I'm investigating how tone potentiometer works on my guitar. It's 500k tone pot with 22nF capacitor and passive humbucker pickups.
As described in this answer, I expected tone control to act as low-pass filter.
With tone control at 0%, I expected it to be:
While in practice it makes a wide cut around 3k, there's still a lot of high-frequency content:
This EQ curve was consistent between measurements and neck/bridge pickups, I did them by playing over the entire fretboard and averaging the spectrum. I was unable to bypass tone control and compared with the control set to 100%.
Here are the spectrums of signals with tone at 0% (red), 100% (gray) and the difference between them (white) that averages to the curve shown above:
Intermediate control positions also resulted to a similar curve with narrower cut.
Is there a good explanation of what's going on there? How tone controls may work in other guitars in practice?
I'm asking here and not at Electrical Engineering because I'm more interested in musical effects rather than technical details.