Using more than one microphone to record the same sound can be a powerful tool. There are some important principles at work that give both advantages and disadvantages. To begin with, recording with two microphones, can make the recording louder. It can also give us the ability to record in stereo. Let’s discuss how the human brain is able to perceive a sound’s direction. Like with our eyes, which, while used in a pair, can give us depth perception and help us to find our way in a three dimensional space, the ears are similar. The shape and position of our ears helps us to filter out unneeded information and extract the truth about a sound. This is done by the power of two. When we were discussing microphones earlier, we mentioned the dual diaphragm capsules being used in condensers and how one signal can be used to focus the other in a specific direction through polar patterns. The contour of the outside of the human ear, with all its ridges, is able to allow only specific polarities to reach the ear first. This is done by both ears being shaped in a reverse fashion with their position being slightly different on opposite sides of the head. The brain is wired by our maker to use that polarity information to identify a sound’s origination point. This is how we know if a sound is coming from in front of us, or behind us, or to the side, or even above. Using more than one microphone on the same sound is nowhere near as accurate as our hearing but the same principles apply and we must take notice of them to get an accurate recording. Phase is very important!
Firstly, the position of space that the microphones occupy is key. If they sit as close to the same position as possible, they will record the sound in pretty much the same way. But if they are far apart, they will record a very different thing. While the track will sound the same from each one separately, they may not sound good together, because there can be a slight delay between what each one picks up. That delay can be very small and yet still have an enormous impact on the combined recording. This is because the waves are not at the same point in the completion of their cycle when they reach the diaphragms. One might pick up the sound while it is in its uppermost position of oscillation; at the crest, while the other picks it up at a downward movement. If they are picked up at opposing momentum which is referred to as opposite phase, or 180 degrees, they can cancel each other out and no sound will be heard.
Secondly, there is a difference between waves that are high like the sound of a pencil tapping on a table, and sound’s that are low, like a person pounding their chest. The higher frequency waves are much shorter than the lower and they take much less energy to travel longer distances. Lower frequencies take a lot of energy in order to move a great distance; this is why subwoofer speakers are so large and require so much power to push the sound. Incidentally, lower frequencies also take a longer time to complete their cycle of up and down. (A typical home studio doesn’t possess enough room for even one of the bass frequencies to complete their travel. This often results in a phenomenon known as bass stacking; more about that later!)
Because sounds consist of a variety of frequencies that all travel at their own speed, and those sounds can cancel each other out if their phases are in opposition, the ability to predict what happens when these sounds are captured in separate points in space becomes an trigonometrist’s nightmare. The best way to minimize this problem is to place the microphones in the same position, or as close as possible to each other. If they must be at a distance, or you desire them to be, then they should be at precisely the same distance from the source. Engineer’s break out a tape measure when setting up a pair of overhead mics for drum sets. Another important detail they are aware of is that if the pair of microphones are identical then the diaphragms will be more capable of recording the same thing. This doesn’t mean they purchase two of the same mic; it means they are matched at the factory by the manufacturer and are sold as a matched pair.
Stereo recording can be done in several ways. There is a method called AB, which does actually place the microphones a distance apart but keeps that distance under two feet so that the phase cancellation will only comb filter the very high notes which are usually not as much of a problem in the typical target sound. In an XY recording, they occupy the same space but are angled apart from each other so they will pick up the identical sound from the same position but at a different phase from one another, which, since they are in the same place, isn’t a problem and can actually simulate the effect of a sound being heard differently by one ear vs. the other. There is a third technique called mid to side and it involves a mic pointed directly at the target, and a dual capsule, or a matched pair pointing in opposite directions in opposite phase, which are then pointed to either side of the room perpendicular from the target. The first mic picks up the sound from it’s center point and the other diaphragms capture the stereo room reflections. The main advantage of mid to side recording is the mixing potential that comes later from being able to process the mono and stereo separately. If not done correctly, however, it can be just as hazardous as bad mic placement.
NOTE: WHEN USING ROOM MICS, THEY SHOULD BE SPACED AT LEAST THREE TIMES AS FAR FROM A SOURCE AS THE SOURCE’S OWN MICROPHONE. THIS ALSO HELPS TO AVOID COMB FILTERING. THE PROS REFER TO THIS AS THE 3:1 RULE.