Comb Filtering
Definition
Comb filtering is the frequency response produced when a signal is summed with a delayed copy of itself: a series of evenly spaced peaks and notches resembling the teeth of a comb. With a delay of t seconds, the notches fall at odd multiples of 1/(2t), so a delay of 1 ms places the first notch at 500 Hz and the next ones every 1 kHz (1.5 kHz, 2.5 kHz and so on). The notches are deepest when both copies have the same level. The 3:1 rule for multiple microphones states that the distance between two microphones should be at least three times the distance from each microphone to its source. The leakage then arrives about 9.5 dB below the direct pickup, which makes the comb filtering much less audible.
In the field
When two microphones on the same source sound hollow together, inverting the polarity of one of them is a quick test. Polarity inversion only moves the notches, so when a time offset remains, the remedy is to change the microphone positions or to delay the closer microphone so that the two signals line up.
Related lessons
Sources
- Shure, « How to Avoid Comb Filtering with Podium Mics » : filtrage en peigne entre deux micros ouverts sur une même source · shure.com
- DPA Microphones, « 3:1 rule » : distance entre micros ≥ 3 fois la distance micro-source · dpamicrophones.com
- AudioTechnology, « Microphones: Comb Filtering 3 » : écart d'environ 9,5 dB obtenu par la règle du 3:1 · audiotechnology.com
- Comb filter (Wikipedia) : creux aux multiples impairs de 1/(2t) · en.wikipedia.org