Reading the title carefully. It says single tones. This is because the non linearity of the human hearing mechanism may hear the intermodulation (not beat) of say 25KHz plus 30KHz as combination subharmonics. A similar nonlinear multiplication such as on how an HF, RF in fact Theremnin can produce audio frequencies. See also Tartini violin tones. See also Taylor series in mathematics of non linear transfer curves ie electron tubes in class A etc.
But back to a single sine wave.
Yes the human ear may not recognize a single 25KHz tone. But an analog tape recorder can demodulate the information and bring it back to the audio band. Theremin non linear reduction of frequency again.
This was by chance verified on a Ferrograph series 6 electron tube analog tape recorder running at 7.5ips.
A 10Hz to 200KHz sine wave generator was connected at the input.
While recording and when instant monitoring was done by the replay head any fairly close to above 20KHz could create clearly heard subharmonics, ie audio band. While input frequency was increased, going down frequemcy sweeps were created. Is this demodulation happening due to the interaction of input frequency with the bias high frequency Thermein way again? Together with non linearity of magnetic tape at higher levels? Interesting and nice sounding glissandos. Also called birdies, see reference.
So although we may not be able to hear the presence of a single sinewave above 20KHz a simple signal path tape recorder let us do hear its presence. The information is not lost. Perception is enchanted.
Is this another reason why analog recording may send for tea or to the cleaners other technologies?
Reference:
https://www.stereophile.com/content/revox-g-36-open-reel-tape-recorder
Further general reading:
Applied Electronics - T. S. Gray - MIT
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