The wah was originally intended to mimic the sound of the human voice but obviously falls a little short. A formant filter is an extension of the wah principle but operates on the concept of formants. Formants are resonances in instruments, cabinets, and, in this case, the human vocal tract that give sounds their character. The human vocal tract generates a handful of formants that produce the vowel sounds we recognize. For example the vowel sound "eee" can be reproduced with a bank of narrow bandpass filters with various frequencies and amplitudes. The Axe-Fx Formant Filter allows "talk-box" effects without the fuss and muss of the real thing.
The Format Filter morphs between three vowel sounds: start, mid and end. The CTRL knob then controls the morphing between these sounds. For example, we can program the Formant Filter to go "ooo - eee - aaahh" as a pedal is moved. The start vowel is generated with the CTRL knob counter-clockwise, the end vowel is generated with the knob fully clockwise and the mid vowel is generated with the knob at 12:00. The sound gradually changes from one vowel to the next between positions. The Formant Filter usually sounds best when placed after distortion although there are no hard and fast rules.
- START - Sets the start vowel sound.
- MID - Sets the mid vowel sound.
- END - Sets the end vowel sound.
- RES - Sets the resonance of the filters. Higher resonance can yield a more dramatic effect.
- CTRL - Controls the morphing between vowel sounds. Normally you would connect this to a controller (i.e. EXTERN1).