Comb Filter Frequency Calculator
When a signal combines with a delayed copy of itself, such as direct sound plus a reflection or two microphones on one source, they reinforce at some frequencies and cancel at others. The result is comb filtering: a regular pattern of peaks and notches across the spectrum. This calculator finds the first cancellation notch, the notch spacing, and the first reinforcement peak from either a delay in milliseconds or a path-length difference and the speed of sound.
Comb filter formula
delay (s) = path difference / speed of sound
first notch = 1 / (2 * delay)
notch spacing = 1 / delay
first peak = 1 / delay
Notches fall at odd multiples of 1 / (2 delay), where the delayed copy is half a cycle out of phase. Peaks fall at whole multiples of 1 / delay, where it is back in phase. A longer delay packs the comb teeth closer together.
Comb filtering facts
- A longer delay moves the first notch lower and spaces notches more tightly.
- The first notch is always at half the notch-spacing frequency.
- The microphone 3-to-1 rule keeps the delayed copy quiet enough to limit combing.
- Absorbing reflections at the listening position reduces comb filtering.
- Very long delays are heard as a discrete echo rather than a tonal comb.
Comb filtering: frequently asked questions
What is comb filtering?
Comb filtering happens when a signal combines with a delayed copy of itself, for example direct sound plus a reflection. At some frequencies they reinforce and at others they cancel, producing a regular series of peaks and notches across the spectrum that looks like a comb.
Where is the first cancellation notch?
The first notch sits at half the inverse of the delay: frequency = 1 / (2 times delay). For a 1 millisecond delay (0.001 s) the first notch is at 500 Hz. Notches then repeat every 1 / delay hertz.
Where are the reinforcement peaks?
Peaks occur where the delay equals a whole number of wavelengths, at frequencies that are whole-number multiples of 1 / delay. The first peak is at 1 / delay hertz, midway in frequency between consecutive notches.
How does path-length difference relate to delay?
If the two signals travel different distances, the delay equals the path difference divided by the speed of sound. Enter the path difference in metres and the speed of sound, and the calculator converts it to a delay before finding the comb frequencies.
How do I reduce comb filtering?
Remove or attenuate the delayed copy: absorb the reflection, move microphones to follow the 3-to-1 rule, or time-align and pan sources so identical signals do not overlap with small delays. Larger delays push the first notch lower and may be heard as discrete echo instead.
Official sources
- National Institute of Standards and Technology: SI units and frequency.
- Audio Engineering Society: audio interference references.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.