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In The Studio: Comb Filtering Demonstrated (Includes Audio Samples)
Looking at the destructive impact of even innocuous-seeming reflective surfaces...
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This article is provided by Audio Geek Zine.

 
Let’s talk about some situations where you’ll encounter comb filtering with just one microphone.

You can run into comb filtering whenever there is some kind of reflective surface near a microphone or your ears.

Many times I’ve noticed the way sound coming from behind me can be bounced back at me while reading. It sounds strange because it had to travel further to reach my ears.

Let’s jump into some demonstrations.

I have some pink noise playing out of a loudspeaker, with a microphone 90 degrees off axis, about a foot away.

I’ll bring up a couple different objects behind the mic, parallel to the loudspeaker.

Notice how the sound gets nasty with notches in the spectrum that change with distance.

 

Wood

  Wood by theaudiogeek

 

Ceramic Plate (similar to tile floor or walls)

  Plate by theaudiogeek

 

What’s the point of this demonstration? Well this comb filtering can happen when recording a guitar amp too close to the floor or walls.

Even worse, if you’re mixing with a reflective high-backed chair or are too close to a reflective wall, it can drastically alter what you’re hearing. Now let’s see what happens when you bring something non-reflective up to the mic.

 

Here is a piece of 2-inch acoustic foam:

  Foam by theaudiogeek

 

There is far less coloration and comb filtering in that example.

Of course you can also use this to your advantage. Some engineers, such as Steve Albini, like to put mics right on the floor when recording drums. Because the distance between the mic and floor are minimal, there is very little phase shift and comb filtering, all frequencies are boosted.

So I guess the point of this is: before you record, have a look around to see if there is anything that is going to interfere with getting the best sound.

Jon Tidey is a Producer/Engineer who runs his own studio, EPIC Sounds, and enjoys writing about audio on his blog AudioGeekZine.com.


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