What do we know about the geometry of "feedback space"

Question

Everyone has seen someone hold a microphone too close to a speaker and get feedback. I've been experimenting with pickup feedback on my guitar recently (where the strings don't vibrate, but the guitar still feeds back at loud volumes).

I notice that I can change the position of the guitar and get different tones for the feedback. There seems to be some kind of invisible cone or sphere in the room with various sections that differ in pitch.

Is there any deterministic standard that allows one to know "which notes are where"? It feels like playing a theremin in three dimensions, but I don't have a clear understanding of what pitches will be created at various positions in the room.

Maybe the types of guitars/pedals/amps/speakers create too much variability to know, but I'm curious if anyone is familiar with this.

Answer 1

Is there any deterministic standard that allows one to know "which notes are where"?

"Deterministic standard" might be too much, but there are some things to consider:

1. Distance between the guitar and the speaker will affect the phase at which the sound reaches the guitar. This affects what frequencies are subject to positive feedback, and what to negative one; you may see it as a comb filter. The distance alone is not sufficient to determine the pitch, because there might be an additional phase shift in the electronics that is between the guitar and the speaker, but in general the farther you are from the speaker, the lower feedback notes you can achieve (but sound can also be reflected from the walls, so it depends also on the room acoustics and your position w.r.t. walls).

2. Eq. If you boost certain frequency ranges and/or mute the others you will increase the likelyhood of feedback occuring at these frequencies. Wah pedal is a low-pass filter with resonance, it may be very handy for controlling feedback.

3. Feedback requires inducing vibrations in something. If it's a string vibrating, this limits the frequencies to the fundamental frequency of the string and its harmonics. In the other question you asked about pickups feeding back, also known as microphonic feedback. Here it may depend on construction of the pickup, in particular its imperfections, and might be very difficult to predict.