Comb filtering occurs when two or more identical audio signals are mixed together with a slight delay between them. The resulting frequency response graph resembles a comb because frequencies that are in phase sum together, while frequencies that are out of phase cancel.
In this article, you’ll learn what causes of comb filtering and how to avoid it.
Comb filtering occurs when two or more identical audio signals are mixed together with a slight delay between them. The resulting frequency response graph of these signals resembles a comb.
Comb Filtering and Phase Interference
To understand why comb filtering happens, it is helpful to first understand the basics of phase interference. I wrote an article about how sound works that I recommend you check out before continuing. Once you have a good grasp of the basics, watch this video.
Take a look at the these sound waves. The waves are the exact same frequency. They are also in phase, meaning the waveforms go up and down together. Because they are in phase and the same amplitude, they add together and create a sound wave that is double the original size. It’s like 1 + 1 = 2. This is called constructive phase interference.
In this image, the sound waves are perfectly out of phase. This means that when one waveform goes up, the other goes down. The waves are the same size and perfectly out of phase, so the result is a complete cancellation. These two sounds add together to equal nothing. It’s like 1 + -1 = 0. This is called destructive phase interference.
It’s rare that the two waves are ever lined up perfectly in phase or perfectly out of phase. Usually, they are slightly offset as you can see below. In most cases, there is a combination of constructive and destructive interference.
It’s important to recognize the difference between phase and polarity. These terms are often used interchangeably, but they are different! I wrote an article with a video that will help you understand how polarity and phase are different and why that distinction is important.
What does Comb Filtering Look Like?
Comb filtering gets its name from the shape it creates on a frequency response graph.
Read this article I wrote to understand how to read frequency response graphs.
A frequency response graph shows the intensity of each frequency in a signal. The x-axis shows frequency, while the y-axis shows level.
This is a frequency response graph of white noise. A nearly straight line from 20Hz to 20kHz is created on the graph, because the level of each frequency is equal in white noise.
The following is a frequency response graph of the same white noise, but duplicated. The duplicated white noise has been shifted in time by 1ms. The line on this graph resembles a comb.
The comb shape forms because some frequencies are perfectly in phase, some frequencies are partially in phase, and some frequencies are perfectly out of phase.
The highest points of the comb are frequencies that are perfectly in phase and sum together.
The lowest points of the comb are frequencies that are perfectly out of phase and completely cancel.
The frequencies in between are partially out of phase.
What Does Comb Filtering Sound Like?
Comb filtering, or phasing, sounds different depending on which frequencies are being canceled. In this section, I’ll provide you with an audio example so that you can hear it for yourself.
Here is an audio clip. First, you’ll hear two copies of the voice perfectly time aligned, and in phase. Then you’ll hear what happens when one copy of the voice is shifted in time, or delayed.
You can hear the phasing that occurs when the signal is delayed by 10ms and 20ms. However, 50ms of delay doesn’t have the same effect. When the signal is delayed by 50ms or more, it begins to sound like an echo.
Time delays from less than 1ms to about 25ms create comb filter effects. Time delays beyond that range create an echo effect. Therefore, for the purposes of discussing comb filtering, we will be working with delay times in the 0ms to 25ms range.
What Causes Comb Filtering & How to Avoid It
You’ve already learned that comb filtering occurs when two or more copies of a signal are mixed together. This section will explain and demonstrate the real-world situations that cause comb filtering. There are three main categories: reflections, multiple speakers, and multiple microphones.
Comb Filtering Caused By Reflections
Any time a sound is created, it radiates outward from the source and bounces off of the surfaces in the room. When a sound takes multiple routes to a microphone, the delay caused by longer routes causes some frequencies to be out of phase.
Imagine you are recording a snare drum. When the drummer hits the drum, the sound travels in a straight line from the drum to the microphone. However, this isn’t the only path the sound takes. The sound of the drum also reflects off of the walls and then to the microphone.
The reflected sound travels a further distance than the direct sound, and therefore arrives at the microphone later. Both signals are the same, but one signal is delayed by a few milliseconds. This causes a comb filter, where some frequencies are cancelled and some are summed.
Tabletops, Floors, & Ceilings
The most obvious example of reflections is the wall example above, but don’t forget about reflections off of tabletops, floors, and ceilings.
If you are recording a podcast at a desk, the sound of your voice will not only travel directly to the microphone, but will also reflect off of the table and arrive at the microphone slightly later. This is also a problem when recording guitar amplifiers on the floor.
It isn’t just the boundaries of your room that can cause reflections. Any surface near the sound source or the microphone can create comb filtering in your recording.
For example, musicians might place music stands in front of themselves for sheet music. A voice actor might be reading off of a script. These surfaces will create another pathway for the audio signal to travel, and this might have detrimental effects.
How to Avoid Comb Filtering from Reflections
Here are a few tips to help you improve the sound quality recorded in a reflective recording space.
Critical Distance (Direct vs Reflected Sound)
Consider that the force of a soundwave decreases as it travels over distance. You can use this to your advantage. Try to place the microphone as close to the sound source as you can so that the direct sound level is significantly louder than the level of the reflected sounds.
