Tones in the 20-21 kHz range are annoying to me because I can just barely hear them.
Frequency: 2730 +/- 500Hz
The 2730 ± 500 Hz is just the location of the first resonance mode, not the entire frequency range. The advertised range is 1800 Hz to 20 kHz. The output will certainly drop off at some point above that, but I suspect that 20 kHz is the top of the chart because that is where they stopped testing, not because the output suddenly drops off. Clearly it will decrease below a usable point at some frequency, but it isn't graphed.