Understanding Phase

This is one of those things that you must always be conscious about during sessions, much like listening for instrument tuning, which just so happens to carry some of the same principals.

It's safe to say that audio signals are made up of two basic principals: time (frequency) and volume (amplitude). These factors make up a sin wave (sinusoidal wave) when displayed on a graph. This is where the term "sound wave" comes from. The sin wave moves in both the positive and negative direction. The height of the wave represents the amplitude, and the length of the wave represents the frequency. Frequency is measured in Hertz (Hz), which is just a fancy way of saying "cycles per second." One cycle is when the wave travels from zero to "its" amplitude in the positive direction, then to its amplitude in the negative direction, and then back up to zero (Fig 1.1). The amount of cycles that happen in one second gives you the frequency. For example, twenty cycles in one second is 20 Hz and 20 thousand cycles per second is 20 kHz, which is also the average audible range for humans. We listen to audio in its analog form because our ears work like mics. A speaker pushes out in the positive direction of the wave, and pulls back in the negative direction of the wave. Our ears react in the opposite direction so on a speakers "push," our eardrums "pull," and vice versa - like talking into a mic.

Why am I telling you about sound waves?

It's because phase, in essence, is the direct comparison of sound waves being played overtop of each other. Let's use a portion of a 20 Hz wave as an example. If we zoom in on one cycle, we see that it begins at zero, and travels to the positive (push) amplitude, and then to the negative (pull) and back to zero. If we put the exact same 20 Hz frequency overtop of it that has the same amplitude, we end up adding the two waves' amplitudes together to make a theoretically louder 20 Hz. If we "reverse the phase" on one of them, we get a "figure eight" type of sin wave (Fig 1.2). All we've done is started the wave from zero to its negative amplitude first, and then to its positive amplitude and back down to zero. It's a mirror image split horizontally. The result is a complete theoretical cancellation of both waves, which means no sound! This comparison can also be made using two waves with different amplitudes or frequencies. Their sum, however, follows some pretty complex physics and mathematics. The result is an altered wave in both frequency and amplitude.

How does this affect us in the real world? Imagine a vocalist that wants to record in the control room using the studio monitors to listen instead of headphones. We can apply the cancellation theory in this case. First we must set up a mic and speaker on the points of an equilateral triangle (Figl.3). The height of the mic must be at the height of the centreline of the speakers as well. We have to switch the control room mix to mono, which means that both speakers are emitting the exact same wave. If we reverse the positive and negative wires on one of the speakers, we have just flipped the phase of that speaker. In theory, at the point where the wave from the left speaker meets the right speaker (exactly where you placed the mic), the sound will cancel out, leaving you with a vocal track that amazingly has only vocal on it. Because the vocalist's ears are further back from the crossing point of the two waves, they can still hear the track while they are singing, it just sounds out of phase. Listening to something out of phase is causing one of your eardrums to push and the other to pull, making it seem unbalanced and unpleasant, but it gets the job done! If you are going to attempt this trick, make sure you double check that you have set it up properly - triple-checking your measurements, and starting off at a very low volume. If you do screw it up, you will probably get the worst feedback loop ever, which could blow up both your eardrums and your speakers, so be careful!

[Sidebar]

DNA Recording Facilities is owned and run by Chris and Dave Tedesco and hosts Steve Chahley as Chief Engineer.