Overhead Microphone Phasing And Urban Legends

Nearly every sound engineer has dealt with phase. I have showed how to cancel an entire signal simply by duplicating and inverting the signal in this video. We’ve spoken about identifying phase by using phase inversion and listening for changes in the low end. But, how do we avoid phase issues?

This article will focus on drum overheads using the spread pair technique, but the principles will hold true for other studio applications.

Spread Pair Overhead Technique.

This is when we spread our two overheads apart from each other. Personally, this is my favorite overhead mic position. It provides an exciting stereo spread of the cymbals and natural tom panning. But, it can cause phase issues! The trick to avoiding phase is to place the microphones equidistant from the sound source. But, there is more than one sound source! Which source should we measure from?

There are two schools of thought on this:

1. Equidistant From The Snare. When you listen to the overheads, the snare drum is likely to be the most audible drum. If you’re like me, you will put a high pass filter on your overheads to better isolate the cymbals. This will cut some bass drum frequencies out, making the bass drum a less important part of the overhead mix.

2. Equidistant From The Bass Drum. If you are not intending to use a high pass filter, this is the method may be for you. This is a very realistic interpretation of a drum kit. The snare is not the center of the drum kit. That position belongs to the bass drum.

The best way to avoid phase issues is to keep the mics low and therefor closer to all of the drums. This is due to the speed of sound and the relative distance the microphones are from each sound source. The greater the height and microphone spread, the greater the time smearing. This discussion will continue below…

Time Smearing Due To Microphone Spread

WARNING! ONLY READ THIS PART IF YOU’RE A SERIOUS AUDIO GEEK:

The speed of sound is 340 meters per second. Meaning if one overhead microphone is 4 inches further from the sound source, it will take the sound 3/10ths of a millisecond longer to reach the furthest mic. 12 inches will get you about 1 millisecond of this time smearing.

I’d like to address an urban legend. Some colleagues have attempted to convince me that I must use the bass drum as the center of my spread overheads. They explain that low frequency sound waves move slightly slower than higher frequency sound waves. And they’re right about the psychics, but they’re wrong about the math.

And here’s why:

A frequency of 10 Hz travels 0.1 meters per second slower than a sound wave of frequency 100 Hz. This however is not a scalable equation. As sound sits in the more normal audible range, this speed change is close to negligible. (And as many of us know, 10Hz is not an audible frequency.)

But, let us walk through their argument as if it were:

Picture a signal generator. 340 meters away from the signal generator is a microphone.

The speed of sound is 340 meters per second.

If the signal generator were to generate a sound wave of 100Hz, it will reach the microphone in 1 second.

Now, imagine the signal generator were to generate a sound wave of 10Hz, it will reach the microphone in 1.0003 seconds.

This is a difference of 0.0003 seconds.

That is 3/10ths of a millisecond slower than the 100Hz signal over 340 meters!

Now think about one of your drum overhead microphones being 4 inches further than the bass drum than the other overhead microphone.

100Hz signal reaches the mic in 0.00029412 seconds

10Hz signal reaches the mic in 0.00029420 seconds

A difference of 0.00000008 seconds or 8/100,000ths of a millisecond

We have now put this urban myth to bed.

Please leave your comments and feel free to contact me directly at Anthony@myrecordinginternship.com. I’ll do my best to respond to you and answer any of your questions. Now, go out there and make great music!

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