Wah-wah (music)
Wah-wah is an imitative word (or onomatopoeia) for the sound of altering the resonance of musical notes to extend expressiveness, sounding much like a human voice saying the syllable wah. The wah-wah effect is a spectral glide, a "modification of the vowel quality of a tone" (Erickson 1975, p. 72), also known as a band pass filter. Although this effect is thought of almost exclusively as the electric guitar wah-wah pedal, it is also used in other contexts, listed here.
Read more about Wah-wah (music): Wah-wah in Trumpet and Trombone Playing, Wah-wah in Electronic Music, Source