When it comes to qualify the nature of audio effects, such as nonlinear distortion, it is not uncommon to observe artists or audio engineers using semantic descriptors such as “warmth”, “smooth”, “rough” or “bright”, to give a specific indication on the timbre itself, altered by those effects. One of the many ways of modifying sound is the nonlinear distortion synthesis technique, also called waveshaping. While a lot of different hardware implementations of distortion effects have their own specificities, each is designed to generate new partials thanks to a nonlinear function. On the other side, it is unclear to which distortion parameter(s) "roughness" and "brightness" refer to, and we don't know yet if those characteristics have a close relation to nonlinear distortion. In this study, we investigate the effect of the nonlinear distortion induced by a guitar pedal, the Boss DS-1, on the perceived brightness and roughness. First, an acoustic analysis reveals how the various controls of the guitar pedal affect the brightness and the roughness of a guitar chord. Then, a listening test confirmed that perceived evolutions of brightness and roughness are coherent with their related presets related to them.