The various harmonics are perceived by the ear in different ways. So THD doesn't tell the whole story, and often amps with low THD might not sound as good as an amp with higher THD. Here's some of why:
The ear does two things with harmonics. One is that it assigns a tonality to all forms of distortion, including harmonics. the lower orders (2nd-4th) cause 'warmth', 'bloom', 'body' and the like and the ear is considerably less sensitive to them. The higher orders (5th and above) are used by the ear to sense sound pressure and so the ear is **keenly** sensitive to them!
For this reason I've felt for a long time that a weighting system should be used on the harmonics, perhaps a '1' assigned to the 2nd and a '1000' assigned to the 7th, that sort of thing. In this way we might be able to tell how an amp might sound simply by looking at the spec sheet.
The second thing the ear does with harmonics I already mentioned- the higher orders are used to gauge sound pressure. As a result of these two things, a system with small amounts of higher ordered harmonic content (but more than the signal has) will sound louder, brighter and harsher than it should.
If these harmonics are kept to a minimum, the system will sound smoother, more relaxed and you may not be able to tell how loud its playing. So that old saw about a small amount of distortion being 'inaudible' is rubbish, if we are talking about the higher ordered harmonics! The ear has to be able to sense sound pressure over a 130dB range or so, so it **has** to be keenly sensitive to tiny amounts of higher ordered harmonics. Add to this the Fletcher-Munson curve, and now its easy to see how brightness due to distortion is so easily heard.
The ear does two things with harmonics. One is that it assigns a tonality to all forms of distortion, including harmonics. the lower orders (2nd-4th) cause 'warmth', 'bloom', 'body' and the like and the ear is considerably less sensitive to them. The higher orders (5th and above) are used by the ear to sense sound pressure and so the ear is **keenly** sensitive to them!
For this reason I've felt for a long time that a weighting system should be used on the harmonics, perhaps a '1' assigned to the 2nd and a '1000' assigned to the 7th, that sort of thing. In this way we might be able to tell how an amp might sound simply by looking at the spec sheet.
The second thing the ear does with harmonics I already mentioned- the higher orders are used to gauge sound pressure. As a result of these two things, a system with small amounts of higher ordered harmonic content (but more than the signal has) will sound louder, brighter and harsher than it should.
If these harmonics are kept to a minimum, the system will sound smoother, more relaxed and you may not be able to tell how loud its playing. So that old saw about a small amount of distortion being 'inaudible' is rubbish, if we are talking about the higher ordered harmonics! The ear has to be able to sense sound pressure over a 130dB range or so, so it **has** to be keenly sensitive to tiny amounts of higher ordered harmonics. Add to this the Fletcher-Munson curve, and now its easy to see how brightness due to distortion is so easily heard.