Editing Total harmonic distortion (section)

==Measurement==
The distortion of a [[waveform]] relative to a pure sinewave can be measured either by using a [[THD analyzer]] to [[Fourier analysis|analyse the output wave into its constituent harmonics]] and noting the amplitude of each relative to the fundamental; or by cancelling out the fundamental with a [[notch filter]] and measuring the remaining signal, which will be total aggregate harmonic distortion plus noise.

Given a sinewave generator of very low inherent distortion, it can be used as input to amplification equipment, whose distortion at different frequencies and signal levels can be measured by examining the output waveform.

There is electronic equipment both to generate sinewaves and to measure distortion; but a general-purpose [[digital computer]] equipped with a [[sound card]] can carry out harmonic analysis with suitable software. Different software can be used to generate sinewaves, but the inherent distortion may be too high for measurement of very low-distortion amplifiers.

=== 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>