Editing Total harmonic distortion (section)

=== Interpretation ===
For many purposes, different types of harmonics are not equivalent. For instance, [[crossover distortion]] at a given THD is much more audible than [[Clipping (audio)|clipping]] distortion at the same THD, since the harmonics produced by crossover distortion are nearly as strong at higher-frequency harmonics, such as 10× to 20× the fundamental, as they are at lower-frequency harmonics like 3× or 5× the fundamental. Those harmonics appearing far away in frequency from a fundamental (desired signal) are not as easily [[Auditory masking|masked]] by that fundamental.<ref>{{Cite web |last=Elliott |first=Rod |date=2009 |title=Valves vs. Transistors (Part 1) |url=https://sound-au.com/valves/valve-trans.html |access-date=2024-12-12 |website=Elliott Sound Products |quote=[Crossover distortion] may barely register on a distortion meter, so the figures looked excellent.  Unfortunately … listeners could hear the distortion - it was plainly audible, and sounded dreadful.}}</ref> In contrast, at the onset of clipping, harmonics first appear at low-order frequencies and gradually start to occupy higher-frequency harmonics. A single THD number is therefore inadequate to specify audibility and must be interpreted with care. Taking THD measurements at different output levels would expose whether the distortion is clipping (which decreases with an decreasing level) or crossover (which stays constant with varying output level, and thus is a ''greater percentage'' of the sound produced at low volumes).

THD is a summation of a number of harmonics equally weighted, even though research performed decades ago identifies that lower-order harmonics are harder to hear at the same level, compared with higher-order ones. In addition, even-order harmonics are said to be generally harder to hear than odd-order.<ref>{{Cite web |url=https://gearspace.com/board/so-much-gear-so-little-time/382595-odd-vs-even-harmonic-distortion.html |title=Odd vs Even harmonic distortion |website=Gearspace.com}}</ref> A number of methods have been developed to estimate the actual audibility of THD, used to quantify crossover distortion or loudspeaker rub and buzz, such as "high-order harmonic distortion" (HOHD) or "higher harmonic distortion" (HHD) which measures only the 10th and higher harmonics, or metrics that apply psychoacoustic loudness curves to the residual.<ref>{{Cite web |title=Raising the Bar for Rub & Buzz Defect Detection |url=https://www.ap.com/news/raising-the-bar-for-rub-buzz-defect-detection |access-date=2024-12-12 |website=Audio Precision |language=en-us |quote=High-Order Harmonic Distortion (HOHD…) – A classic method for rub & buzz detection, HOHD uses the THD (Total Harmonic Distortion) ratio but only of harmonics above the 10th, 10-35, 20-200, etc., which is a simple way to account for frequency masking effects. … Rub & Buzz Loudness … Applies a psycho-acoustic loudness model to the residual signal to calculate the perceived level of the rub and buzz}}</ref><ref>{{Cite web |title=RTA Window |url=https://www.roomeqwizard.com/help/help_en-GB/html/spectrum.html |access-date=2024-12-12 |website=www.roomeqwizard.com |quote=HHD (higher harmonic distortion for harmonics from the 10th up to at most the 50th)}}</ref>