Calculation of phase shift distance for guitar feedback?

Question

Pioneering guitarist Robert Fripp utilized a system in which he placed a strip marked with notes between himself and the amplifier. Each marked distance from the amplifier was ideal for generating infinite sustain (feedback amplification) for a single note.
How might I calculate each distance, given the fundamental frequency of the desired note?

Answer 1

The fundamental equation is:

distance = velocity of propagation / frequency

The velocity of propagation is that of sound in the atmosphere which is 1128 ft / sec (under normal temperature, pressure conditions).

For example, G4 (391.99 Hz for A=440) would be:
1128 / 391.99 = 2' 10.5"

Now, this is a very simplistic approach. In reality sound velocity varies with temperature and atmospheric pressure (you'd have to recalculate every distance for Denver, versus LA) . Plus, amplifiers change signal phase, and sometimes that phase is dependant on frequency -- this will mess up the distance strip, also. Reflections off of walls will mess up a nice neat strip, too.


That's kind of a neat concept -- I'm going to try it at home.

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