Comb filter calculator
Two mics on one voice, or a direct sound plus a reflection: enter the time or distance difference and see which frequencies cancel.
Time offset
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First notch
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Notch spacing
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Peak-to-notch depth
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Notches up to 20 kHz:
How it works
notches at (2k + 1) ÷ (2 × delay) peaks at k ÷ delay
depth = 20 × log10((1 + a) ÷ (1 − a)), a = 10−difference/20
depth = 20 × log10((1 + a) ÷ (1 − a)), a = 10−difference/20
When a signal is added to a delayed copy of itself, frequencies whose half-period equals the delay cancel and frequencies whose period equals it double. On a frequency plot the pattern looks like the teeth of a comb. A 1 ms offset puts the first notch at 500 Hz, the next at 1.5 kHz, and so on every 1 kHz.
If the second arrival is quieter, the notches get shallower: 6 dB down still leaves about a 9.5 dB dip; 10 dB down, under 6 dB.
Field notes
- Two mics, one source. Two goosenecks on a podium, or a lav plus a handheld on the same pastor, comb filter whenever both are open. Use one, or an automixer that gates the other.
- Close beats equal. Making the second mic much further away (so it's quieter) shrinks the notches. That's the idea behind the "3 to 1" spacing rule for multiple mics: keep other mics at least three times as far from a source as its own mic.
- Tiny offsets hurt the highs; big ones hurt everything. Half an inch of difference notches around 13 kHz. A few feet puts notches all through the vocal range.
- Time-align instead of fighting it. For mics or speakers that must both be on, delaying the earlier one to match the later one moves the first notch out of the way. See the delay calculator.