Root-mean-square (rms) refers to the most common mathematical method of defining the effective voltage or current of an AC wave.

To determine rms value, three mathematical operations are carried out on the function representing the AC waveform:

(1) The square of the waveform function (usually a sine wave) is     determined.

(2) The function resulting from step (1) is averaged over time.  

(3) The square root of the function resulting from step (2) is found.

So is it basically the average peak to peak voltage of a signal * .707, or do I need to keep in mind that the guitar's signal isn't a perfect sine wave?

-Colin

RMS represents the average power-equivalent delivered by a waveform, which is different than the average voltage. It is calculated by taking a number of samples, squaring each one, taking the mean (average) of the squared values, and then taking the square root.

In a sampled system (digital audio), you can do it just like that (square root of the mean of the squares of the samples). In analog, this can be done with logging circuits and a low-pass filter (without doing any sampling).

There is no way to convert from average voltage to RMS that works for all waveform. Cheap meters (that aren't "true RMS") use an approximation that works for single sine waves. They can be WAY off for other waveforms. "True RMS" is a good feature for audio and absolutely essential for any kind of pulse-waveform work (switching power supplies, stepper motors).

Here's something that can help you determine RMS for different waveforms. If you have a scope, you should be able to figure it out if you can determine the P to P value.

http://www.diyguitarist.com/Images/RMS-Calcs.jpg

Hope that helps.

Yes, it is best not to assume that a guitar is a pure sine wave (especially in the case of high gain distortion)

There's a trick called Parseval's theorem that allows you to find the RMS value of each frequency component, and sum them together and this is equivalent to the RMS power (or RMS voltage if voltages are added) in the signal.  

To guess at the relative harmonics in a guitar signal, a form of A*exp(-kF)
is a decent guess for a clean signal.  

For a heavily distorted signal, assume a near square wave for the fundamental, and carry it out to the 7th harmonic.

This is by no means precise.  It is only a "ball-park" estimate that will give you at least a clue of what power you may expect from a guitar signal.

Quote from: Paul MarossyHere's something that can help you determine RMS for different waveforms. If you have a scope, you should be able to figure it out if you can determine the P to P value.

http://www.diyguitarist.com/Images/RMS-Calcs.jpg

Hope that helps.

Thanks, that helped a lot.  Accuracy isn't that important, I just need to get it in the ballpark, and the guitar shouldn't be distorted so it should be reasonably sinosudal.

-Colin

Glad to be of service, Colin.