Relative to recording audio, phase tuning is a technique used to alter frequency content without an equalizer. This alteration is achieved purposely, by way of the constructive and destructive properties of the phase difference of two audio signals derived from the same source.

How it's done

To phase tune a source, set up the first microphone where you would like it to pick up the sound, e.g. if the source was a vocalist, set the microphone up near his or her mouth. Note how the source sounds through this microphone alone; does your vocalist have piercing sibilants? Does that guitar you are recording need a little more presence? Keep your goal in mind as you proceed to the tuning process.

Next, put on a pair of cans and grab the second microphone in hand. If you have someone assisting you, have them wear the cans and grab the microphone while you direct from the console. Make sure you are listening to a mono mix of the signal from the first and second microphones; they can be panned left or right, but must both be panned to the exact same direction. Have the source make some sound, and begin to move the second microphone around slowly, changing its distance from the first microphone. The mono mix of the two signals should sound remarkably different as you move the second microphone. Certain frequencies will be emphasized, others will drop away. When you find just the right mix of constructive and destructive interference you were looking for to make the source sound ideal, stop moving! Mount the microphone exactly in that spot and move on to recording.

How it works

If two audio signals are summed into mono and are exact duplicates of one another, the resulting waveform will have an amplitude twice that of the singular signal. Although impossible to achieve in reality, if the capsules of the two microphones being used to phase tune are in the exact same place relative to the source, the summed mono signal would exhibit this doubling in amplitude. Moving the second microphone causes the arrival time to differ between the first and second microphone, i.e. they are out of phase. Summing similar signals simultaneously that are out of phase causes simultaneous constructive (gaining amplitude) and destructive (losing amplitude) interference. By changing this distance while listening to the resultant signal, a beneficial mix of constructive and destructive interference can be achieved.

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