Shape the frequency balance of an audio file with three filters: a low shelf, a mid bell, and a high shelf. Each gain covers ±18 dB, and each band sets its own frequency.
The three bands
Low shelf, 30 to 250 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. Where the corner sits decides which of those you get: 50 Hz reaches only the sub range, while 250 Hz takes the bottom of a voice with it.
Mid bell, 100 Hz to 10 kHz with adjustable width. 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. Width runs from Q 0.4, wide enough to act as a tone tilt, to Q 8, narrow enough to notch out a single ringing frequency.
High shelf, 2.5 to 16 kHz. Cymbals, sibilance, and air. Small moves go a long way, and boosts here raise tape hiss and encoder artifacts along with the detail. Moving the corner up past 10 kHz keeps sibilance out of a treble lift, since most of it sits between 5 and 9 kHz.
All three frequency sliders are logarithmic, so equal travel covers equal musical distance rather than equal hertz. On the mid, the lower half of the slider covers 100 Hz to 1 kHz and the upper half covers 1 kHz to 10 kHz.
Boosting what is not there
A shelf raises the content already in the file. If a recording rolls off below 60 Hz, a low shelf at 50 Hz finds nothing to lift and only raises noise. Generating the missing octave is a different operation, and the bass booster is the tool for it.
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. Moving a frequency or the width while the gains sit at 0 dB stays on that same path, because a band at 0 dB is a passthrough wherever it is placed.