Auto-wah

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Auto-wah (also known as an "envelope following filter" or just "envelope filter") is a type of wah-wah effects pedal typically used with electric guitar, bass guitar, clavinet, and electric piano etc.

The distinctive choppy rhythm guitar sound on many funk and disco recordings from the 1970s popularised the effect.

Instead of the effect being controlled by a foot pedal, as on a standard wah-wah, the effect alters in response to the volume of the input signal. Since the electronic circuits in the effect can respond much faster than a human musician can physically move a foot pedal, certain effects that a standard wah cannot achieve are only possible with auto-wah.

A typical auto-wah circuit uses an envelope follower to produce a voltage representing the overall volume of the input. This signal is then used to sweep the cutoff frequency of a filter. The filter usually has a lowpass or bandpass response.

Typical controls on an auto-wah include a sensitivity control to adjust the input level to match the level expected by the envelope follower and other circuitry, a control for the initial cutoff point of the filter, and a control for the depth of the filter sweep. Some more sophisticated units offer controls for the resonance of the filter, multiple filter types, and options for sweeping the filter up or down.

Commercial examples include the MXR Envelope Filter, the Musitronics Mutron III and the Boss AW-2 Auto Wah.

Examples of the use of this effect include the songs:

"Industrial Disease" by Dire Straits; "What I Am" by Edie Brickell & New Bohemians; and "Estimated Prophet" by The Grateful Dead.