How Feedback Occurs and How You Can Avoid It

Audio feedback occurs when a sound ”loop” is created between an input (a microphone or guitar pickup) and an output (an amplifier or loudspeaker). In other words, signals received by the microphone are amplified and passed out of the loudspeaker, then the sound from the speaker is picked up by the microphone again, amplified further, and then sent through the loudspeaker again.

This is what is called positive feedback. The sound’s determined primarily by resonant frequencies in the microphone, amplifier circuitry and loudspeaker, but is also affected by the room acoustics, the particular pick-up and emission patterns of the microphone and loudspeaker, respectively, and the distance between the “oscillating” elements.

Main Causes of Feedback

Audio feedback generally results in a high-pitched squeal, very familiar if you listen to bands at house parties and other locations where the sound setup is less than ideal. The problems begin when live microphones are placed in the general direction of the output speakers, and too close, too.

Professional sound engineers avoid feedback by placing the main (or “house”) speakers at a distance from the band or speaker, and then using smaller speakers (called “spot monitors”) pointing back at the performers or speaker.

An old “handyman” trick for handling feedback is called "ringing out" a microphone (or instrument pickup). If you increase the level of a microphone or guitar pickup until feedback occurs, you can then turn down frequencies on a “equalizer,” preventing feedback at that particular pitch but allowing maximum volume. Professional sound engineers can "ring out" microphones and pick-ups by ear but you can use a graphic equalizer connected to a microphone to show the offending (“ringing”) frequency.

The Higher-Tech Solution

Automatic anti-feedback devices are also used to handle this problem. The technology goes by the name "feedback destroyer" or "feedback eliminator," and many of them work by slightly shifting the ringing frequency. This will result in in a "chirp" sound instead of a squealing sound, due to “upshifting” the frequency of the feedback.

Other devices use what are called “notch filters” to eliminate the offending frequencies, which can be “tuned” with other circuitry if necessary. There are also devices known as “hush” devices that reduce both feedback and ground-loop noise (otherwise known as the dreaded “60Hz hum”).

Steps to a Solution

The troubleshooting procedure for eliminating microphone-based feedback in a projector-amp-mic-monitor setup would be:

1. Turn down the volume on the offending mic
2. Use as few mics as possible.
3. Use directional mics that pick up less feedback; hypercardioids and supercardiods are best at rejecting feedback, while omnidirectionals are best avoided for live applications but are good for recording.
4. Place the mic close to the sound source for higher gain before feedback.
5. “Equalize” the room with a 3rd-octave graphic equalizer.
6. Keep mics as far behind main speakers as possible.
7. Place spot monitors directly behind mics.
8. Use the same kind of mics where possible.
9. Turn off mics not in use, using the mic power switch if there is one.
10. Use the mixer channel’s on/off buttons to guarantee that mics are either live or not.