Another person asked this too. There's obviously no way to directly measure it, since it is simply a perception of something that does not actually exist. I only "hear" it because I have either neurological damage or inner ear damage. But I know musical notes, and it's a D2. A C#2 is too low, and a D#2 is way too high. It might be more like a 72Hz instead of 73Hz (D2), but it's close. It's a "guess" in the same way that any singer might sing any other particular note. I can hum it, in tune with itself, although the sound of my own voice destroys the perception. I can use that tone generator I linked above, starting it and stopping it, and compare until I hit the right note. Just like tuning a guitar by ear. I do have to have absolute quiet to hear it, but I can reliably hear it 100% of the time when in absolute quiet.