Auto-wah

The distinctive choppy rhythm guitar sound on many Funk and Disco recordings from the 1970s popularized the effect.

A typical auto-wah circuit uses an envelope detector to produce a voltage representing the overall volume of the input.

There is a variant of auto-wah that utilizes a low-frequency oscillator (LFO) instead of an envelope detector to alter the effect.

The song "Falling Into Grace" by the Red Hot Chili Peppers is an example of an LFO controlled auto-wah applied to the bass guitar.

Grateful Dead guitarist Jerry Garcia is known for extensive use of an envelope filter (particularly the Mu-Tron III), examples being his playing on "Estimated Prophet" and "Shakedown Street".