in layman's language, what is Total Harmonic Distortion

[Deleted member 375592]

What exactly is LTI distortion?

How about cones in an open baffle? On second guess, they might not be any different for the harmonics.

A planar is constructed differently so it may not have such interference, huh?

LTI = Linear Time Invariant
An open baffle could be bad because the cone excursion is higher than in the closed box.
Planar has its own troubles, mostly because it is not Time Invariant. Heat dissipation is a big problem, the membrane changes resistance rapidly and "eats" into waveform fronts.