Explainer: Phase
Understanding waveforms’ interaction is essential for producers – but it’s easier to grasp than you think…
As the word phase is used in so many contexts, it’s no surprise that its meaning gets misunderstood. To get to grips with it properly, let’s imagine a waveform, which consists of peaks and troughs that oscillate above and below the centre median line. These peaks and troughs represent the way the signal pushes and pulls air molecules, which then vibrate our eardrums, resulting in the sound we hear. And in exactly the same way, those air oscillations vibrate a microphone’s diaphragm; movements that are converted into an electrical signal, and then to digital where they’re recorded into software.
Polarity refers to the back-and-forth oscillations above and below that centre median line. Many mixing consoles and plugins feature a confusingly-named ‘phase invert’ button, which is actually polarity. Invert the polarity of a signal by 180 degrees, and you’ll flip the up-and-down shape of the waveform. More on this shortly…
Phase, on the other hand, refers to the position of a waveform in time, and is most important when dealing with two or more identical (or almost identical) signals. All waveforms consist of peaks and troughs, right? Well, if you layer an identical copy of the signal below the first, in exactly the same position in time, the peaks and troughs of the two waveforms will be perfectly aligned: they’ll be
in phase, resulting in a straight-up doubling (+6dB) in volume. Flip the polarity of the duplicate copy by 180 degrees, however, and the waveforms will now be completely out of phase – the peaks and troughs of the two waveforms are polar opposites, and cancel each other out, resulting in complete silence.
Let’s now go back to our two duplicate, in phase signals. Delay the second waveform later in time, and you’ll shift the alignment of the waveforms’ peaks and troughs, causing certain frequencies to cancel out (destructive interference) and others to reinforce (ie, constructive interference). When observed on a spectrum analyser, the sharp spikes and dips of these constructive and destructive frequency interferences look a bit like a comb, hence why this effect is known as comb filtering.
Phase is a common issue when recording instruments or real-world sound sources – explained in more detail opposite – and phase problems and comb filtering are also environmental considerations. In an untreated studio room, the signal from your speakers will hit room boundaries (the ceiling, side walls etc), then bounce back and combine with the dry signal to create an undesirable comb-filtered response. Acoustic treatment counteracts these reflections and phase issues, minimising comb filtering and ultimately giving you an optimum signal at the listening position.