Fatten Up The Bottom End With Physics
While discussing unusual ways to record drums with Ex’pression College for Digital Arts’ Advanced Recording instructor George Borden and student Jonas Alt, a secret technique from the recording magicians of yore came to light. This rather scientific approach involves the phase relationship between overhead and close mikes to enhance the bottom end oomph of the drum set. So don your lab coat and check this out.
Close mike all the drums in the traditional manner and position two large diaphragm cardioid condensers (such as Shure’s KSM32, AKG’s C3000B, Audio-Technica’s 4050, etc.) directly above the floor tom and snare drum mikes in a split pair configuration. Angle them similarly to the close mikes. To determine how high to place the overheads above the kit, divide the speed of sound (1,130 feet per second) by the distance the overheads are from the close mikes. This will give you the frequency that will be boosted by a phase phenomenon called comb filtering (an effect one usually tries to minimize under ordinary circumstances). The laws of physics decree that the frequency double the length of the boosted frequency will be negated by the same phenomenon.
If you place the condensers five feet from the close mikes on the source, the equation would go like this: 1130/5 = 226Hz. Now this frequency tends to muddy up a mix, so here’s the trick: flip the phase on both the overhead mikes at the preamp or console (or by using reverse-wired cables). This effectively cuts the frequency being reinforced by half because the overhead signals are now shifted 180 degrees out of phase, and when mixed in with the close mikes, the enhanced frequency will now lie somewhere around 100Hz, giving the kit that big bottom end. In addition, the muddy frequency between 200-250Hz will be minimized, clarifying the sound.
As you geek out on this approach, keep in mind that phase is a frequency dependent phenomenon and will be affected by the dimensions and reflective characteristics of the room, the size, tuning, and setup of the drums, the complexity of the waveforms around the fundamental frequency, and the dynamics of playing style. Also, the higher you place the overhead mikes, the more room you’ll be hearing, the less defined the drum sound, and the more diffused the stereo image will be. Therefore, experiment a bit with placement and angle to maximize the effect of this technique. Use your ears!