Although perhaps best known from the electric guitar's wah-wah pedal, the sound is much older, having been significantly developed by trumpet players using mutess in the early days of jazz.
Joe "King" Oliver recorded "Wawawa" in the 20s and Bubber Miley of the Duke Ellington Orchestra was another innovator. The effect was used in the 30s on "Sugar Blues" by commercial Dixieland trumpter Clyde McCoy, who built a long career around the sound. "The Fat Man", the first hit by Fats Domino features vocal wah-wah.
The electronic version, which sweeps the peak response of a filter up and down in frequency to create the sound, was first heard in 1945 on a pedal steel guitar created by Leo Fender and in the early 60s on Vox amplifiers (under the name wah-wah) and Thomas electronic organs (as the Crybaby). None of these innovators patented the effect. The variation in the peak response frequency of the filter resembles the change in formant frequency in the human vocal tract when saying the word "wah", making the wah-wah pedal a crude form of speech synthesizer.
Jimi Hendrix did much to popularize the wah-wah in the late 60s using his own modified effects pedal, as heard on his "Machine Gun" and the Electric Ladyland album (the track "Voodoo Child (Slight Return)" in particular). There is still a commercial wah-wah pedal named after him.
Eric Clapton first played wah-wah with Cream on "Tales of Brave Ulysses" on the Disraeli Gears album and used it for both background riffs and an extended solo on "White Room". (Clapton sometimes used a wah-wah named for Clyde McCoy, of all people).
George Harrison recorded his own song "Wah-Wah" on his solo album, All Things Must Pass, on which Clapton also appeared.
Melvin "Wah-Wah Watson" Ragin played wah-wah on some notable singles by The Temptations in the early 70s, as well as with Martha Reeves and the Pointer Sisters. Hendrix acclaimed blues guitarist Earl Hooker the "master of the wah-wah".
Other notable guitarists using wah-wah include Robert Fripp, Larry Coryell, Anson Funderburgh, and Carlos Santana. A great many guitarists use it from time to time.
In electronic music, 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.
Wah-wah effects can also be achieved by using a vocoder to modulate an instrument sound, and speaking "wah-wah" into the modulation control input of the vocoder. The vocoder then impresses the formants of the spoken sound into the musical sound.Wah-wah in trumpet playing
Wah-wah in guitar playing
Wah-wah in electronic music