How to Stop Feedback in Live Sound
Feedback is the bane of every live sound engineer—a piercing squeal that can ruin a performance and damage equipment. Understanding its root causes and applying systematic fixes will keep your system stable and your audience happy. This guide covers the physics of feedback, gain structure, EQ, mic technique, and monitor placement, with insights from SSOUNDS engineering.
Key takeaways
- Feedback is caused by a loop between mic and speaker; gain structure, EQ, mic technique, and monitor placement are the four pillars of control.
- Set gain structure so that preamp levels hit -6 dBFS peaks, then adjust monitor sends conservatively.
- Use narrow notch filters on feedback frequencies, but limit cuts to preserve sound quality.
- Position microphones close to the source and use directional patterns; place monitors in the mic’s rear null.
- SSOUNDS DSP and controlled directivity loudspeakers provide superior gain-before-feedback, reducing the need for drastic EQ.
- Always ring out the system before the show and use feedback suppressors only as a backup.
What Causes Feedback?
Feedback occurs when a microphone picks up sound from a loudspeaker, which is then re-amplified in a continuous loop. The system becomes unstable at frequencies where the gain exceeds the acoustic isolation between mic and speaker. Room acoustics, microphone polar patterns, and loudspeaker coverage all play a role.
In practical terms, feedback is a gain-before-feedback problem. The closer a mic is to a speaker, the louder the monitor must be before feedback sets in. Similarly, a microphone with a cardioid pattern rejects sound from the rear, but if the monitor is placed in its rear null, feedback risk drops. SSOUNDS engineers analyze these variables using predictive modeling to optimize system gain before a show.
Gain Structure: The Foundation of Feedback Control
Always set monitor levels after the main PA levels are established. Use a slow, methodical approach: bring up the monitor send until you hear the first hint of feedback, then back off by 3-6 dB. This gives you a safety margin. With SSOUNDS line arrays, the controlled directivity minimizes spill into stage areas, improving gain before feedback.
EQ: Surgical Notching and System Tuning
System EQ should be used to flatten the overall response of the room, not to chase feedback. Use pink noise and an RTA to measure the PA’s response at the mix position, then apply gentle shelving or peaking filters to correct room modes. A flat system is less likely to excite feedback frequencies.
Microphone Technique: The First Line of Defense
Microphone placement and selection dramatically affect feedback susceptibility. Use directional microphones (cardioid, supercardioid) for vocals and instruments. Place the mic as close to the sound source as possible—this increases the direct-to-reverberant ratio, allowing lower gain before feedback.
Teach vocalists to stay on-axis and maintain consistent distance. If a singer moves off-axis, the frequency response changes and feedback may occur. For instruments, use close miking techniques. SSOUNDS engineers often recommend using a high-pass filter (80-100 Hz) on vocal mics to reduce low-frequency rumble that can cause feedback.
Monitor Placement and Coverage
Monitor placement is critical. Position wedge monitors so that the microphone’s rear rejection lobe points toward the speaker. For cardioid mics, the null is at 180 degrees; for supercardioid, it’s at 120 degrees. Place the monitor directly behind the mic’s rejection zone.
Avoid placing monitors in corners or near reflective surfaces, which can amplify certain frequencies. Use subwoofers sparingly in monitor mixes—low frequencies are omnidirectional and harder to control. SSOUNDS stage monitors are designed with optimized dispersion to minimize bleed into unwanted areas, and our line array systems can be flown to keep sound off the stage entirely.
Advanced Techniques: Feedback Suppressors and System Alignment
Digital feedback suppressors can automatically detect and notch feedback frequencies, but they should be used judiciously. Over-reliance can lead to a muddy sound. Instead, use them as a safety net during shows with multiple quick changes.
System alignment—time-aligning subwoofers with tops and ensuring coherent coverage—reduces phase cancellation that can cause feedback. SSOUNDS systems include FIR filters and delay management to achieve coherent summation across the listening area, reducing the need for excessive EQ.
Frequently asked
Why does feedback happen more at certain frequencies?
Feedback occurs at frequencies where the room’s acoustic response has peaks, or where the loudspeaker and microphone have high sensitivity. These are often room modes or system resonances. Notching these frequencies with EQ is the standard fix.
Can I use a graphic EQ to stop feedback?
Yes, a graphic EQ is effective for ringing out monitors. However, parametric EQ is preferred because you can adjust the Q (bandwidth) to cut only the problematic frequency without affecting neighbors.
How much gain before feedback should I expect from a typical monitor system?
With proper placement and EQ, a well-tuned monitor system can achieve 10-15 dB of gain before feedback. SSOUNDS systems often exceed this due to their controlled directivity and DSP optimization.
Does the type of microphone matter for feedback?
Absolutely. Dynamic microphones like the Shure SM58 have a tighter cardioid pattern and are less prone to feedback than some condenser mics. Supercardioid mics offer even more rear rejection but have a rear lobe that must be considered.
Should I use a feedback suppressor on every channel?
No. Feedback suppressors should be used sparingly, typically on the main outputs or monitor mixes as a safety net. Overuse can degrade sound quality. Focus on good system tuning and mic technique first.
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