Mains hum is a fixed-frequency problem, which makes it one of the few noises that can be removed without a noise profile. A recording made near a dimmer, an unbalanced cable, or a ground loop carries the supply frequency and a stack of harmonics above it.
Frequency
Set the fundamental to match the grid the recording came from, 50 or 60 Hz. The control moves in 0.1 Hz steps because a supply is not perfectly stable and an old tape transfer may be slightly off speed.
If the hum is still audible after a notch is placed, the frequency is likely a fraction out. Raise narrowness first so the notch gets easier to aim, then nudge the frequency until the tone disappears.
Harmonics
Harmonics adds notches at exact multiples of the fundamental: 2 gives you 100 and 150 Hz on a 50 Hz supply, 4 goes up to 250 Hz.
Buzz and hum are different symptoms of the same fault. A low hum is mostly the fundamental. A harsher buzz is mostly harmonics, so it needs the higher settings.
Narrowness
Narrowness is the Q of every notch, between 5 and 60. At Q 40 a 50 Hz notch is roughly 1 Hz wide, so nothing musical is lost. At Q 8 it is wide enough to audibly thin a bass note that sits on the same frequency.
Depth
Depth is how far each notch drops, from 0 to 48 dB. At 0 dB the filters are an exact passthrough, which is what the step does when it sits in a chain with the control at zero.
Full depth is rarely necessary. 20 to 30 dB usually pushes hum below the noise floor of the recording, and a shallower notch is less likely to leave an audible hole in a sustained low note.
Where notching stops working
Notches remove a frequency for the whole file, including the parts where the wanted signal shares it. A bass line with a note on 50 Hz will lose energy on that note.
Hum that changes pitch, buzz that only appears on some passages, or broadband noise like hiss and air conditioning need a different approach.
Example: a guitar DI with a ground loop
Frequency 50 Hz, harmonics 4, narrowness 30, depth 26 dB.
The fundamental and the buzz at 100 to 250 Hz drop out, and the notches stay narrow enough that the instrument’s low strings are unaffected.