Cut everything above a frequency, everything below it, or everything outside a band. Four filter types share one cutoff, resonance, and slope, and the mix control blends the result back against the untouched signal.
The four types
Low-pass keeps the low end and removes what is above the cutoff. It darkens a source and is the type behind most filter sweeps.
High-pass does the opposite: it keeps the top and removes what is below. At 80 Hz on speech it clears rumble, footsteps, and handling noise that carries no useful content.
Band-pass keeps only what surrounds the cutoff. Narrowed with resonance, it is where the small-speaker or telephone sound comes from.
Notch removes a narrow band and leaves the rest. It is the type to use for a single hum tone, set to 50 Hz or 60 Hz depending on the mains frequency where the recording was made.
Cutoff
The cutoff slider sweeps 20 Hz to 20 kHz on a logarithmic scale, so an equal drag moves an equal musical interval rather than an equal number of hertz. Musical hearing works the same way, which is why the region below 1 kHz gets most of the travel.
The cutoff is the -3 dB point, not the point where sound stops. A low-pass at 2 kHz still passes 4 kHz, just 12 or 24 dB quieter.
Resonance
Resonance sets the Q of the section. On the low-pass and high-pass types it raises a peak at the cutoff itself, so the corner sings rather than rolling smoothly away. Around 0.7 the response is flat into the corner; higher values emphasise it.
On band-pass and notch, Q controls width instead. A high value makes the band narrow, which is what you want when removing a single tone without gutting the material around it.
Slope
12 dB/octave is one filter section. 24 dB/octave adds a second section, which halves what survives an octave past the cutoff.
The second section runs at a fixed Butterworth Q, so raising resonance still produces one peak at the corner rather than a doubled one. The steeper option removes more but shifts phase more around the cutoff, which matters if the result will be mixed against the original.
Mix
The mix control blends the filtered signal with the untouched one, which is not something a filter is usually given. It is useful for parallel filtering: a heavily resonant band-pass at 30% mix adds a coloured layer under the source without removing anything from it. At 0% the audio passes through bit for bit.
Removing mains hum
Hum sits at 50 Hz or 60 Hz plus its harmonics. A notch at the fundamental with high resonance removes the strongest component and leaves the rest of the low end alone.
Harmonics at 100/120 Hz and 150/180 Hz survive that single notch. If the hum is still audible after the fundamental is gone, a second pass through the audio workflow can place another notch on the harmonic.