and reduce frequencies that are most likely to feed back.
- Slowly increase the system level until feedback begins to occur.
- Identify the offending frequency and reduce it by approximately 3 dB using a graphic equalizer.
- Listen for the type of feedback: Hoots or howls often occur around 250-500 Hz. Singing tones often occur around 1 kHz. Whistles and screeches typically occur above 2 kHz.
- Raise the system level again until another frequency begins to ring.
- Repeat the process until the desired operating level is reached, taking care not to over-equalize the system.
Pro Tip #3: Identify and Reduce the Problem Frequency
Sometimes feedback is caused by a specific frequency range that is being over-amplified. In these situations, reducing the offending frequency with equalization can be more effective than simply lowering the overall system volume.
"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 |
Key takeaway: Not all microphone feedback is caused by excessive volume. In many cases, identifying and reducing the specific frequency causing the feedback can improve gain before feedback without significantly lowering overall system volume.
Pro Tip #4: Use EQ Carefully and Consider Automatic Feedback Reduction
"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 |
Key takeaway: Feedback can often be traced to a specific frequency or gain-stage change. Careful use of EQ can help reduce microphone feedback while maintaining overall system volume and sound quality.
Automatic Feedback Reducers
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: Train Your Ear to Identify Feedback Frequencies
"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 |
Remember the Fundamentals
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.
The Bottom Line
Audio feedback is caused by a feedback loop between microphones and loudspeakers, but it can often be prevented through proper microphone placement, loudspeaker placement, gain structure, and system tuning.
Whether you're troubleshooting microphone feedback in a small room or controlling feedback in a large live sound system, the most effective approach is to address the root cause rather than relying on a single tool or device. By combining good microphone technique, thoughtful system setup, equalization, and feedback-control tools when needed, you can improve gain before feedback and maintain clear, reliable sound.
Learn more about Live Performance and House of Worship products.
FAQ: How to Control Feedback in a Sound System
What causes microphone feedback? Microphone feedback occurs when sound picked up by a microphone is amplified through a loudspeaker and then picked up again by a microphone connected to the same sound system. As this process repeats, a feedback loop is created, producing a ringing, squealing, or howling sound
How can I prevent feedback from microphones? Proper microphone placement, keeping microphones away from loudspeakers, reducing the number of open microphones, and maintaining appropriate gain levels can all help prevent microphone feedback.
What is a feedback loop in a sound system? A feedback loop occurs when sound travels from a loudspeaker back into a microphone, is amplified again, and continues repeating through the system until an audible feedback tone develops.
How do directional microphones help reduce feedback? Directional microphones focus on the intended sound source while rejecting sound from other directions, helping increase gain before feedback and reducing the likelihood of feedback problems.
What is "ringing out" a sound system? Ringing out a sound system is the process of identifying frequencies that are prone to feedback and reducing them with a graphic equalizer to improve system performance and gain before feedback.
Can EQ eliminate feedback completely? Equalization can help reduce feedback-causing frequencies, but it works best when combined with proper microphone technique, loudspeaker placement, and sound system setup.
Are automatic feedback reducers a replacement for proper system setup? No. Automatic feedback reducers can help suppress feedback quickly, especially in wireless microphone applications, but they are most effective when used alongside good microphone placement, gain structure (setting appropriate input and output levels throughout the sound system), and sound system design.
