The first thing that made me sit up and think was listening to someone playing sine waves and square waves at different frequencies. At the highest frequencies they sounded the same of course, but drop down a bit and I could hear the difference between sines and squares at 6 or 7 kHz. A square wave is composed of the fundamental plus just the odd order harmonics - 3rd, 5th, 7th etc. That meant I was hearing the difference that 18 and 21 kHz harmonics were making. But I can't hear single tones that high.
I knew that infinite baffle speakers sounded better because they were phase linear to lower frequencies than other subs. I was convinced that Storm and Trinnov prices were exorbitant until I realised they corrected phase as well as amplitude. This improved time domain performance, especially in the mid-range. Wondering if high frequency time domain performance was being compromised by amplifiers, I looked at the phase measurements performed by SoundStageNetwork, and I found nothing. However, they also tested DACs, and despite flat amplitude response, there was a clear correlation between sample rate and pass-band phase linearity, like this
RME ADI-2.
Some research at Kyoto University by Shibasaki & others was published by the Journal of Neurophysiology where music was played to a large number of listeners with different bandwidth filtering applied. The 50kHz bandwidth was broken into two halves - audible and ultrasonic. By various means, the listeners could distinguish the audible sound from the audible + ultrasonic combined. The interesting bit is the ultrasonic sound was completely inaudible,
when it was played on it's own, without the audible sound.
The other was the DBT by googlebot on hydrogenaudio mentioned above. He freely admitted that he couldn't hear (sinusoidal tones) above 17 kHz, but he could hear the difference between a 20 kHz speaker and a 50 kHz speaker. That test was indeed done with robust scientific principles - level matched, double blind, and randomised. Those are the rules on hydrogenaudio. Yes, the test material was taken from the same master, and even Arnold Kruger himself (RIP) said of the test methodology: "I don't see anything obvioiusly wrong with the samples." Googlebot performed multiple tests and got a foobar2000 test result with a high degree of confidence, and other people also got positive tests with over 95% confidence.
People often try to answer positive results like this by attributing it to intermodulation distortion, and foldback of high frequencies into the audible band. In googlebot's case, he was using the same DAC and amplifier, and if they were causing the IMD, then that would have been audible with the 20 kHz speakers. Of course the speakers themselves could have been causing the IMD, but it would probably have been
even more audible with the inferior speakers, working outside their normal operating range.
Some people have said there were two variables at play - both different bandwidth and different depth of modulation (16 vs 24 bits). If that was the explanation for the results, then it wouldn't have made any difference which speakers were being used.
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