Whether implemented in its classic pedal form or created by an analog or digital synthesizer or effects processor, a wah is simply a bandpass filter or a lowpass filter that exhibits a resonant peak just at its lowpass rolloff frequency. The resonant peak can be moved up and down in frequency by the player, often by means of a foot pedal, which makes for a striking emulation of the human voice making a “waaaah” tone (or its tonal inverse, “aaaooow”).
Several kinds of circuits have been used through the years to produce the wah sound. One of the originals was the Vox-style inductor-based wah circuit. Other common wah circuits employed notch filters or op-amp active filters. In electronic instruments, wah-wah effects are easy to produce by applying a modulation envelope to the voltage-controlled filter in an analog synthesizer. Digital synthesizers can also simulate this effect.
