Ryan on the Three to One Rule


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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,

however if they are out of phase (anywhere around 180°) then the waves will cancel each other out, nullifying certain frequencies. So why do the microphones have to be placed three times apart? Well, phase cancellation largely occurs when the two signals being recorded are around the same level. When the level or amplitudes are the same then any peak will be cancelled out by a trough and the audible result is a poorly recorded stereo track. By distancing the microphones at least three times should adequately prevent this because amplitude reduces with distance and at this multiple it reduces so much we get no cancellation even if they are out of phase. That's 3:1 rule in a nut shell! It's not a fail proof solution to phase cancellation but it will help prevent it, always spend time fine tuning your mic placements to credit the sources they will be capturing.

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