To me, dissonance is a musical and subjective term, and it is in the ear of the beholder, rather than being explainable based on harmonics etc. I would define dissonance as a voicing of notes that sound unpleasant to the listener.

It depends on the style being played, as well as the people listening to it. For example, the flat 5 in desafinado (slightly out of tune) was humorously referred to as "out of Tune" in the title likely because many people would hear it as dissonant. Yes that same flat 5 is heard everywhere and prominently, including pop tunes, and it is not heard as dissonant or "out of tune".

There are some intervals that can make a nice chord sound dissonant though. An example is a CMaj7 voiced from low to high as B,E,G,C. The B to C creates a flat 9 interval, and it sounds really bad. Change that C to a D and it sounds sweet again - even though we have introduced a "ninth" which some people would consider dissonant because it is a non chord tone.

Have Fun!
Peter Gannon
PG Music Inc.