There's more to decent mic technique than knowing how different mics work and what directions musical instruments sound good from, because the moment you use two or more mics simultaneously, you'll find that their recorded signals don't simply add together — they can also subtract and interact in complex and sometimes counter-intuitive ways. To understand why this is, you need to grasp the concept of phase and how it applies to different miking setups — which, conveniently, is what this article is all about!

Let's start with a sine wave — the simplest audio waveform there is. Every other audio waveform can theoretically be broken down into a collection of sine waves at different frequencies, so by dealing with the concept of phase in terms of sine waves first, we can extrapolate to how they affect the more complicated real-world audio signals you'll find coming out of the back end of a mic.

A sine wave generates only a single audio frequency, according to how many times its waveform shape repeats in a second. For example, a 1kHz sine wave repeats its waveform 1000 times per second, with each waveform repetition lasting 1ms. Imagine that you have two mixer channels, each fed from the same sine-wave source at the same frequency. The peaks and troughs of the two waveforms will be exactly in line, and mixing them together will simply produce the same sine wave, only louder. In this situation we talk about the two sine waves being 'in phase' with each other.

If you gradually delay the audio going through the second channel, however, the peaks and troughs of the two sine waves shift out of alignment. Because of the unique properties of sine waves, the combination of the two channels will now still produce a sine wave of the same frequency, but its level will be lower than if the two channels were in phase, and we say that partial phase cancellation has occurred. When the second channel is delayed such that its peaks coincide exactly with the first channel's troughs (and vice versa), the two waveforms will combine to produce silence. At this point we say that the waveforms are completely 'out of phase' with each other and that total phase cancellation has occurred.

When total phase cancellation occurs, you sometimes hear engineers say that the signals are '180 degrees out of phase'. This is a phrase that's not always used correctly, and it can therefore be a bit confusing. In order to describe the phase relationship between two identical waveforms, regardless of their frequency (that is, how fast they're repeating), mathematicians often quantify the offset between them in degrees, where 360 degrees equals the duration of each waveform repetition. Therefore, a zero-degree phase relationship between two sine waves makes them perfectly in phase, giving the highest combined signal level, whereas a 180-degree phase relationship puts them perfectly out of phase, resulting in total phase cancellation — and therefore silence as the combined output. All the other possible phase relationships put the waveforms partially out of phase with each other, resulting in partial phase cancellation.

What's confusing about the '180 degrees out of phase' term is that it is sometimes used to refer to a situation where the second channel has not been delayed, but has had its waveform flipped upside down, so that the peaks become troughs and vice versa — a process more unambiguously referred to as polarity reversal. This scenario also results in silence at the combined output, hence the common confusion in terminology, but it's very important to realise that the total phase cancellation here is brought about by inverting one of the waveforms, not by delaying it. In this example, it might seem like we're splitting hairs, but in practice the distinction between time delays and polarity becomes much more important.

Let's scale things back up to deal with real-world sounds, made up as they are of heaps of different sine waves at different frequencies, each one fading in and out as the pitch and timbre change. If we feed a drum loop to our two mixer channels, instead of a single sine wave, any delay in the second channel will have a dramatic effect on the tonality of the combined signal, rather than just altering its level. The reason for this is that, for a given delay, the phase relationships between sine waves on the first channel and those on the second channel depends on the frequency of each individual sine wave. So, for example, a 0.5ms delay in the second channel will put any 1kHz sine-wave components (the waveforms of which repeat every 1ms) 180 degrees out of phase with those on the first channel, resulting in total phase cancellation. On the other hand, any 2kHz sine wave components (the waveforms of which repeat every 0.5ms) will remain perfectly in phase. As the frequency of the sine wave components increases from 1kHz to 2kHz, the total phase cancellation becomes only partial, and the level increases towards the perfect phase alignment at 2kHz.

Of course, above 2kHz the sine wave components begin partially phase-cancelling again, and if you're quick with your mental arithmetic you'll...