What is phase?
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What is Phasing?
Sound waves are cyclical, that is to say that they repeat. The phase of a waveform refers to the point at which the waveform is at during its cycle.
This waveform starts at 0°, peaks at 90°, reaches neutral pressure at 180°, has a trough at 270° and returns to the zero-crossing at 360°, which also marks the beginning of another cycle.
If a waveform doesn’t start at 0° and starts at 90° for example, this is known as a 90° phase shift.
Phase difference is “when two waves are not in sync with one another that has an audible effect on the audio but does not completely cancel out."
If you combine two waves with the same frequency and amplitude but a difference in phase, or a phase shift, you get what is known as partial phasing.
Phase cancellation occurs when “two audio signals are out of phase with each other resulting in a reduction in the level of the combined signal. The audio loses certain frequencies (or all frequencies) as the waves cancel each other out."
In other words, if two waves with the same frequency and amplitude are combined with a phase difference or shift of 180°, they will cancel out and you will be left with no audible sound.
The main effect that phasing has on sound is the change in amplitude. When two waves are combined, whether they are in phase or not, constructive and destructive interference will occur. When two waves that are in phase are combined, constructive interference occurs which results in the doubling of the amplitude of the waveform. When two waves are out of phase, destructive interference occurs, which results in the loss or reduction of amplitude.
Causes of and Solutions to Phasing
Cause 1
Phasing is most commonly introduced into a recording when two microphones are being used to record a single sound source. For example, you might want to record an acoustic guitar with two microphones, record a full orchestra using the spaced pair technique, or use complex multi mic’ing techniques on a drum kit.
The reason for phase issues when using multiple mics is that, unless carefully measured, the sound from the sound source will reach the mics at slightly different times, meaning that when the microphone outputs are played simultaneously, one will be slightly behind the other, causing partial phasing.
Solution 1
By placing the microphones at equal distance from the sound source, you should be able to almost completely eliminate any phasing. You could use the 3:1 microphone placement rule, which states that the distance between the two microphones should be 3 times the distance between each microphone and the sound source.
Cause 2
Phasing can also occur when recording a sound in an untreated room/reflective environment, like a church for example. When recording in a space like this, the microphone not only picks up the original sound source, but also the reflections of that sound from surfaces such as walls, doors, floor etc. The reflections arrive at the microphone a short while after the original sound is picked up which, much like the problem above, causes partial phasing.
Solution 2
A common way of minimising phasing issues in an environment like this is to use boundary mics. Boundary mics have plates attached to them, which act as a reflective surface. This means that all the ‘versions’ of the sound (the original sound and multiple reflections) arrive at the microphone at more or less the same time, therefore reducing phase issues.
Cause 3
Electric bass and electric guitar can be recorded using DI or by capturing the output of an amp. Sometimes you might want to record both to give your recording a bit more ‘oomph’, but this is yet another place where phasing can become a problem. The great thing about DI is that there is hardly any latency, ie. there is very little time difference between the performer plucking the strings on the instrument and the signal being recorded to your DAW. Its not quite the same for amp recordings; you’ve got to take into account the time that it takes for the signal to be boosted by the pre-amps in your amp, the time it takes for the speakers to produce the sound, the time it takes for your microphone to pick up the sound etc. This means that its going to take longer for this signal than your DI signal to reach your DAW, causing partial phasing. It may sound like there would be a massive, audible time difference, but in reality the non-DI signal is only 1 millisecond behind the DI signal per foot travelled.
Solution 3
Knowing what the time difference is, you can then use your DAW to add a 1 millisecond (or whatever time is required) delay to your DI signal so that the two line up and eliminate the phase issue.
Uses of Phasing
Although phasing can be a massive problem when it comes to recording, it can also be very useful and be used creatively.
In balanced cables, phasing is used to eliminate interference. In a balanced cable you have two signal cores and a ground. The two signal cables carry the same audio signal along the cable but one is inverted so that they are out of phase and cancel each other out. Once the signal reaches the other end of the cable, the phase is inverted again. The reason for this is, if at any point there was any interference, the unwanted noise will affect both signals simultaneously. When the phase is inverted at the end of the cable, the signals are back in phase but the interference is out of phase with itself and therefore cancels out.
Phasing is also used as a creative effect. In a phaser the original signal is copied, and this copy is passed through a series of all-pass filters which alter the phase of certain parts of the frequency spectrum. The phase-shifted signal/frequencies can be shifted to varying degrees. This phase-shifting around certain frequencies cause notches to form, which are narrow bands of the frequency spectrum that are completely cut/made inaudible. A series of these notches are called a comb filter. An LFO moves the positions of theses notches which is what makes this filter a modulation filter. The phase difference between the original signal and the phase-shifted signal causes some frequencies to be amplified and some frequencies to be attenuated or cut completely.