Spectral, bispectral, and dual-frequency analysis of tube amplified electric guitar sound

We have analyzed the sound of an electric guitar that has been amplified by a high-quality all-tube amplifier, and emitted by means of a speaker cabinet. We re-amplified a recording of a clean guitar through a state-of-the-art all-tube amplifier at three different preamplifier gain settings: one clean, one half-distorted, and one massively distorted. Spectral analysis of recordings of the three signals exhibited a remarkably rich overtone spectrum, and we observed that only the high frequency part of the spectrum was boosted by an increase in the distortion levels. A bispectral analysis of the amplified guitar sound showed that quadratic nonlinearities are responsible for coherent phase coupling among the partials, and that the fraction of the total power which is due to quadratic nonlinearities is larger for the clean sound than for the distorted sound. Finally, a dual-frequency analysis showed that the sound, even for the sustained part of a single string pluck, is in fact a nonstationary random process. Our analysis showed that the guitar tone should be classified as an (almost) cyclostationary random process.