Comb filtering is the result of two or more identical signals interacting at a slight
delay. Each signal contains many frequencies, each frequency with a different wavelength.
When the signals are aligned in time every frequency lines up perfectly and the two sounds
add together to create a louder sound. When the signals are slightly shifted in time some
frequencies add together and some cancel out. The resulting graph resembles a comb. Hence the name,
comb filter. In this video you'll hear for yourself what a comb filter sounds like,
you'll learn the common causes of comb filtering, and how to eliminate comb
filtering. But if you're new to this channel, my name is Kyle. Welcome to Audio University.
You've certainly heard the effects of comb filtering in the past.
Maybe you didn't notice it or maybe you didn't know what to call it, but severe comb filtering
can ruin the quality of a recording so it's really important to learn to identify it by listening.
To learn what comb filtering sounds like, let's start with a very extreme example. In addition
to the fundamental frequency of my voice, my voice is also made up of many other frequencies.
So what does comb filtering sound like? If I copy this recording of my voice to another track,
the two copies will work together and the overall level will increase by six decibels.
So what does comb filtering sound like? However, listen to what happens when I delay one copy.
So what does comb filtering sound like? Due to the fact that the two copies of the signal
are equal in level, this example is very extreme. The closer in level the two signals are, the more
extreme the interference will be. If I turn down the delayed copy of the signal by six decibels,
the comb filter effect becomes much more subtle. So what does comb filtering sound like?
As I said before, comb filtering occurs when two or more identical signals interact at a slight
delay. In this section, I want to go through the four most common scenarios in which comb filtering
occurs in the real world. The first cause of comb filtering is latency or delay caused by digital
signal processing. All digital systems add some bit of latency to the signal and this could be a
big problem. Imagine a situation where you want to add parallel compression to the snare drum.
In a parallel compression setup, there are two copies of the original signal.
The dry snare signal passes through from input to output with no additional processing.
The wet snare signal passes from the input to a compressor and then mixes with the dry signal at
the output. Because the wet signal is processed by the compressor and the dry signal isn't the
dry signal could reach the output before the wet signal, which would result in comb
filtering. Fortunately, most modern digital audio systems have delay compensation which ensures a
fixed latency across all audio channels. In an effort to keep all signals aligned,
the system will intentionally delay each signal so that the latency is constant across the board.
Another cause of comb filtering is reflections.
Right now you're mostly hearing the direct sound of my voice to the microphone.
However, when I bring a barrier closer there are now two pathways that my voice can take
one directly to the microphone and another from my voice, to the barrier, to the microphone. Now of
course the distance that the reflected path takes is longer than the distance that the direct sound
takes, meaning that there are two copies of my voice reaching the microphone, one at a slight
delay. If you listen closely you can hear that the frequencies affected most by the comb filter
shift depending on the distance away from the barrier. This problem can occur any time that
a sound takes multiple pathways from the source to our ear or from the source to a microphone.
For example, the sound from a guitar amp cabinet takes a direct path from
the speaker to the microphone and also takes a reflected path from the speaker, to the floor,
to the microphone. The sound from your speakers might be perfectly accurate at the source but by
the time it reaches your ear, it could be wildly inaccurate. Not only do you need to consider the
sound reflecting off of the walls, but also the desk, the ceiling, and any other nearby sources.
To avoid comb filtering from reflections it's important to remember what I said earlier.
The closer in level the two copies of the signal are, the more extreme the interference between
those signals will be. One way we can widen the difference in level between the two signals
is to add distance to the reflected path. The further the reflected sound has to travel,
the quieter it will be by the time it reaches the listener. One of the most effective ways to
improve the sound quality of your speaker setup is to use absorption panels to break up the
early reflections from the speakers. To do this, you'll determine the shortest indirect pathways
from each speaker to the listening position and place absorption panels in those spots.
Comb filtering can also occur when placing multiple microphones on a single sound source.
Right now both of these microphones are about the same distance away from me,
so it takes the sound of my voice about the same time to reach each microphone.
However, if I start to move one mic further away, now the sound of my voice has to travel further to
reach that microphone than it does to the closer mic and so, when these two signals are mixed
together in your headphones, there will be some interference which will result in comb filtering.
The same thing can happen when you place multiple microphones on a single instrument.
Let's take a guitar amp for example. I'll record a short bit of playing.
