Originally Posted by bigshot
when we are talking about harmonic distortion, the noise is following the signal, right? I would guess is the fundamental gets loud, the harmonic distortion gets loud and quieter goes quieter.
Yes. Not only that, our hearing acuity improves at louder levels too. Further, if the noise floor in your room is 30 dB SPL, that 1% or 0.1% must be louder than the noise floor in order to hear it.
So there would be two factors making it more difficult to hear THD than a steady sine wave.
A bass solo with distortion is mostly just a few sine waves. This is why I usually refer to "artifacts" rather than distortion.
the masking effect of the loud signal drowning out the low level harmonic distortion
Yes, though masking is reduced when the frequencies are farther apart.
the fact that the noise is going up and down in volume and pitch, not maintaining a steady and consistent tone. That's why 1% THD is usually cited as the threshold. Am I correct in this?
It depends entirely on the source. Again, distortion on a bass solo might be more noticeable than a complex mix. Further, whenever you have THD you also have IMD, and IMD is more noticeable. Earlier I mentioned that circuits don't generally create a strong 30th harmonic and no other content. But IMD can easily occur at distant frequencies. For example, two flutes (which create fairly pure tones) playing a high D at 1,175 Hz and the E above at 1,318 Hz, when distorted, generate sum and difference frequencies at 2,493 Hz and 143 Hz. The lower frequency will certainly be noticed because it's not masked by the source tones at all.
--EthanEdited by EthanWiner - 2/18/14 at 10:54am