Originally posted by charlie:
I believe scientific evidence. Got any?

For example, there is an apocryphal story about Rupert Neve that tells of a console channel that sounded particularly "bad". It was later discovered that it was oscillating at some ultrasonic frequency, like 48 kHz. Rupert Neve is rumored to have seized upon this as "proof" that the ear can hear well beyond 20 kHz. However, there exist an entire range of perfectly plausible mechanisms that require NO ultrasonic acuity to detect such a problem. For example, the existence of ANY nonlinearity in the system would result in the production of intermodulation tones that would fall well within the 20 kHz audio band and certainly would make it sound awful. Even the problem that was causing the oscillation itself could lead to massive artifacts at much lower frequencies that would completely account for the alleged sound of the mixer in the complete absence of a 48 kHz "whistle."

Rupert Neve does a test where he changes sine waves to square waves with high fundamentals, and people can hear the difference when they should not theoretically be able to, as the only difference is in harmonics that are above the commonly accepted audible range. He also tells a story of Geoff Emmerick correctly pointing out a couple of improperly terminated channels just by listening to the console output when the differences were only a few db down at around 50 kHz. In both cases above, there may be other distortions at work that explain the differences heard, but it remains interesting nonetheless. It has also been pointed out that trumpet with a harmon mute has a harmonic near 50 kHz which is near the amplitude of the fundamental, thus the argument of the upper harmonics being so low as not to matter is not an entirely accurate statement.

There are less-formal experiments that purport to show that we can hear above 20 kHz, and perhaps the best known of these is the one that Rupert Neve — whom I have a tremendous amount of admiration for, although I think he's completely wrong on this — does. He plays his audience a 10kHz sine wave and then a 10kHz square wave, and everyone in the place agrees that the two waves sound different. Therefore, he concludes, because the lowest harmonic above the fundamental in a square wave is the third, we are hearing 30 kHz!

Of course this is, as the English say, “tosh,” and many before me have pointed this out. There are a lot of reasons why we can hear the difference between those two tones, none of which have anything to do with ultrasonic sensitivity. One is simply that the energy of a square wave is higher than a sine wave at the same nominal amplitude, so the square wave sounds louder. Another is that any transformers in the signal path, unless they are exquisitely designed and constructed for passing such high frequencies, will introduce slewing and intermodulation distortion from the square wave — not only from the third harmonic, but from all the odd harmonics above it — that will have products well inside the audible range. And, if somehow a perfectly amplified 10kHz square wave were to make it all the way to the speakers, then the speakers would create their own distortion, which would be quite different from the distortion a sine wave would make.