Today's Lesson

There are two ways comb filtering is created in a physical space with a sound system. 

The first is by what is called reflections. Any time a sound is produced, it radiates from the source and bounces off of nearby hard surfaces. 

For example, when a drummer hits the snare, the emitted sound travels to the microphone as well as to nearby walls in the room, which reflects them back into the microphone. As reflected sounds travel a further distance than the direct sound, they arrive at the microphone later than the original. Both signals are identical, but one signal is delayed by a few milliseconds, causing a comb filter, where some frequencies are cancelled and some are summed.

Reflections can also occur from tabletops, floors and ceilings, windows, etc.

One way to avoid this is to place microphones as close to the sound source as possible. This way, direct sound level will be significantly louder than that of the reflected sounds. Sound wave energy decreases rapidly over distance, so by making sure we are getting as close as possible to the original source, we can take advantage of this.

The other effective method is to focus on absorbing or diffusing early reflections with appropriate sound treatment in the room. This is not always an option however. There are many ways to diffuse and absorb reflections.

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