Phase

You probably hear about it a lot but it’s often confusing or misunderstood.  What is it?  Phase refers to the relationship between sound waves as they travel from the sound source to a microphone.  One mic – no phase issues.  2 mics or more mean definite potential for phase issues so let’s delve into a quick overview of phase and how to deal with it.

We’re looking at phase here as it applies to recording a sound source with more than one microphone.  You may hear polarity mentioned as a term interchangeable with phase but we’ll stick with phase for our purposes here.

When a sound wave travels from a source, different frequencies travel at different wavelengths.  If you think of a wavelength like a wave in the ocean you’re not far off.  High frequencies have very short wavelengths.  Wavelengths are the distance between the peaks with a “valley” in between.  Something like this:  ~  Low frequencies have much longer wavelengths.  In fact, the frequencies put out by a low instrument like bass guitar can be several feet long whereas very high frequencies are in inches.  When a sound wave reaches a microphone it will arrive at the mic either at the peak, the bottom of the trough or somewhere in between.  This is no problem for one microphone but let’s say we have two mics.  If the sound wave hits mic A at the peak but reaches mic B at the trough, that particular frequency will be fully out-of-phase.  The result of this is a strong cancellation of that frequency between the mics.  Summing the two together will give this:  - a flat wave because the trough and peak interact to meet at the null point or middle.  When you work with more than one microphone you must consider phase as part of the equation.  The 3-1 rule (place a second mic 3x further from the sound source than the first) will help alleviate some of the phase issues commonly encountered and works well with two mics.  What about when you have a dozen mics on a drumset though?  This one is more complex.  Some would argue that you cannot completely eliminate some sort of phase issue but you can work to minimize it.  One way to approach the problem is to measure the distance of the overhead mics to the center of the snare.  By making each overhead mic the same distance from the snare you will ensure the sound will reach each mic simultaneously.  The closer each individual drum mic is to its particular drum, the less chance there is for phase problems with its neighbor. 

- How do you identify phase issues?  This can be tricky at times but if you follow the 3-1 rule or measure mic distance you’ll already be in a good position regarding phase potential.  However, you can often identify phase problems in a couple of ways.

- Solo the mics on the sound source at the mixing board.  Listen for differences in tone if you mute one mic.  Phase often appears as a “thin” sound lacking body.  A “swishing” or “washy” effect on cymbals is a definite indicator that there are phase problems. 

- You’ll almost always want to invert the phase if you use two mics on a drum with one on top and the other on the bottom.  In this case, the head will be moving towards the bottom mic when the drum is struck but the top head will be moving away from the top mic.

- Look at the waveforms in your DAW if you’ve recorded something with multiple mics.  If you zoom in on the waveforms for those particular channels, phase will be exhibited in either the peaks occurring on each channel at a different time or by seeing one channel having a trough where the other has a peak.  By simply sliding the tracks back and forth in time you can align the peaks and then just invert the phase on the channel that is showing a trough versus a peak.