Taming the Howl: A Musician's Guide to Stage Feedback Control

It’s the uninvited guest at every amplified performance. It can arrive as a low, ominous hum that slowly builds, or as a sudden, piercing shriek that sends the audience grabbing for their ears. It cares nothing for your perfect solo or your heartfelt lyric. It is acoustic feedback, and it has humbled even the most seasoned musicians.

For many artists, especially those playing hollow-body acoustic instruments, fighting feedback feels like a mysterious, unpredictable battle. But it is not magic or a curse. Feedback is a simple, predictable phenomenon of physics. Understanding its anatomy is the key to taming it, and transforming you from a potential victim into the master of your stage sound.

Anatomy of a Feedback Loop

At its heart, feedback is a closed loop of sound reinforcing itself. It’s a classic positive feedback loop that follows a clear path:

1. Your instrument or voice creates a sound.
2. A pickup or microphone captures that sound and turns it into an electrical signal.
3. An amplifier boosts this signal.
4. A speaker turns the amplified signal back into sound waves in the air.
5. Those sound waves travel back to your instrument or microphone, get picked up again, and are sent back to step 2, but this time a little louder.

This loop repeats itself hundreds of times per second. When the total amplification in the loop at a specific frequency is greater than the loss of sound traveling through the air, that frequency begins to run away, rapidly escalating into an overwhelming howl or hum.

[Figure: A simple circular diagram illustrating the feedback loop, with arrows pointing from "Guitar/Vocal" to "Pickup/Mic," to "Amplifier," to "Speaker," and back to the "Guitar/Vocal."]

Why are acoustic guitars so susceptible? Because their hollow bodies are designed to resonate. The body of your guitar acts as a natural amplifier, especially for certain frequencies determined by its size and shape (a phenomenon related to Helmholtz resonance). When the sound from the speaker matches one of these resonant frequencies, the guitar’s body begins to vibrate sympathetically, feeding an already strong signal back into the pickup with incredible efficiency. It’s a perfect storm for a low-frequency hum that can feel impossible to control.

Understanding that feedback is just a runaway loop of a specific frequency is the first step. The next is knowing you have a powerful arsenal at your disposal to break that loop.

Your Arsenal: Three Weapons Against Feedback

You have three primary methods to combat feedback, ranging from physical defense to surgical electronic intervention.

Weapon 1: Position (The Physical Defense)

This is your first, best, and most important line of defense. Since feedback relies on the speaker’s sound reaching the pickup/microphone, simply changing the physical relationship between them can often solve the problem instantly.

• Never point a microphone directly at a speaker. This is the most direct path for a feedback loop.
• Move away from your monitor or amplifier. Increasing the distance reduces the intensity of the sound waves reaching your instrument.
• Change your angle. Don’t stand directly in front of your amp. Turning your body or the amp slightly can be enough to disrupt the resonant frequency’s path.

Remember, every stage and every room is different. Before you touch a single knob, try taking one step to the left or right. You’ll be surprised how often this simple act is a complete solution.

Weapon 2: Phase Inversion (The Polarity Trick)

This is a wonderfully elegant electronic solution to a physical problem. Sound travels in waves, with peaks and troughs. When two sound waves meet, if their peaks align, they combine and get louder (constructive interference). If a peak aligns with a trough, they cancel each other out (destructive interference).

[Figure: A simple sine wave diagram. The top shows two waves in-phase, resulting in a single, taller wave. The bottom shows two waves 180 degrees out-of-phase, resulting in a flat line (cancellation).]

A phase inversion switch, like the [PHASE] button found on many preamps and pedals such as the BOSS VE-8, simply flips the electrical polarity of your guitar’s signal by 180 degrees. While this has a very subtle effect on the tone you hear, it can have a dramatic effect on feedback. If the feedback loop happens to be occurring because the waves are aligning constructively, flipping the phase can force them into a state of destructive interference at the critical frequency, killing the feedback instantly. It won’t always work, as it depends on the specific phase relationship in the room, but it’s a zero-effort, single-button press that can be a magic bullet, especially for low-frequency body resonance.

Weapon 3: The Notch Filter (The Surgical Strike)

When moving isn’t an option and the phase switch doesn’t help, you need a more precise tool. Enter the notch filter.

A standard EQ knob (like “mids” or “treble”) affects a broad range of frequencies, and cutting enough to kill feedback often guts your tone, making it sound thin and lifeless. A notch filter is different. It is a highly specialized EQ designed for one purpose: to surgically remove a very narrow band of frequencies.

Imagine your sound spectrum as a loaf of bread. A regular EQ cut is like pressing down on the loaf with your hand—you affect a wide area. A notch filter is like taking a razor blade and cutting out a single, paper-thin slice.

When feedback is howling, you can slowly turn the [NOTCH] knob, which sweeps the frequency of this filter. As you sweep past the exact frequency of the feedback, the howl will suddenly disappear, with minimal impact on the rest of your guitar’s sound. This is the most precise and tonally transparent way to eliminate a stubborn feedback problem.

The Battle Plan: A Practical Workflow

Knowing what these tools do is one thing. Knowing when and why to use them during a chaotic soundcheck is another. That requires a battle plan. To make it easy under pressure, remember the “M-P-N” Rule: Move, Phase, Notch.

Value Asset: The Stage Feedback Emergency Workflow

[Figure: A flowchart/decision tree]

1. Feedback Occurs. -> Is it possible to move?

   • Yes: Change your position/angle relative to your speaker/monitor. -> Problem Solved? -> Yes (End). / No (Go to 2).
   • No: Proceed to step 2.

2. Press the [PHASE] Invert button. -> Did the feedback stop or significantly reduce?

   • Yes: Success. You can fine-tune your position if needed. (End).
   • No: The feedback is not phase-related. Turn Phase off and proceed to step 3.

3. Engage the [NOTCH] filter. -> Slowly turn the Notch frequency knob while the feedback is happening. -> Listen for the point where the feedback disappears.

   • Found it: Leave the knob there. You have surgically removed the problem frequency. (End).
   • Can’t find it / Multiple frequencies: You may have a more complex issue. Lower your overall volume and consult with the sound engineer.

Conclusion: From Victim to Master

Acoustic feedback is not an arbitrary force of nature; it is a direct and logical consequence of physics. By understanding the simple concept of the feedback loop, you can begin to see it not as a threat, but as a puzzle to be solved.

Your body, your gear, and the stage itself are all parts of this puzzle. By learning to use your position as a physical tool, the phase switch as a clever trick, and the notch filter as a surgical instrument, you change your relationship with feedback. You are no longer its potential victim, waiting for the howl to strike. You are the engineer of your own sound, equipped with the knowledge and the workflow to control the physics of the stage.