Four bands run in series at the source sample rate, each one a second-order section. A band left at 0 dB with a bell shape passes the signal through unchanged, so an unused band costs nothing and colours nothing.
Shape decides which controls matter
Bells and shelves respond to gain. High-pass, low-pass, and notch do not: they take out everything past a corner, or everything inside a narrow slot, and their only settings are frequency and Q.
Set a band to Off to take it out of the path entirely.
Q on a bell, Q on a filter
On a bell, Q is width. The relationship is bandwidth = frequency / Q, so a bell at 1 kHz with a Q of 2 works across roughly 500 Hz and one at Q 10 across roughly 100 Hz.
On a high-pass or low-pass, Q is resonance at the corner. Butterworth is 0.707, and anything above that lifts a peak right where the slope begins. Values near 8 ring audibly, which is either the effect you want or a mistake.
The two shelves ignore Q. They run at a fixed slope, the same one every hardware channel EQ shipped with, because a shelf’s steepness is a separate parameter that this tool does not expose.
Cut narrow, boost wide
Resonances are narrow, so remove them narrowly: find the frequency with a high-Q boost, sweep until the problem jumps out, then invert the gain and lower the Q slightly.
Tone is wide, so add it widely. A 2 dB high shelf at 6 kHz reads as air. A 2 dB bell at Q 8 reads as a phase-shifted whistle at 6 kHz.
Example: a dull interview recording
Band 1 high-pass at 75 Hz, Q 0.7, to take out desk rumble. Band 2 bell at 320 Hz, -3.5 dB, Q 1.6, for the boxiness a small room adds. Band 3 bell at 2.6 kHz, +2 dB, Q 1, to bring the consonants forward. Band 4 high shelf at 9 kHz, +1.5 dB. Output gain -1 dB to make room for the boosts.
Sample rate sets the ceiling
Every band’s frequency is clamped to 45% of the source sample rate before its coefficients are built. On a 44.1 kHz file that ceiling is 19.8 kHz. On a 16 kHz voice recording it is 7.2 kHz, so a band parked at 12 kHz lands at the ceiling instead.
Moving a control during playback
Coefficients ramp to their new values over 20 ms rather than jumping, so sweeping a frequency while the file plays sounds like a sweep instead of a series of steps. The export runs the same code as the preview, so what you hear is what the file gets.