Comb Filtering Demonstrated

Let’s talk about some situations where you’ll encounter comb filtering with just 1 mic or just your ears.

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. That reflected sound would sound 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 speaker. I have a microphone 90* off axis about a foot away. I’ll bring up a couple different objects behind the mic, parallel to the speaker. Notice how the sound gets nasty with notches in the spectrum that change with distance.

Book

Wood

Ceramic plate (similar to tile floor or walls)

What’s the point of this demonstration? Well this nasty comb filtering can happen when recording a guitar amp too close to the floor or walls. Even worse, if your mixing with a reflective high-backed chair or too close to a reflective wall, it can drastically alter what you’re hearing.

Now lets see what happens when you bring something non-reflective up to the mic. Here is a piece of 2″ acoustic foam.

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, see if there is anything that is going to interfere with getting the best sound.

Also Interesting:

1. Phase Cancellation And Comb Filtering
2. Free Sound Of The Week 07