The lower trace shows classic crossover distortion, where one transistor imperfectly hands over to the other in a push-pull pair. Crossover in large-ish quantities (above 0.3%) is audible, roughening the sound.
What it tells us
A distortion of a sound is, in the broadest sense, any change that affects the original. Distortion that can be heard as a nasty ripping coarseness is a particular form that comes from non-linearity where the output, or what we hear, ceases to follow the input signal in fairly gross fashion. A measurement of distortion is supposed to tell us about the purity, or lack of it, of the sound. It certainly does do this when there are large quantities (5% or more) of low order harmonics, which will add muddle. Just how much muddle depends upon the correlation of the harmonics to the original though; uncorrelated produces audible muddle; correlated produces a subtle change in timbre. Second harmonic above about 5% lightens timbre; third harmonic above 1% sharpens the sound (these are necessarily very approximate values).
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