Now let's take a look at the waveforms from the two mics inside the DAW.
You can see that the sound from one mic is a little behind the other mic.
To fix this, I'll adjust the distance of the mics until the waveforms are aligned.
Now you can see that the waveforms from both mics are more closely aligned.
Another scenario where this might be a problem is when you're recording two nearby sound sources
with separate microphones. The sound from the trombone reaches the trombone microphone but
it also bleeds into the trumpet microphone, and vice versa. Of course, it takes longer for the
sound from the trombone to reach the trumpet mic than it takes for it to reach the trombone mic.
When both of these mics are mixed together in your final mix, you'll hear some bit of comb
filtering. You may have heard of the 3:1 rule in audio. In this situation, the 3:1 rule would
tell us that the distance from the trombone to the nearest mic (in this case the trumpet mic)
should be at least three times the distance of the trombone to the trombone mic. The idea
is that by the time the sound from the trombone reaches the trumpet mic, it will be so quiet that
it won't cause too much interference. Remember - the closer in level the two signals are, the more
extreme the interference will be and therefore the more extreme the comb filtering will be.
The last cause of comb filtering I want to show you is multiple speakers. This one is
also extremely common because we almost always use two or more speakers in a sound system.
In a mixing studio environment, as long as the listener is in the right position
the sound from the left and right speaker will reach the listener at the same time. However,
what happens when the mixing engineer steps over to the equipment rack to make some adjustments?
Now the sound from the left speaker takes a longer path
than the sound from the right speaker. I want to offer you a speaker placement guide
to make sure that you get this right in your listening space. You can download that for free at
audiouniversityonline.com/speaker-placement-guide/. This is even a bigger problem in live sound
reinforcement. You've got a left speaker, a right speaker, and maybe some front fill speakers at
the front of the stage. You can do your best to align all of these speakers, but you can really
only align them at one point in space. Taking one step to the side will change the distance
between the listener and each speaker. The experience can't be the same for every listener
because they're each in a different position. Unfortunately, the best you can do is try to
direct the speakers so that their dispersion patterns overlap as little as necessary.
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Before you go, I want to help you better understand what exactly happens within a comb
filter. To demonstrate this for you, I'm going to use some test signals. The first signal I'll use
is called white noise, which is made up of equal energy per frequency. When you play white noise,
the frequency graph shows a more or less straight line
from the lowest frequencies we can hear to the highest frequencies we can hear. I'm
also going to add another copy of the white noise, but I'll delay it by 5 milliseconds.
When I play those two copies of the white noise together, we'll get a comb filter.
That's because white noise is made up of all frequencies and, based on the amount of delay
between the two copies, certain frequencies will be cancelled and certain frequencies will be
summed together. The second test signal I want to use is called a sine sweep. It sounds like this.
This sine sweep test signal sweeps from 20 hertz
to 20 kilohertz. Again, I'm going to copy this sine sweep and place it on the next
track. I'm also going to delay this by 5 milliseconds. Let's hear what happens.
That was a little fast. Let's slow it down a little bit.
These two signals are the same sine sweep, but they're over 30 seconds rather than just 1 second.
Let's take a closer look at the sign sweep signal to get a better understanding of why
comb filtering is happening. You can see that at the beginning, the peaks
and troughs of both copies of the signal line up.
When we get to our first dip at 1 kilohertz
you can see that the peak and the trough are lined up, which makes a complete cancellation.
Again, the peaks and troughs are more or less lined up,
and then again we get to the point where a peak and a trough are lined up. This pattern continues,
notching a frequency at each octave. You'll notice that because the delay between the
two signals is still five milliseconds, the frequencies that are notched by the
comb filter are the same as we saw with the white noise demonstration. Check this out. So
as we can see when there's 5 milliseconds of delay between the two signals,
the first frequency that is notched is just over 1 kilohertz. Let's see what happens
if we increase the amount of delay to 10 milliseconds.
With 10 milliseconds of delay rather than 5 milliseconds of delay between the two signals,
we can see that the first frequency to be notched by the comb filter has gone down a full octave,
now to just over 500 hertz. With that logic, if we move the delay to about 2.5 milliseconds we
should see that the first notched frequency will be an octave higher than 1 kilohertz. Let's see.
If you enjoyed this video,
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