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One of the sure ways for things to sound bad is a result of the phenomenon known as Phase Cancellation. It's important to understand how phase cancellation happens and how to avoid it. One of the most important and overlooked aspects of recording is to make sure that all the mics are in phase if more than one is to be used at the same time. The reason is because with only a single out of phase mic, a multi-mic instrument like a drum kit will just never sound right, and if not corrected will never be able to be fixed. So just what is phase anyway? Without getting into an intense technical explanation it just means that the outputs of all microphones used on the session are pushing and pulling together as one.
If one mic is pushing while another is pulling they cancel each other out at certain frequencies, when Mic number 1 signal peaks, Mic number 2 signal valleys. They cancel each other out at that frequency and the result is a very weak sounding signal when mixed together. Here both mics are pushing and pulling together. The signal peaks happen at the same time as does their valleys, as a result the signals reinforce one another. There are two types of phase cancellation problems it can happen, electronic and acoustic.
An acoustic phasing problem occurs when two mics are too close together and pick up the sound from the same instrument, only one is picking it up a little later than the first because it's a little farther away. With acoustic phase problems, the sounds won't cancel each other out completely, only at certain frequencies. When the two are mixed together, this usually makes them sound either hollow or just lack depth and bottom end. The way to eliminate the problem is by moving Mic number 2 a little further away from Mic number 1, and if the mics are directional, make sure that each one is pointing directly at the source that they are trying to capture.
The 3-1 principle states that in order to avoid phase cancellation between microphones, a second mic should never be within three times the distance that the first mic is from its source. For instance, if a pair of microphones are placed over the soundboard of a piano at a distance of 1 foot, the separation between the two mics should be at least 3 feet. If the distance from the source was 2 feet, the distance between the mics should be at least 6 feet. This principle is not a hard and fast rule, but it certainly is a good guideline for eliminating phase problems.
Remember, if you record something with the phase problem no amount of EQ or processing can ever make it right afterwards.
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