I know what a wah pedal is and what it sounds like, but what does a wah pedal actually do to your sound? Someone told me it's like a mids controller. I don't think that's what it is, though.

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it's an adjustable filter that controls the frequency of your sound via an expression pedal
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Wah accentuates the highs and lows of your sound. So toe down, high, heel down, low. hence you get a "wah" sound.
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A very rudimentary example would be rolling your tone knob back and forth while playing.

Not exactly it, but gives you an idea.