In this video, you'll learn about comb filtering. I'll explain how it works and
I'll give you practical examples so that you can see and hear it for yourself.
But if you're new here, welcome. My name is Kyle and I've created
Audio University to help you learn the fundamental concepts of audio and how
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Comb filtering occurs when two or more identical audio signals are
mixed together at a slight delay. The resulting frequency graph of
these signals resembles a comb. Hence the name, "comb filtering".
To understand comb filtering, it's first important to understand the basics of phase interference.
These two audio waves are identical and perfectly in sync, or in phase. They add together to create
a bigger wave. These two audio signals are also identical but they're perfectly out of sync,
or out of phase. They cancel when mixed together. In addition to waves being perfectly in phase
and perfectly out of phase, waves can be slightly out of phase to varying degrees.
Here's what a wave like this would look like on a frequency graph. Most sounds,
however, are made up of many frequencies. Here's a frequency graph of white noise, which contains
all frequencies. Let's pretend that these tracks are the various frequencies in white noise. I'll
copy these frequencies and paste them below. Now there are two copies of each frequency. When the
audio clips are in sync, all of the frequencies are in phase. However, when I shift all of them
by just one millisecond, look what happens. Some frequencies are still in phase, some frequencies
are slightly out of phase, and some frequencies are completely out of phase. Some frequencies
will add together, some will cancel out, and that forms a comb on a frequency graph - a comb filter.
My voice is also made up of thousands of frequencies. Here's what my voice looks like on
a frequency graph. Listen to the effect that even a slight delay can have on the sound of my voice.
Zero milliseconds of delay. Ten milliseconds of delay. Twenty milliseconds of delay.
There are three main causes of comb filtering in the real world - reflections,
multiple speakers, and multiple microphones.
Let's start with comb filtering caused by reflections. The sound from this speaker
takes two paths to the microphone - a direct path from speaker to microphone
and a reflected path from speaker, to wall, to microphone. The reflected sound travels a
longer distance and will reach the microphone slightly later than the direct sound. Listen
to the way the comb filter changes based on how far away the wall is from the speaker.
Another cause of comb filtering is multiple speakers. The sound from the left speaker
reaches the microphone before the sound from the right speaker. Imagine this microphone is
your ear. The sound changes depending on which speaker is closest to you.
And finally, comb filtering can occur when you're using multiple microphones. Listen to the way the
sound changes as the distance changes when I mix the sound from each microphone together.
In future videos, we'll take a look at how to avoid phase cancellation and comb filtering,
but we'll also take a look at how phase cancellation can be harnessed
in technology. If you got value out of this video, click the "Like" button, subscribe
to Audio University, and check out the website at AudioUniversityOnline.com. Thank you for watching!