The Comb Filter is a distortion effect created when an audio signal is summed with a slightly delayed copy of itself.

When the direct sound and the delayed sound mix, some frequencies add up (becoming louder) and others cancel each other out (becoming quieter), depending on how the waves align.

This interaction creates a series of very narrow and evenly spaced peaks and valleys across the frequency spectrum. When viewed on a spectrum analyzer, this pattern looks like the teeth of a comb, hence the name. The resulting sound is a "hollow," "phased," and metallic timbre, similar to that of a phaser or flanger effect. Although it can be used creatively, the comb filter is generally an unwanted problem that occurs in situations such as: Reflections: When a microphone picks up the direct sound from a source and, milliseconds later, the reflection of that sound off a nearby wall or surface. Bleed: When an instrument is picked up by two microphones at different distances, and the signals are summed.