
Wah Wah effect works by modulating the peak frequency of a resonant filter. Wah Wah effect is ussualy a pedal that allows guitar player to manualy control the peak frequency of the effect. This pedal was made famous by Jimmy Hendrix, and a lot of modern musicians use it, for instance Kirk Hammet from Metallica. Wah wah pedals are not expensive, and
you can get one cheap at Ebay.