An expression effect pedal used primarily with the electric guitar. Originally developed by the British Vox Sound Ltd. company, makers of amplifiers and occasional guitars, in 1967. All Vox products were imported into the United States by the Thomas Organ Company who began manufacturing a wah wah pedal of their own and called it the CryBaby. The famous CryBaby was eventually licensed to the Jim Dunlop company, which sells it to this day.

This effect was first created to allow guitarists to mimic the sound of a muted trumpet. The effect is also reminiscent of a human voice, and can sound even more voicelike if two pedals are connected in series.

A wah wah pedal is essentially a band pass filter with a resonant, variable center frequency. Rocking the pedal sweeps a contact across a potentiometer with a semi-logarithmic taper, which varies the voltage feeding the ground side of a capacitor (czeano sez: "there tends to be a really large store the energy instead of a capacitor.") to make the capacitor "appear" variable to the rest of the circuit.1 This variable capacitance changes the resonant frequency of the circuit, and sweeping the resonant frequency combined with the filtered signal from the guitar makes that nifty "wahhh" sound come out of your amplifier.
NOTE:This is an extremely simplified version of wah wah pedal circuitry. Anyone with a better head for electronics should feel free to correct me or add to this node.

When the pedal is flat (i.e. parallel to the ground), the filter's center and resonant frequencies are relatively high, and the sound produced is thin, and biased towards the treble. Depressing the pedal toward the heel lowers the center frequency of the band pass filter, drops the resonant frequency, and makes the guitar sound more "thick", although most of the bass tone from the guitar is still cut out.

The wah wah pedal was popularized by Jimi Hendrix who, one assumes, became aware of it during his time gigging in the UK. An excellent example of his wah style can be found in the song Voodoo Child (Slight Return), which contains the de rigeur riff for guitarists who've just plugged in a wah for the first time. czeano feels that Eric Clapton's wah work on Cream's White Room may be more familiar and equally instructional. Miles Davis was known to play his trumpet through a wah wah pedal on his later albums, particularly Get Up With It; the effect coming full circle, back to its roots.

GangstaFeelsGood reminds me that there are wah wah pedals made specifically for bass guitars as well. A normal wah wah pedal would cut practically all the frequencies in a bass' lower range, which is rather undesireable. As such, a bass wah will be tuned to affect a different range of frequencies than a guitar wah.

1. TeknoHog elucidates: "A single component (capacitor, resistor or inductor) does not have a characteristic frequency. A combination of them may have one. For a simple example, a low-pass filter made of a resistor and a capacitor has a cutoff frequency around 1/RC. Thus you can change the frequency by merely altering the resistance, which is nice as variable resistors are simpler and cheaper than variable capacitors.
"Similarly, the capacitance in a wah wah pedal is not changing. The resonance frequency is changed simply via the variable resistance."

This writeup appears, in somewhat edited form, at

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