Reflections from nearby surfaces cause comb filtering.
In film sound, it can be a problem to record the dialog of people talking facing a window, whether they be in a room, car or other setting. In addition, loudspeaker cabinets and amplifiers can generate comb-filtering challenges.Ĭhoosing directional microphones can help, but you should always prioritize better positioning. Even the musical instruments, such as a piano, a cello, a double bass, a kick drum, etc., that you are recording may act as reflectors. When making a recording setup, it is important to look for reflecting surfaces near the microphone. Sound is reflected when it hits hard surfaces, like a floor, a wall, a window - or even when the sound hits a water surface. This filter function could be the result of a single reflection picked up by the microphone or it could be the result of adding the signals of two microphones of which one is 0.6 m (1.9 ft) further apart compared to the other. This curve shows the resulting filter response of a direct sound and a delayed sound when added. This phenomenom is called comb filtering because the effect acts as a filter with a frequency response looking like a comb for the hair. In addition, in order for comb filtering to occur, the levels of the signals must be within 10 dB from each other. Because more than one microphone is open and picking up the same signal at different positions.This interval typically ranges from less than one ms to approximately 25 ms.
Comb filtering occurs when a sound adds to itself within a short time interval.
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