2 to 1 Microphone Rule

Most audio engineers know about the 3 to 1 rule, when dealing with 2 microphones and 2 sound sources. The rule is that if the sound sources are 1 foot way from the microphones then the two microphones need to be 3 feet apart from each other. In this article we are going to look at the 2 to 1 rule, when dealing with microphones and boundaries.

2 to 1 rule

The 2 to 1 rule is that if the sound source is 1 foot way from the microphone then the microphone needs to be 2 feet way from any boundary surface that will reflect the sound wave back towards the microphone. These are things like walls, gobos, music stands, ect.

In order to understand the 2 to 1 rule we have to understand phase distortion and reflections. Lets first define reflection; this is when a wave hits a boundary like a wall and the wave will then bounce off the wall and travel outwards away from the wall.

Phase distortion is when a sound is duplicated and is not in sync with the original sound source. When it comes to phase distortion, reflections and the 2 to 1 rule, this is what is happens. The original sound hits the microphone then travels past the mic, then hits the wall and bounces back towards the microphone and causes phase distortion.

Why is this a problem? Next time you get a chance to find a quiet room, go next to the wall and talk into the wall and slowly step back words until you can't hear your voice change tones. This exercise will give you a hint as to what is happening to the tone of the sound, as it bounces off the wall. It colors the sound and can be unnatural and undesirable. This exercise can be used to determine how close a microphone can be to a boundry. This may look odd to the recording artist, however; it will make a significant difference in the recording.

Problem spaces that ignore the 2 to 1 rule

Sound booths are probably the worst offender of the 2 to 1 rule. These tiny rooms can be too small to get a nice sound recorded. I went into a recording studio once to help them set up their sound booth. The booth was probably a 5' x '5 x 8' room. There was foam on the walls and when I went in there to listen to my voice I was extremely annoyed with the reflections in the mid range. I told them that this would not be the ideal place to record vocals. I couldn't imagine putting a opera singer in there or  any recording artist for that matter. I would be embarrassed. A better option would be to record the vocals in a larger space.

The next thing that comes to mind are gobos (go between). A gobo is an acoustic baffle that usually has a slanted window. This makes it so the players can see each other, but the sound is blocked. It would be a good idea to listen to the microphones with the gobo in between and with out the gobo in between the two players, to see if it makes a difference on the tonality of the recording. In some cases having a gobo there makes the recording warmer and better.

Conclusion

It's important to think about boundaries and how they will affect the recording. When choosing mic placement one should always consider the boundaries around the microphone. The worst offender of 2 to 1 rule is a flat wall that runs perpendicular to the angle of the microphone and the sound source. The reason is that the sound wave will bounce right back into the microphone and cause phase issues.

If you have any questions about microphone placement or recording email me at kyle.tronaaudio@gmail.com

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