I've read about THD and THD+N due to interest in audio quality.
Some audio devices (ej. speakers) report %THD or %THD+N. %THD+N is greater than %THD. I've found that sometimes the difference is huge, which makes me think that a significant part of the noise is due to large generation of spurious frequencies (which are not harmonics of the reference input frequency).
I think that in similar circumstances non-harmonic frequencies should generate to the hearer a perceived distortion greater than those due to harmonics frequencies (Am I right?). If this is true, the %THD+N must be much more relevant than %THD. In such case %THD can only be relevant for comparing two audio devices if it is positively correlated with %THD+N.
Q1 Are %THD+N and %THD correlated in practice in the case of common audio devices (speakers, headphones, amplifiers)?
Q2 How important is %THD in the reproduction of the input signal in speakers or headphones?