HiFi Equipment - What is Total Harmonic Distortion (THD) And Do I Need to Worry About It?

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Total harmonic distortion or THD is one of the more regular little T.
s (or Three letter acronyms) you are likely to see in most manufacturers specifications.
But what exactly is THD, is it all that important and what level of THD is acceptable for Hi Fi and AV equipment? In audio reproduction between the input stage and output stage of a hi fi system there will be a certain amount of noise and distortion introduced to a signal.
Some of this distortion is created by the circuits and physical elements introducing signal harmonics at double and triple the frequencies of the original signal.
These added signals are technically known as Harmonic Distortion.
THD is a measure of the differential between the input stages of the system and the audible extra signal at each harmonic.
The THD calculation doesn't consider the relative level of each harmonic.
EG: One amplifiers output at the 2nd harmonic could be higher than the output at the 3rd.
Another could create equal levels at the 3rd and 5th.
The two amps would sound very different yet could have the same measured overall THD.
In most hi fi circuits such as amplifiers and pre amps one should expect a value of less than 1% THD as it is relatively easy to remove a large amount of THD from amplifier circuits using various techniques.
Unfortunately it is far harder to stop significant levels of THD from being produced by the physical elements of a setup such as the hi fi speakers.
Much higher levels of THD are common in cheaper end speakers especially at higher volumes.
Speakers with high levels THD tend to cause "smearing" of audio, and result in poor imaging and a "muddy" sound.
In reality one should test a component ideally in the room it is to be used in and at a volume level it is likely to commonly be heard at to ascertain if the system is acceptable for purpose.
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