ACOUSTICS - BACKGROUND INFO
Comb Filtering
The filtering function that arises when a signal is added to itself after having been delayed in time is called a comb filter. The resulting frequency response resembles a comb, hence the name.
Two 500 Hz sinusoidal tones added. The second tone is delayed 1 ms hence the sum is zero
Two 1 kHz sinusoidal tones added. The second tone is delayed 1 ms hence the sum is the double (+6 dB).
The comb filter function is almost never intentional, but it is heard all the time in sound productions, where it can arise both acoustically and electrically.
Acoustically, it typically occurs when the sound on its way from source to recipient takes in part a direct path and in part an indirect path via a single reflective surface. The reflection must be attenuated at least 10 dB and preferably 15 dB in order for it not to have an effect on the sound field at the recipient position. Electrically, the phenomenon arises when two microphones with a certain distance between them capture the same signal and the level from each microphone is of the same order of magnitude.
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