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Ear-piercing wail or magnificent noise? Dig into the hows and whys of feedback.
By Ken Pearsall
We all know that sound. An uncontrollable, high-pitched, squealing feedback that hurts the ears of everyone in the building. It can be the worst (or best) thing that can happen during a live performance.
How did it happen? What went wrong? How could it have been avoided? And can it be controlled?
The title track from David Bowie’s Heroes, Lou Reed’s divisive Metal Machine Music, Frank Zappa’s live performances and the caustic beauty of My Bloody Valentine and other shoegazers all make great use of feedback, so, why didn’t that electric terror-scream make me sound like Jimi Hendrix?
Let’s break it down.
When you plug in your electric guitar, one way or another you are amplifying the signal so you can hear it. If that amplified signal happens to make it’s way back into the guitar signal, it’s amplified again, and again, and again, in a constant loop. It gets louder and louder until your amplifier is overloaded, producing the loudest signal it is capable of. And this ear-piercing circulation can all happen in an instant.
Feedback happens more easily by using gain-based effects like distortion, overdrive, fuzz, or compression. These effects allow low-level signals picked up by your guitar to be heard much louder, making it simpler for external sounds to find their way back into the guitar. One surefire way to create feedback is simply by turning your amp’s volume up really loud. But how is the signal getting back into the guitar?
Normally, the signal from your guitar is created by using your fingers or a pick to vibrate the string. This vibration causes a change in the magnetic field of your pickup. The pickup converts that change of vibration into an electrical signal, which you can hear through your amplifier.
You’ve probably heard about guitar pickups “going microphonic”. This means that the actual wire wrapped around the poles inside the pickups has loosened and can now vibrate. The speaker in your amp vibrates to create sound waves that we can hear, and now those same sound waves can cause your pickup windings to vibrate. This is exactly how a microphone works. If your pickups are microphonic, they are acting as a microphone for sound waves in the room. Since the wire in your pickups is extremely thin—much thinner than you’re your high “E” string— they will vibrate at a much higher frequency, and that's the piercing squeal some dread and others desire. Most pickups are dipped or “potted” in wax to prevent this from happening, but with age the wax can break down and allow the windings to loosen.
There are a few things you can do to get rid of this ear-killing feedback. In the above example, a pickup acts like a microphone. But it’s not a great microphone. It can’t pick up low-level talking in the room or anything like that with any kind of clarity. You need a lot of volume for feedback of this nature to occur, so you may never even experience this at quiet bedroom levels, while turning it up on stage or at practice will sound like a screeching nightmare.
The easiest fix is to turn the amp down and get the guitar further away from it. The soundwaves that are feeding back through your guitar lose energy the further they have to travel. Moving your guitar away from the amplifier prevents these soundwaves from having enough energy when they get to your guitar to feedback.
Unfortunately, the best solution is to have your pickups repotted with wax. You’ll need to visit a reputable repair shop to do this, and it can be expensive, though some pickup manufacturers will allow you to send in your pickups to be repotted for a nominal fee.
You aren’t out of luck just yet, though. This feedback occurs because that high-pitched sound is caught in a loop getting amplified and fed back through the guitar over and over again. Most amplifiers have at least a simple EQ built into them. If you can turn down the high frequencies, this will affect this feedback loop. You just have to turn it down enough to cancel out the large boost of the feedback frequency, hopefully without losing tone.
Another avenue are feedback suppression stompboxes that control feedback automatically by reducing the feedback frequency with EQ or automatically lowering the volume until the feedback stops.
Acoustic and hollow body guitars loaded with pickups are another monster altogether.
These guitars are designed to make the entire instrument body vibrate in order to amplify the sound acoustically. Acoustic guitars in particular are known to be volatile feedback magnets. These guitars often have what is called a “piezo” pickup installed for stage use. This pickup is attached directly to the body wood and turns the vibrations of the body into an electrical signal for an amplifier.
The feedback is created in the same way as an electric guitar, but now there is a pickup that is specifically designed to reproduce vibration. While you may have luck controlling this feedback with EQ or feedback suppressing stompboxes, you will usually lose too much treble or tone for it to be useful.
The best way to handle these issues for acoustics is to use a rubber soundhole mute to keep the soundwaves from entering the body to resonate it. Hollow body and semi-hollow body guitarists have often resorted to stuffing the body cavity with foam through the F-holes.
One of the first uses of feedback in popular music was by the Beatles on their single, “I Feel Fine”. John Lennon set his acoustic (with an electric pickup) guitar down in front of the amp, and that famous raspy feedback was the result. Thankfully, they were still recording and loved it so much they used it as the intro to the track.
So, feedback can be a royal pain, but can we have fun with it? Of course.
In fact, many artists use feedback as just another tool in the shed. Enter "harmonic feedback".
Harmonic feedback happens in much the same way as microphonic feedback, but more volume is necessary to make it squeel to life. Instead of making the tiny pickup wires vibrate, enough volume energy is required to make your guitar strings vibrate. You will notice as this starts to happen that the note seems to fade into the octave above the note you are playing. This is because it is much easier to cause a string to vibrate at a natural harmonic.
No guitarist has ever mastered the use of feedback quite like Jimi Hendrix. He was a genius with a guitar and used feedback as a natural extension of the instrument. On Live At Woodstock ’69, Hendrix’s rendition of “The Star-Spangled Banner” made great use of this, letting almost every musical phrase explode into feedback before starting the next.
I mentioned before about using EQ to kill feedback, but it works both ways. Being able to selectively increase EQ for a certain frequency can be used to push your volume just over the edge to cause feedback.
There’s no better way to do this than with a wah pedal. Sure, a wah is a filter designed to cut frequencies, but it also has a strong boost. You can stomp on a wah during your solo, hold a note and sweep the wah to find the perfect spot to get that note feeding back. You can also control feedback with a volume pedal, but this can be tricky. You’ll need to keep the pedal dialed back a bit most of the time for normal playing, and then kick it all the way up to start the feedback. Watch out, though, as it can be difficult to roll the volume pedal back to the exact spot you want your playing volume to be at.
If you want to create feedback at the touch of a button, you can also use a clean boost stompbox. This will provide a clean volume boost that can put your volume over the edge and open the floodgates for some monstrous feedback. You can kick it on for solos to stand out above the band and let your notes ring out until the feedback kicks in. (Frank Zappa used to have these circuits installed directly into his guitar so he didn’t have to be standing over his pedalboard all the time.)
But there’s no more surefire way to create feedback than walking right up to your amp and shoving your guitar right up against the grill. With even moderate volume, your amp’s speaker will create more than enough soundwave energy to trigger raucous feedback. You can often see guitarists doing this onstage, in live performance videos of Jimi Hendrix, Sonic Youth, Kurt Cobain, J Mascis, and many others; Mascis, in particular, has developed feedback into his performance style and used it to pioneer his own form of expression on the guitar.
Feedback can help or hinder. Knowing how to manage that is the key. In the end, it’s up to you to decide if it’s an annoyance to be dealt with, or if it’s “all part of the show”.