Feedback and microphonic noise are two terms that seem to be used and misused as people seem fit. I want to take a look at the mechanics behind feedback and microphonic feedback and how to use feedback in your playing, or simply get rid of it all together.
Feedback and microphonic feedback are terms used to denote a vibration, initially unwanted, self sustaining (to some degree) and kept in motion by other means of energy than the primary source of energy (in the case of our guitar: the fingers or pick). It must be stressed that feedback and microphonic feedback use the same mechanism to create and sustain the vibration: it’s just a different element that vibrates. I spoke before about microphonic feedback, but that was more aimed at solving the problem. Now I want to talk about what causes it on a more basic level.
To understand the issue at hand, let’s investigate how feedback in general is caused. After we hit the string, via electromagnetic physics a signal goes to our amp and through our speakers, making air move which in turn moves our tympanic membrane in our ears, which sends messages which our brains perceive as sound. The string loses its initial energy due to various reasons (like air or magnetic resistance), and the output of the pickup – and thus the volume – decreases, causing us to hit the string again. But if the sound(wave) produced by the speaker works in some unique fashion in conjunction with the guitar and strings, that wave might set the strings themselves in motion. If that’s the case, the note will sustain as long as you wish. This phenomenon is what we call feedback, and it uses resonance as its mechanism to power the string. Because it takes less energy to power the first harmonic, feedback is often perceived as the octave and not the root note. Just take a look at this clip. Here you see the Tacoma Narrows Bridge, also affectionally known as Galloping Gertie, moving in the wind. A perfect example of resonance. The bridge started to vibrate due to various reasons and the wave just seem to amplify itself.
If you’ve tried to conjure feedback yourself, you may have noticed that it happens when a series of conditions have been met. Firstly, you need power. Feedback goes best with a lot of volume. High volume means a large sound wave, and it’s that wave that powers the mechanism of resonance. Secondly, the exact position of you and your guitar. The way the sound wave distributes across the floor is also important for the way you get feedback. Just try to get it, and when move away 45 degrees to the left or right you’ll soon notice that the feedback gets less (or sometimes even more!).
As mentioned, feedback means that the string gets ‘new’ energy to vibrate and since it’s ‘easier’ to make it vibrate at one of the upper harmonics (an octave, to be precise), the sustaining feedback we get is in that said octave. Microphonics works in a similar fashion namely something vibrates in a way it’s not intended or supposed to do. In this case, it’s the windings of copper wire around the coil that vibrate. This sound is a hard, harsh, shrieking sound that isn’t pleasant to the ear and doesn’t go away if you change positions. Only turning down the volume helps in this case. Just take a quick look at this clip to see how annoying microphonic feedback is.
What could be considered ‘normal’ feedback can be used in a musical way, though. Jimi Hendrix, The Who, even Iron Maiden used it for contrast and tonal flavor. Listen to Ted Nugent’s Homebound to get a sample on how feedback can be used. The longer notes in the intro slowly move into a first-harmonic feedback. I find that tone to be spectacular! This is of course aided by the fact that Ted Nugent played a full hollowbody thinline, the Gibson Byrdland!
To stop the layers of copper from shifting (thus in turn creating their own signal, which suppresses the signal we actually want), pickups are dipped in molten wax, to fix the coil wrappings in place. Slight movement of the wires can be perceived is pleasant though. It gives the pickup an ‘aliveness’ that’s very hard to describe. Some pickups are, therefore, less intensely wax-potted than others and some aren’t even wax potted at all for this exact reason! Thankfully, modern winding techniques allow for a very precise, tight wind ensuring that even the most vintage of single coil or PAF-replica doesn’t exhibit the shrieking noise of microphonic feedback at decent volumes and levels of distortion. Seymour Duncan’s Seth Lover set is a prime example! The Seth Lover humbucker isn’t wax potted (for the sake of historical accuracy) but can be easily used in high-volume situations. If you have a great sounding pickup but it has huge amounts of microphonic feedback you don’t want, you can try to wax pot it yourself. In case you are unfamiliar or hesitant to do this process you can always contact the Seymour Duncan Custom Shop. Those guys are happy to help you out: they can pot any pickup you like for just $10!