Non-linear distortion generates new frequencies as level-dependent behavior shifts.

Evidence

Non-linear distortions are additional frequencies generated when signals or programs interact with a device that changes its behavior as a function of sound level—the moment-by-moment input-to-output relationship is not a straight line. The non-linearity might be in, for example, a woofer cone suspension that at a point requires proportionally greater effort to move as the cone displacement approaches it limit. It may sound clean at low sound levels, but as the volume is turned up, the timbre changes as additional sounds (distortion products) are generated by the non-linearity. There are also other causes. — Toole, Floyd E, Sound reproduction- the acoustics and…, 108

Categorization

• DEFINE: Precise definition of non-linear distortion as level-dependent input-output curvature generating extra frequencies.