A comb filter is a filter that has a series of very deep notches in its frequency response with the spacing of all of the notches at multiples of the frequency of the lowest notch (they are all harmonically related). It got its name from looking like a comb when plotted on a frequency response graph. Comb filters are produced when a signal is time delayed and added back to itself. Some frequencies will cancel and others will be reinforced, which can dramatically change the tonal color of the sound. In practice this is common problem that occurs when a stereo mix is collapsed to mono because many stereo effects, such as chorus and flanging, achieve their stereo imaging by using some form of the Haas effect. A static comb filter will make its audio sound kind of hollow or “phasey” depending upon how severe it is. Add some modulation and you have a flanger. Comb filtering is one of the main ingredients in the distinctive sound of a jet aircraft passing overhead. The difference in the time arrival to your ears of the direct sound versus the sound reflected off of the ground causes various frequencies to be cancelled or reinforced. As the plane moves these distances are all changing, thus causing the coloration of the sound to change. Again, it’s the same principle used in a flanger.