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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?

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Q1 Are %THD+N and %THD correlated in practice in the case of common audio devices (speakers, headphones, amplifiers)?

This really depends. THD is just a contributor to THD+N. If the noise dominates the numbers can be vastly different and if the THD dominates the numbers are basically the same. Passive components (passive speakers or headphones) have no inherent electrical noise so THD tends to dominate unless you hit a point where you trigger mechanical or acoustic noise as well. A good example of non-harmonic noise is port noise on a vented speaker.

Q2 How important is %THD in the reproduction of the input signal in speakers or headphones?

Depends on your definition of important on what you care about, but specifically in speaker design we keep a very close eye on THD since speakers are fairly non-linear.

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  • $\begingroup$ Thank you. I would appreciate if you could elaborate a little about your last sentence, that is in the relationship on THD and non-linear noise. In any case, I marked your answer as the accepted one. $\endgroup$ – user1420303 Aug 7 '19 at 23:06
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    $\begingroup$ Harmonic distortion is almost always caused by a non-linearity in the system. Electrodynamic loudspeakers have plenty of those: the most common ones are force-deflection of the suspension, magnetic force & inductance as a function of displacement and inductance as a function of current. $\endgroup$ – Hilmar Aug 8 '19 at 16:21
  • $\begingroup$ Thank you. That infromation is very interesting, and suggest that at %THD decreaces with the volume. $\endgroup$ – user1420303 Aug 8 '19 at 16:35

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