OK.... simple explanation.
A pure tone (like a whistle) consists of a single frequency of sound.
The sounds produced by most musical instruments consist of a mixture of several tones (which is why a "middle C" on a guitar sounds different than a "middle C" on a violin).
And, with most instruments, most of those sounds are "harmonically related" to the main tone - which means they are multiples of the main frequency.
If your main tone is 500 Hz, the second harmonic is 1000 Hz (500 x 2), the third harmonic is 1500 Hz (500 x 3) etc.
So, with most instruments, their characteristic sound is determined by the mix of harmonic overtones they generate.
With a perfect amplifier that would be the end of the story - it would reproduce the original mix of tones exactly and there would be nothing to talk about.
However, no amplifier is absolutely perfect; they all generate a little bit of distortion.
Distortion, as a general term, refers to
ANYTHING that is added or removed from the original.
(We tend to refer to stuff that is added all the time, and has nothing to do with the music, as "noise" rather than distortion, but it qualifies as distortion too.)
Now, amplifiers are imperfect in many different ways, and so they produce many different types of imperfections.
This matters mainly because different types of distortion may be far more audible or annoying than others.
Harmonic Distortion refers specifically to distortion that is related harmonically to the primary frequency.
If the primary tone you're playing is 500 Hz, and your amplifier also produces some extra 1000 Hz, and some 1500 Hz,
THAT DON'T BELONG THERE, then that extra stuff is harmonic distortion.
It is
HARMONIC distortion because it is at frequencies that are harmonically related to the original.
Now, if you played a
PURE 500 Hz tone, and got back a 500 Hz tone with lots of 1000 Hz added to it, the extra tones would be very obvious.
However, because most instruments produce a mix of harmonic tones already, a little extra at one or another frequency will tilt the balance a little bit, but won't be glaringly obvious.
(Also, because most instruments produce mostly lower order harmonics - 2x and 3x - we tend to notice a few extras of those less than extras at higher harmonics.)
TOTAL Harmonic Distortion is simply the total of all the extra harmonically related stuff that shouldn't be there - and it is usually expressed as a percentage of what
SHOULD be there.
(So "0.5% THD means that, when you play music, 0.5% of what comes out is harmonically related junk that shouldn't be there.)
Note that the amount of
TOTAL Harmonic Distortion does not specify which harmonics are involved.....
This matters because lower harmonics (like x2) tend to be innocuous, while even much lower levels of higher frequency harmonics tend to sound audibly wrong.
Note that there are
OTHER types of distortion..... such as IM Distortion - which refers to a modulation product between two frequencies.
IM distortion occurs at frequencies that are
NOT harmonically related to the original frequency, and so can be much more obvious.
Various digital systems can produce noise/distortion that, while related mathematically, is not related harmonically to the music.
This sort of distortion can be very obvious, and can sound very unpleasant, which is why it is important that it be held to very low levels in order to be inaudible.
The other catch is that we usually specify THD at one power level..... which may not tell the whole story.
For example, a typical 100 watt tube amplifier may have 0.1% THD at 1 watt, 1% THD at 10 watts, and 10% THD at 100 watts.
But a typical solid state amplifier may be 0.05% THD at 1 watt, 0.07% THD at 10 watts, 0.1% THD at 90 watts, and 10% THD at 100 watts.
Even though both have the same THD at 100 watts, it's clear that, at
MOST power levels below that, the THD on the solid state amp will be
FAR lower.
(So, even though both have the same specification at 100 watts, the solid state one will have far less distoriton for virtually all of the time you're listening to it.)
The bottom line is that harmonic distortion can be a complex subject......
But what you should take away from the discussion is that some harmonic distortion sounds worse than other....
And lower is always better...
And, fortunately, if it's
LOW ENOUGH, then the other details become unimportant.
(People may argue whether 0.5% second harmonic sounds less annoying than 0.1% third harmonic.... but most agree that 0.01% of either is going to be inaudible.)
As a
VERY broad generalization,
Most people I know would consider anything over about 0.5% THD to be "bad enough to possibly be audible"...
and anything below about 0.1% to be "good enough that you probably won't notice it"...
Does anyone have a good resource, link or youtube video that can explain to me what Total harmonic distortion ?
Need something easy to understand.... I'm a simple person.
or maybe its simple to explain in a qucik post.
Why is this important in amplifier design... What is a good number... Do the specs just ridiculous as to not even matter to the human ear...and how is it measured... etc
Chris
