treadle

On a wah, the treadle controls the center frequency of a filter band, boosting frequencies inside the band and attenuating those outside it¹, and rapidly sweeping it generates a "waka" sound common in funk². Treadles also appear on other effects, such as phasers, where a switch can select treadle control of either phaser speed or manual phase position³, and fuzz-wahs, where the treadle's top plate controls the fuzz sound⁴. Treadle designs vary, with some offering a longer sweep and more tapered filter curve than others⁵.