More on duet dissonance

See Duet dissonance: the experts speak for an update

I mentioned that two violins playing in unison usually sound out of tune. I've heard a rule of thumb that "any small, even number of stringed instruments will sound terrible."

Is this true? If so, why does it happen?

Much of the energy in a violin note isn't in the fundamental tone, but in the overtones. When two violins play the same note, the fundamental tone is in tune (we hope!), but some of the overtones are most likely not in tune. The frequencies and relative strengths of the overtones are affected by the violin string and the structure of the violin, among other things. A listener will hear "beats" as the out-of-tune overtones move in and out of phase with one another. Adding a third violin greatly increases the complexity of the overtones (we could call this "richness of the sound"), making any given pair of overtones less obvious.

I suspect there are some deeper physics and psychoacoustics in this phenomenon. Helmholtz roughness curves are a promising approach. There is quite a bit of research out there on consonance and dissonance (possibly because it's a realistic topic for undergraduate-level laboratory research). But in my casual reading and Googling, I haven't found any research which directly addresses this particular problem.

Comments are welcome.