When recording audio in any studio sound engineers have a knowledge of audio that can seem almost like a dark art. They often hold onto their knowledge but I'm here in this article to dispel some of the illusions. If you've ever worked with a sound engineer then you've probably come across the term: 'the three to one rule' at some time. Engineers love to blind artists with their amazing knowledge of audio recording and this idea is a classic one that can baffle and bemuse. It is a concept related to correct microphone positioning. Its association with multiple microphones and their proximity with one another and the source or sources being captured
is often misunderstood and can be confusing adding to the mystique of the audio engineers job. When we look at why this rule is so practised and popular it all relates to phase differences in the audio.When you use more than one mic to record a source the sound will be picked up by two terminals at two different times depending on their distances from the source(s). Ideally you want the sound waves to reach both microphones with as little time delay as possible. Why? Because time delays between microphones will lead to phase cancellations which null certain frequencies picked up by the mics. Essentially, whatever distance from the source or sources to the microphones, these microphones must be three times that distance apart.
Why this is important in any recording where more than one microphone is used is explained here. The image left shows the interaction of multiple waves with varying phases. The wavy lines represent the pressure gradient of a sound wave, a peak is a positive pressure gradient (compression) and a trough is a negative sound wave (rarefaction). Each rise and fall of the pressure is a complete cycle and the progression of each cycle is measured in degrees, 90° (¼ of a cycle), 180° (½ of a cycle), 270° (¾ of a cycle) and 360/0° (a complete/beginning of a cycle). The energy of the sound wave is related to the amplitude of the wave or in other words how high the peaks are and how low the troughs are. So if sound waves are being picked up from two different mic's the waves will be added together, if their phase is similar then the two waves will combine accurately,