Shape the frequency balance of an audio file with three filters: a low shelf, a sweepable mid bell, and a high shelf. Each covers a range of ±24 dB.
The three bands
Low shelf, fixed at 120 Hz. Everything below the corner moves together. Raising it adds weight to bass and kick; cutting it removes rumble, air-conditioning hum, and microphone handling noise.
Mid bell, sweepable from 100 Hz to 10 kHz. A peaking filter centred on the frequency you choose, tapering off either side. The 200 Hz to 500 Hz region carries body and, when overdone, boxiness. The 2 kHz to 5 kHz region carries consonants and attack, which is where speech intelligibility lives.
High shelf, fixed at 8 kHz. Cymbals, sibilance, and air. Small moves go a long way, and boosts here raise tape hiss and encoder artifacts along with the detail.
The mid frequency slider is logarithmic, so the lower half of its travel covers 100 Hz to 1 kHz and the upper half covers 1 kHz to 10 kHz. That matches how pitch is heard.
Cut before you boost
Every boost adds energy to the signal. Three bands lifted at once can push the peaks past 0 dBFS and clip on export, and the result also sounds louder, which is easy to mistake for sounding better.
Removing the frequencies you do not want is usually the more reliable direction. Cut first, judge the balance, then raise the level afterwards if the file ends up quiet.
Filter behaviour during playback
Moving a slider while the file plays crossfades the filter coefficients over about 20 milliseconds rather than switching them on the next sample. A hard coefficient switch inside a running filter produces a click, and the crossfade removes it.
Flat is bit-exact
At 0 dB, a shelf and a bell both reduce to a passthrough. The tool detects that case and copies the samples through without running them through the filters at all, so a flat EQ leaves the render identical to the source.