If your mix has ever sounded a bit off but you couldn’t quite place why you probably had experienced phase cancellation. It occurs when two or more waves with the same frequency are combined and cancel each other out due to them being out of phase. This basically results in eliminating specific frequencies, thus making the sound flat or hollow. This post intends to help you better understand phase – what it actually is, why it is important and what it means to be out of phase.
In physics terms
Phase is fundamentally a part or a stage of a process of change in a waveform cycle. The term typically refers to sound waves or waves that are produced by air movements and vibrations. We hear the changes in air pressure in the process of movement. A rippling effect is caused by the vibrations of the air, the same way as in water, and so waves are produced and spread with their highs and lows. These sound waves are picked up by our eardrums which makes them also vibrate and this process is translated into sound in our brains. Likewise, the diaphragms in the microphones vibrate in accordance with the waves. The peaks of the wave cause the microphone’s diaphragm to move in one way and the troughs create movement in the other, opposite direction.
If there are two signal or two channels of a signal are in phase, we hear the sound at the same range level in both in our ears at the same time. Here’s an illustration showing the channels in phase:
Provided that one part of the stereo signal is reversed like shown in the second figure, both signals will cancel each other out and combining them would not only remove some frequencies but might even result in total silence, if a pure sine wave is used.
Virtually, we can hardly hear pure sine wives, as most of the music there is and the instruments actually produce a composite combination of numerous waves and sounds and thus the outcome of phase cancellation would be just as complex.
Recording in the studio
When it comes to recording, phase can quickly become an intricate issue, making it difficult to combine more than one channel to record a single source, like for example when stereo miking a guitar, multi-miking a drum set or using a microphone/DI combo for bass. There is a great chance of one microphone to receive a positive phase while another receives a negative, because of the different timing of sound waves with different frequencies reaching different mics. As the number of mics increases, so does the possibility of some sort of phase problems. That happens very often during recording sessions with drum kits. The top and bottom drum heads are usually moving in opposite sides (inward and outward) and this leads to the two microphones recording out of phase signals.
Issues might occur also in other scenarios, such as a stereo recording of an acoustic guitar. Usually, in this case, there are two microphones set up – one pointed at the sound hole to record the lower frequencies and one directed at the fingerboard to pick up the attack. With the guitar frequency interval being several octaves, it creates a wide variety of different audio wavelengths. Consequently, those waves will be caught up by the mic at different points of time, because the distance from the mics to the source is fixed. As it may be expected in that situation one or more tones or harmonics will end up weaker than the rest.
Certain delay settings, including pre-delays within a reverb patch, can cause a delay of the original signal and wind up being out of phase. Also, a bass track recorded direct (DI) can be too clean sounding, so it might need an extra mic to be put on the bass amp cabinet to give it more colorful sound, but it can also introduce phase issues.
The phase cancellation phenomenon might also occur by connecting speakers incorrectly, thus reversing the polarity of a channel. What is worse is that it is more often than not so vague it can hardly be detected, without careful listening. Technically, it denotes a polarity problem, although it is mostly regarded as “out of phase wiring”. The polarity reversal and the phase cancellation actually sound the same way.
What does it actually sound like?
One way to detect phase issues is to check the mix in mono, instead of stereo, in other words, combine the left and right channels into a single channel. Phase cancellation becomes apparent when in mono, the sounds come out thin and dull. Of course, if the signal sounds hollow in stereo, it’s a sign of phase cancellation.
Another common indication of out-of-face stereo sounds is when the central signal is eliminated and the side ones remain – this is the case when the lead vocals or instrument solos vanish.
As phase cancellation is so common, there are many microphones, microphone preamps, and consoles offering a phase flip switch.
How it can be fixed?
- First, the issue must be identified in time. Tip: Sum up in mono every chance you get.
- If you identify the problem during the recording process, you can fix it by flipping the phase switch or move the microphones.
- If you come up with the phase cancellation issue during mixing, you can edit, move and zoom the track in your DAW, or change a track’s EQ or add reverb. There are also some very useful phase alignment plug-ins, which can be used to clean it up.