Wah & Envelope Filter
A wah pedal is just a bandpass filter being swept. Three sweep sources: manual, LFO, envelope follower.
The Bandpass Sweep
A narrow bandpass filter emphasizes one frequency region while attenuating everything above and below. Move that region up and down and you get the classic "wah" sound. The narrower the band (higher Q), the more dramatic the sweep.
Three Ways to Sweep
Manual: you control the filter position directly with a slider, like a rocker pedal. LFO: a low-frequency oscillator moves the filter automatically on a repeating cycle. This is "auto-wah" in the most literal sense. Envelope follower: the louder you play, the higher the filter sweeps. Quiet notes stay dark, loud notes open up bright.
The Quack
High resonance on the bandpass creates the distinctive vocal "quack" quality. At extreme Q values the filter rings at its center frequency, adding a pitched quality to the sweep. Back off the resonance for a subtler, smoother wah.
Try it: start with "Classic Wah" and sweep the Position slider by hand. Then switch to LFO mode for an automatic cycle. Try "Auto-Wah" and play keys at different velocities to hear the envelope follower respond.