Comb filtering causes constructive and destructive interference that can ruin the frequency response of sound. It occurs when a wave combines with a delayed version of itself. This results in constructive and destructive interference.
Comb filtering is a common issue and important consideration when recording music with multiple microphones. When sound reaches one microphone first, then travels further to the next microphone there is a slight time delay between the two instances of sound captured and this causes some phase interactions at the various frequencies of the sound.
A wave delayed by 180 degrees results in the fundamental frequency of cancellation.
Odd harmonics of the fundamental frequency of cancellation will cancel
Even harmonics of the fundamental frequency of cancellation will reinforce
A wave delayed by 180 degrees (half a wavelength) results in the fundamental frequency of cancellation. Frequencies delayed by half wavelengths will cancel, while those delayed by whole wavelengths will reinforce.
Figure 10: Flat frequency
Figure 12: Comb filter response on a linear frequency graph
Figure 13: A delayed version of pink noise interacting with the original version causing strong comb filtering peaks and nulls.
Common causes of comb filtering:
- Two microphones at different distances from the same source.
- Reflective nearby surface causing a reflected sound to arrive at the microphone slightly after the direct sound.
- Two identical sounds in a DAW with a 1 to 10ms delay
- The interference tube on shotgun microphones causes comb filtering to attenuate sounds from the side in order to provide a more focused pickup response from the front of the microphone.
How to calculate the fundamental frequency of cancellation of a comb filter:
Find the longest wavelength that can occur between two points (a 2m room will have a 4m wavelength), from that find the frequency of the full wavelength. This is your fundamental frequency of cancellation.
If one mic is positioned 2 metres from a sound source and another mic is 3.5 metres from a sound source, what will be the fundamental frequency of cancellation if they are combined?