Diffusion & Absorption
The sounds we hear everyday are a mix of direct sound and reflected sounds. If you completely remove the reflections, it might sound unnatural. Focus on absorbing the early reflections, or the first reflections that reach the listener or microphone.
Then try to break up, or diffuse reflections. A diffusive surface reflects sound in many directions rather than focusing all of the energy into one particular direction.
There are many ways to diffuse and absorb reflections. I wrote a whole article here on the Audio University website about the best ways to treat a room for better acoustics. In that article, you’ll learn how to use bookshelves and other things you already own to break up reflections.
Comb Filtering Caused By Multiple Speakers
Any time you send the same signal to multiple speakers, you run the risk of causing comb filtering.
Multiple Speaker Examples
In professional audio, we do this a lot. Here are a few examples of situations where comb filtering is caused by multiple speakers producing the same signal.
Stereo is a very popular format for mixing music and sound for video. However, there are some problems caused by this format in practical applications.
In an ideal mixing environment, the speakers are equally spaced from the listening position. This equidistant spacing of the speakers allows the sound from each speaker to meet your ears simultaneously. If you set up your studio in this way, all frequencies will be perfectly in phase at your listening position, and no comb filtering will occur.
However, the consumers listening to your track will almost never be in such a controlled environment. It’s guaranteed that most listeners will be sitting closer to one speaker than the other, and thus comb filtering will occur.
If you send the snare drum to both speakers evenly, it will sound like it’s directly in the center in a properly spaced stereo listening environment. When the consumer listens to this track in their car, the snare signal from the speaker closest to them will arrive at their ears sooner than the snare signal from the other speaker, causing some frequencies to cancel and others to sum together.
Live Sound PA Systems for Large Audiences
The stereo problem above is also present in live sound applications. However, there is another problem presented in live sound. That is the problem of time-aligning speakers for large audience areas.
In live sound, multiple speaker systems are required for some venues. Often there is a main array of speakers at the stage and additional speakers beneath balconies. The sound from the main speakers will take time to reach the balcony speakers.
In an attempt to align these speakers in time and prevent comb filtering, live sound system technicians delay the signal going to the balcony speakers.
How to Avoid Comb Filtering from Multiple Speakers
Here are some techniques you can use to reduce the comb filtering effect caused by multiple speakers.
In live sound, technicians attempt to synchronize sounds from all speakers using different delay times for each line array. This is very effective. However, time alignment between speakers can only ever be perfect in one spot.
If you are standing right between the left and right speaker, you will be in the sweet spot where sounds are perfectly in phase. But if you move even one step to the left, the signal from the left speaker will reach your ears slightly before sounds from the right speaker.
It can never be perfect for everyone. Although this is a sad truth, it is a fact of life. Sound system technicians do what they can to tune systems to reduce comb filtering as much as possible, but these issues will always exist with multiple sound sources.
If the goal is to cover a large area of space, multiple speakers will be required.
When using multiple speakers, be aware of their polar pattern. Place speakers in a way that allows the least possible overlap while maintaining adequate coverage.
Comb Filtering Caused By Multiple Microphones
Another cause of comb filtering is the use of multiple microphones near the same sound source.
Multiple Microphone Examples
Stereo microphone techniques are a great way to add realism to your recordings. When recording orchestral music, stereo microphone techniques are very commonly used. Spaced pairs of microphones are one of the most common techniques.
This technique creates a recording with a wide stereo image that provides a very immersive listening experience. If not done with care, however, this technique can cause serious phase issues which result in comb filtering.
Comb filtering can also be caused when recording drums. Usually drum kits are recorded using several microphones.
Each microphone is located in a different point in space. This means that the sound of any given drum will arrive at each microphone at slightly different times, causing comb filter effects.
Another example of when multiple microphones can cause comb filtering is panel discussions.
This scenario usually consists of multiple people speaking at board meetings or on stage with several microphones placed in front of them.
Even though each person has a microphone, their voices will also be captured by their neighbors’ microphones and then mixed together in the speaker. This can cause phase cancellations.
How to Avoid Comb Filtering from Multiple Microphones
There are a few methods you can use to reduce the comb filtering caused by multiple microphones.
The 3:1 Rule
The 3:1 rule can help ensure that you don’t create significant phase cancelation in your recordings when using multiple microphones.
If you have a microphone that is one meter away from a sound source, the 3:1 rule states that there should be at least three meters of space between any nearby microphones.
This allows enough space for sound to be reduced to a level that won’t cause significant phase cancelation.
If you cannot use separation in space to reduce the level of sound leakage between microphones, you can try using gain automation.
For example, if you are mixing a panel discussion with multiple microphones, you can ride the fader for each microphone to raise the level when someone is speaking into the microphone and decrease it when the microphone is not in use.
This will reduce the comb filtering effect of using multiple microphones. However, this will not be effective if both microphones are in use simultaneously.
Putting Theory Into Practice
The truth is that comb filtering is unavoidable in a 3D space. Comb filtering and phase cancellation are facts of life. All we can do as audio engineers is try our best to learn what causes comb filtering so that we can minimize its effects.