Room+Reflections

Comb filtering: a series of many peaks and nulls (looks like a comb) Flanger - artificial comb filtering Also used to create pseudo-stereo sound from a mono source

If you have a microphone and a speaker (which is playing what the mic picks up), and you move the microphone closer to a wall, the frequencies it is picking up will be hitting the mic twice and slightly out of sync with each other, so certain frequencies might be either doubled or eliminated. When the mic gets closer to the wall, it seems to be higher-pitched, which is the high frequencies being amplified.

You can place the mic in a spot where the frequencies being doubled sound nice with the music being played. You usually want to avoid comb filtering by absorbing some of the sound.