I was recently glancing over some of the articles popping up on home recording blogs and came across this so called “Recorderman” overhead drum mic placement technique. A good technique built on sound principles. This brings to light some interesting thoughts. This technique can be expanded and applied to any multiple microphone situations. The process of keeping both mics equal distances from the main sound source (in the given example the kick and snare) eliminates what is commonly referred to as phasing.

Phasing occurs when the same sound arrives at two different mics at different times. Sound travels at a particular speed, but always at the same speed (in the same medium, in most cases in the studio: Air). Therefore if the microphones are the same distance from the source and point at generally the same part of the source, they should yield a similar sound (especially if they are a matched pair).

When used in this way multiple microphone techniques can yield good “images” of a sound source with minimal phase issues. The “image”, referring to the stereo placement of sounds, is the sound coming from the left or right side. This also is the logic behind coincident mic techniques i.e. M+S and XY where the mic capsules are placed as close together as possible (which in turn makes them equal distances from the source).

The downside of phasing and the use of multiple microphones is a phenomenon known as comb filtering. When out of phase signals are summed (mixed together) parts of each wave form will cancel, resulting in silence. This is not a general silence but the elimination of specific frequency depending on how out of phase the signals are. Frequencies very near the canceled frequency will also be affected to less and less of a degree as you move away from the canceled frequency. This is very much like applying a notch filter to a sound to eliminate a particular frequency. To make matters worse this notch effect will also occur at each harmonic of the canceled frequency. For example, if 800 Hz is canceled, then 1600, 3200, 6400, and 12800 Hz will also be affected. If you can imagine an EQ with a notch filter on each of the above frequencies you should get an idea of why they call it “comb” filtering. This effect can be simulated and sometimes overlooked when using a delay plug-in. Sweeping the delay time of a delay plug-in will sound similar to sweeping a notch filter up and down the frequency spectrum. Try it!

In conclusion, may I offer two additonal pearls wisdom? 1. Don’t use more mics than necessary. 2. If you want to use more than one mic to create a stereo image be very intentional about how you do it. (A tailors cloth tape measure can be very good investment in a studio.)