Guitar Feedback

I’ll try this is Cafe:

I’m reading a book about Electric Ladyland etc. It says that guitar feedback is produced by the volume of the amplifier vibrating the strings and producing a note in a loop. It also says that there are a couple of variations where the note is pitched up or down an octave and one that pitches to an octave and a half higher.

Does anyone know the mechanics or engineering of this? I find it hard to picture the strings responding to the volume, feeding back at pitch, systematically, and with musicality to it. Maybe it’s like a theremin where you are playing by changing your angle toward the amp.

I am a guitarist not a scientist, but I can say that any note is comprised of its Fundamental (e.g., if you play an A 440, then the fundamental is A vibrating at 440hz) along with a bunch of harmonic overtones above and below that note. A amp feeding back, adding a fuzz box to the mix, emphasizes those overtones. When a player is manipulating feedback, they can use their fingers, the guitar’s controls, etc, to manipulate the feedback - you can dampen some overtones, which leaves the others to be heard.

Ted Nugent, twit extraordinare but a feedback guy, figures out where to stand with his hollow-body Byrdland guitar to trigger certain feedback loops - his position relative to the amp gets it going. For that approach, he is often pulling out notes lower than the fundamental…

It’s not just responding to the volume, it’s responding to the volume of its own note. So say you’ve got your low E string vibrating away which makes an E note and the amplifier makes the E note much louder. Now that note can make the string vibrate in exactly the same way the string produced the note in the first place. The air vibrations that we call “sound” are exactly in sync with the string’s vibrations and if loud enough will make the string vibrate more. As for bringing out different harmonics, it’s all just variations on the same principle.

Also remember we’re talking about notes produced by oscillating strings. Different oscillations can produce different harmonics of the same note.

Re: Nugent. He likes to play Byrdlands, hollow body electrics. Besides the strings getting vibrations from the sound coming from the amplifier speakers, the soundboard of the guitar is probably picking up vibrations too. In the post-war 1940s, guitar amplifiers became powerful enough to induce feedback. Both Merle Travis and Les Paul (individually) were concerned enough about feedback from their hollow body electrics to try to do something about it. Les built his “Log” and in doing so, invented the semi-hollow (or semi-solid) body guitar he showed the Gibson folks who managed to keep a straight face until Les had left the conference room and the door was closed before they broke up in hysterical laughter at it. Merle asked a friend who liked to tinker and craft things, Paul Bigsby (who later became famed for the Bigsby Vibrato Tailpiece) if he could come up with a solid body guitar for him. He did.

Les’s log was a one-off proof of concept creation. If you were a friend of Bigsby and asked him nicely enough, he’d build you a solid body too. He’s built more over the years, but he found his niche and fortune in the vibrato tailpieces.

Ranger Jeff - nice summary. Yep, Gibson laughed at Les Paul until they saw the success that Leo Fender was having with his Broad/Telecaster (of course based on Paul Bigsby’s designs translated to mass produce-ability) and went running back to Les to get the first Les Pauls with the whopper-jawed tailpieces out by '52.

Have you read The Strat in the Attic by Deke Dickerson? He’s a Bigsby nut, so a big chapter is him hunting down a guitar. I haven’t played a Bigsby - that would be fun. There’s a Bigsby mandolin in a case at one of my favorite guitar shops, next to their original Theremin ;).

As for the Nuge, yeah, I am sure you are right that the soundboard of the Byrdland contributes to the howls he extracts from his guitars.

Okay, hasty post:

Quick experiment you could try: Take your phone and call someone in your immediate vincinity and let them answer. Ask for their phone and push it close to yours, mic to speaker and move them around. No guitar needed as the speaker/mic membrane oscillate in place of a guitar string.

Feedback can be manipulated in many ways, with distance, direct interference with the oscillator(finger on string, volume-control on an oscillator in a synth etc.), air moisture, temprature, amplitude, equipment etc.

A feedback loop can just as well stay as an electronic signal, being fed back in before it goes out through a speaker. This, called an internal feedback loop, or no-input mixing, and is much easier to isolate from the other sounds in a chain, making it possible to limit and mix like any other sound signal. Many of the same parameters, like physical distance(as in length of cable, or how far the signal has to travel), affect an internal loop as well as an external one. External and internal loops can also be combined for further exploration. If you have a stereo guitar pedal, you can try running it in mono, with the guitar going into the right input, going out of the right, then going into left in and then out again. Or you can cross the left/right

An internal feedback loop in a mixing console can act much in the same way as an oscillator in a synthesizer can. Many oscillator circuits applicable for both sound and/or control is based on or incorporates positive feedback.
Just as an example of what can be done with feedback: I recently amplified an audio-signal with several positive feedback-loops running through a series og vegetable batteries powering a low power schmitt trigger oscillator circuit, to a degree where you can get a shock by touching the signal path.

I’m sorry if I’m making myself difficult to understand, and I’ve probably made some errors, but I hope this helps a bit.

Thanks. I didn’t mention that Gibson had 5 prototypes already built before Les was brought in to discuss marketing the things. Les’s only contributions to the design was to suggest

  1. tilting the neck (or the headstock) back 13° or so to keep the strings from popping out of the nut,
    B) slanting the bridge/tailpiece to improve the intonation,
    III) a flat top instead of a carved top,
    4th) a gold top would look classier than Fender’s oars (Hey, I perform in gold lame dinner jackets and they’re classy),
  2. people would think a black guitar would be classier than a Fender oar if you called it “Tuxedo Black” (not to mention if he was in a black tux playing a black guitar the TV audience would be fascinated by his fingers dancing up and down the fretboard under a spotlight in a close shot).
    Sixth) Low impedance pick ups, not high impedence.

Gibson ignored III and Six until the early 70s with the LP Recording model. They mostly ended up collecting a lot of dust on dealers’ shelves.

Yep. He also wanted the black Custom to be the premium model, but they made it with the solid mahogany body, whereas the gold top Standard was the mahogany with the maple cap.

Yes, while Les Paul was an innovator in solidbodies, they were using him more for his name as a hit guitarist of the day. And they still screwed up the tailpiece; sigh.

For some reason, my Fender Squire Protone (a FatStrat) has the most awesome and stable feedback on the Seymore Duncan Humbucker I fitted in the bridge position - I just choose a high gain drive, get moderately close to speaker and away it goes. Just a bit of tweaking on the volume knob and I can dial it in and out as I like, and I can goose it with the whammy for variation. Dial it back a bit and I can just enjoy the sustain as it goes on and on.

It is really just awesome and fun.

Really a great post.

One other fun thing about this experiment is that it also makes a dandy, if difficult to control echo/delay, due to the time required for the signal to route. Two phones on speaker phone work even better.

Dang, I never thought about using the hands free setup on my car to call a phone that was inside the car. I bet that’d have a little better fidelity than two speaker phones.

Try it out!

I think the way most people are exposed to that exact feedback delay effect, is via talking to people on skype who have no idea how to adjust their levels.

Sorry for a bump and if a slight hijack, but I think it’s relevant to the topic of feedback in general, if not exactly guitar feedback:

Some may not enjoy this, but I think it’s quite beautiful.