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Comb filtering is a phenomenon that occurs when the same signal arrives at the listener’s ears (or a microphone) at different times with a very small delay between the signals. The delayed signal can be created acoustically with sound reflections from hard surfaces, such as walls or glass panes, or electronically by using delays or latency. The superposition of delayed sounds can lead to certain cancellations and amplifications in the frequency spectrum that look like a comb

Physical Background

Phase Interference

To understand the comb filter effect, it is useful to define phase interference, and more specifically, destructive phase interference. When combining two waves with the same phases, the resulting wave has a greater amplitude (constructive phase interference), whereas two signals, which are 180° out of phase, result in a complete cancellation (destructive phase interference).

The superposition of a delayed version of the original audio signal onto itself leads to phase interference and thus an amplification or attenuation of certain frequencies creating comb a filter effect.

Frequency Response

The resulting frequency response curve of a comb filter consists of a series of regularly spaced notches.

The frequencies of attenuation (notch frequencies fn) can be calculated with:
f_{notch,n} = c_{0}\frac{2n-1}{2ds}

c0: speed of sound in the air (c0 = 343 m/s)
ds: difference of path lengths between direct sound (sd) and reflected sound (sr)

Sound Pressure Calculations

To generate comb filtering, the sound pressure levels of the original signal and the delayed signal need to differ by no more than 10 dB. The resulting sound pressure can be calculated as:

p = p_d + p_r = \frac{p_0}{s_d} exp\biggl\lgroup-j\frac{2\pi f s_d}{c_0}\biggr\rgroup + \frac{p_0}{s_r} r exp \biggl\lgroup-j\frac{2 \pi f s_r}{c_0}\biggr\rgroup

p: total sound pressure
pd: direct sound pressure
pr: reflected sound pressure
p0: reference sound pressurep0 = 20 μPa = 2 × 10−5 Pa
r: reflexion factor
sd: direct sound path
sr: reflected sound path
f: sound frequency
c0: speed of sound in the air (c0 = 343 m/s)

Critical Time Window

The form and strength of the comb filter effect depends on the length of time between the original and the delayed sounds. The typical time delay for comb filtering ranges from less than one ms to approximately 25 ms. For delays greater than 35 ms, the human ear manages to separate the two sound events and the comb filter effect becomes weakerOccurrence and Reduction of Comb Filtering

Occurrence and Reduction of Comb Filtering

Comb Filtering caused by Sound Reflections

Sound is reflected when it hits hard surfaces, like a floor, a wall, a window - or even when the sound hits a water surface. The original sound and the reflected sound are identical, but the reflected sound is delayed by a few milliseconds, causing a comb filter.

To avoid comb filtering caused by reflections, reflecting surfaces should be removed or turned. Further, as sound wave energy decreases rapidly over distance, placing microphones as close to the sound source as possible may reduce comb filtering. This way, direct sound level will be significantly louder than the one of the reflected sounds. The other effective method is to focus on absorbing the early reflections (or first reflections) that reach the listener or microphone. Alternatively, diffusing first reflections by scattering them in many different directions will clearly attenuate the amount of energy coming back to the listener or microphone.

Comb Filtering caused by Multiple Loudspeakers

Comb filtering can occur when two or more loudspeakers play an identical signal and the listening/recording does not take place precisely at the defined listening/microphone position causing a delayed arrival of sound.
For live sound applications in large venues, sound technicians attempt to synchronize sounds from all loudspeakers using different delay times for each line array loudspeaker system to avoid comb filter effects. Another solution is to place each loudspeaker in such a way that there is the least possible overlap between their coverage.
Illustration of comb filtering due to multiple loudspeakers.

Comb Filtering caused by Multiple Microphones

Stereo microphone techniques are a great way to add realism to a recording. However, locating multiple microphones at different positions can cause delays in arrival time at each microphone leading to comb filter effects.
To avoid comb filtering, a spatial separation or increase of distance between nearby microphones may help.


Sources and references

    1. Fuchs, H., “Schallabsorber und Schalldämpfer“, 3. Aufl. Springer Berlin, Heidelberg, 2010, ISBN: 3642014127
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