Comb filter - A distortion produced by combining an acoustical (or electrical) signal with a delayed replica of itself (offset in time). The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response.
In acoustics, comb filtering can arise in some unwanted ways. For instance, when two loudspeakers are playing the same signal at different distances from the listener, there is a comb filtering effect on the signal. In any enclosed space, listeners hear a mixture of direct sound and reflected sound.