1) A transparent material that is over a light that can affect the intensity, color, and focus of the light. 2) Circuit that selectively attenuates portions of the audio frequency spectrum. A filter is the opposite of the traditional equalizer, which selectively boosts, but for the purpose of modern convenient control of sound on mixes and equalizer units, the circuits of tone-altering controls usually incorporate the dual abilities to equalize and filter by simply rotating a knob one way or another.