Tape noise is white noise, which is most noticable at the high end of teh audio spectrum. It is reduced in the demphasis process be cause it is already on the tape, not recorded with the signal.
Well actually, it is supposed to have a 120 uS time constant, which means that the signal has changed 3dB at 1325 hz.
To 2274 hz (70uS).
Starting at 500hz, with maximum effect above 1500 hz.
Get it at a drugstore. Never use rubbing alchohol, which contains mineral oil.