A wah is a filter: it boosts a narrow band of frequencies and lets the rest fall away. The pedal moves that band up and down. Heel down, the band sits low and the sound is dark and throaty. Toe down, it sits high and the sound is bright and cutting. Sweep between the two and the guitar seems to say "wah".
Rocking the wah
The classic way is to rock the pedal in rhythm. Try heel down on the beat and toe down on the off-beat while you strum muted sixteenth-note funk, or slow, wide sweeps under a bluesy lead. Jimi Hendrix's wah playing is the textbook reference, and the Dunlop Cry Baby and Vox wahs are the best-known models.
Two habits help: move your whole foot smoothly, not in jerky stabs, and don't feel you must sweep the full range every time. Short sweeps in the middle often sound more musical.
Cocked wah
Leave the pedal still, somewhere around halfway, and you get a fixed filter: a nasal, honky, vocal tone that cuts through without moving. Mark Knopfler's tone on "Money for Nothing" is often described as a "cocked wah" sound, though accounts of how it was made differ. Try it for leads: rock the pedal slowly until a solo jumps out of the mix, then stop there.
Auto-wah and envelope filter
An envelope filter moves the filter for you, based on how hard you play. Pick softly and it stays dark; dig in and it sweeps up with a squelchy "quack". It's a funk staple, with the Mu-Tron III the vintage name everyone mentions.
Typical knobs:
- Sensitivity (or Sens, Range): how much your picking moves the filter. Start at noon and adjust until hard notes quack and soft ones don't.
- Q (or Resonance, Peak): how narrow and vocal the filter is. Higher is more extreme.
- Up / Down: the sweep direction.
Some auto-wahs sweep on a timed cycle instead, more like a modulation effect.
Where it goes
Most players put wah and envelope filters early, right after the tuner and before the drives. There, the drive amplifies the wah's sweep for a classic rock sound. Wah after a drive sounds more extreme and synth-like; try both. An envelope filter almost always works best before drive, since distortion flattens your dynamics and the filter has nothing left to react to.