During recent recording sessions, we were reminded of an important fact: you have to be careful of what you’re measuring and lining up when you’re aligning microphones for proper phase. This became clear when we were miking guitar cabinets. We had a Sennheiser MD 421, a Mojave MA-100, and a Royer R-122 set on a single speaker in a cabinet. You would think that just setting the front of the mics the same distance from the cabinet would achieve proper phase alignment. But, if you look a bit deeper, there are two factors that could impact the actual distance the microphone is seeing from the speaker.
1. A speaker cone isn’t flat across its front, it’s cone shaped. This means that if you mic the edge of the speaker, the distance to the cone is shorter than it is if you mic the center of the speaker. If you have two or more microphones in a straight line in front of the cabinet (seeming all aligned), the actual distance each sees from the speaker may be different, due to the way the cone-shaped speaker driver recedes into the cabinet.
2. Where the diaphragm is located in a particular microphone capsule may not be the same in each mic. In some mics, it may be closer to the front of the grille. In other mics, it may be set back more from the front of the grille.
Taken together, these variances could result in a difference of an inch or even more of distance between the actual location of each mic’s diaphragm and the actual plane of the speaker cone. And an inch can be a substantial difference when you’re dealing with phase!