Back in the old days (1982) It was FTC mandated for the manufacturer to provide a comprehensive single criteria power specification. However, with the de-regulation craze of the 80's, this requirement was dropped. This left it up to manufacturers to determine how to advertise and display their product specifications with no commonly accepted standard for emasurement.
I'll attempt to explain how THD is measured.
Total harmonic distortion (THD):
Of a signal, the ratio of (a) the sum of the powers of all harmonic frequencies above the fundamental frequency to (b) the power of the fundamental frequency.
The THD is usually expressed in percent as distortion factor or in dB as distortion attenuation.
Measurements for calculating the THD are made at the output of a device under specified conditions.
Now, there are several ways to measure THD that are commonly used. One is defined by the FCC, and another by the EIA.
Years ago a number of papers were written on human hearing and harmonic distortion. What they found was that the human ear is very insensitive to harmonic distortion that is close to the main signal, and increases in sensitivity to harmonic distortion further away from the main signal. The second harmonic, which is an octave away from the main signal, is the hardest to hear, especially when you are driving a loudspeaker.
The best estimates that I can give you is that we can detect somewhere between 1% and 3% of second order harmonic distortion. Which is why you can't hear it. If the sum total distortions were farther away from the main signal you would be able to hear it. Some solid state designs can have pretty low distortion but they can get to be aggravating after awhile. That's because the distortion generated by the amp is further away from the main signal where the ear is more sensitive.
Without going into too much detail, there are many factors in how THD can be measured, including but not limited to:
is the signal being used to measure THD a notch frequency, of full 20Hz-20KHz at equal power?
Over what unit period of time is THD being measured?
What kind of signal is being used to measure the distortion?
Point being, very few manufacturers specify this data, so THD is helpful at times, but again, not something to base a purchase on.
Remember, some of the best amplifiers in the world have an advertised THD of between 1% to 10%.
It's commonly agreed that distortion below 1% is inaudible, and in a car, below 10% is inaudible to the human ear.
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