We’ve all heard it before: A deafening squeal coming from the speakers at a concert or other event that causes everyone in the audience to plug their fingers in their ears and shout, “Make it stop!” Feedback like this is a constant problem for audio engineers, and is a phenomenon that’s as annoying as it is interesting. Though there are many instances of feedback and some seriously complicated math equations about it—like the intimidatingly named Barkhausen Stability Criterion—as well as a whole host of confusing technical terminology like “gains” or “oscillation,” we’re going to keep it simple and explain what causes that unbearable feedback sound you hear in common public address systems.
There are three main components in a typical PA system: The microphone to capture the sound signal; the amplifier to increase the power of the signal itself; and the loudspeaker that projects the signal outwards at the desired audible frequency. Depending on a few factors such as the relative distance between or the position of the speakers and the microphone, and the acoustics of the particular room where the PA system is set up, the main problem lies in the audio loop created by the three components.
If a person taps on the microphone in front of the speakers, the reverberation travels through the mic, into the amp, and out of the speakers. Simple enough, right? But the key here is that the mic is in front of the speakers. The continuous sound created by the initial resonance that ends up coming out of the speakers is then picked up by the microphone, which creates a circular audio loop that amplifies its own frequency—and creates the uncomfortable, high-pitched screech known as feedback.
The sound guys at your favorite music venue know it’s possible to add a sound mixing board or an equalizer to the PA system as a way to technically balance the loop and cut off the amplification that causes that overwhelming noise. Bands can also put the speakers at a far enough distance away from the microphones, and instead use on-stage monitors to hear themselves, to avoid feedback. The other idiot-proof way to cut down on feedback is to always position the loudspeakers in front of and pointing away from the microphone so the loop can never complete itself.
Feedback is mostly seen as a nuisance, but it has been intentionally used by various musicians over the years onstage or on their records. The first instance of intentional feedback on a song was in the introduction to the Beatles classic “I Feel Fine,” where Paul McCartney plucked an A on his bass guitar while John Lennon’s guitar pick-ups were directed toward his amplifier to make a prolonged twang before launching into the song itself. Jimi Hendrix used feedback to his advantage in his famous rendition of “Wild Thing” at the Monterey Pop Festival, while Queen guitarist Brian May designed and built a guitar called the “Red Special” in order for it to feedback. And then there's Lou Reed’s infamous 1975 record “Metal Machine Music,” which is made up entirely of feedback loops.