How to calculate the similarity of two electric waves I have two electric signals from different sources and I think that are similar.
I have the FFT of the signals and I have their spectrums of frequency  in separate arrays. 
How should I proceed to find the percentual of similarity the two signals?
You could measure the Cross-Correlation between the two signals.  

Here's an almost identical thread, with a filling answer:

How do I implement cross-correlation to prove two audio files are similar?

Some general info on cross-correlation from wiki:
https://en.wikipedia.org/wiki/Cross-correlation

Hope that helps.