A Guitarist’s Guide to In and Out of Phase

Phase pedals are commonplace on countless guitar players’ pedalboards, and we 6-stringers love the cool, swirly sound these stompboxes create — Edward Van Halen alone has created a myriad of musical magic with his beloved ol’ script logo MXR Phase 90.
That stated, when it comes to the terms “in phase” and “out of phase,” I don’t mind admitting that I’m not 100% sure exactly what that means. They are most commonly used with regard to the pairing of pickups and speakers.
Hopefully this article will help clarify the basics in “guitar-player speak.” In order to achieve this as succinctly as I can, I’ll refer to definitions (in italics below) from Sweetwater’s Glossary.
What Is Phase Exactly?
When it comes to sound waves, here’s the skinny:
Audio waveforms are cyclical: that is, they proceed through regular cycles or repetitions. Phase is defined as how far along its cycle a given waveform is.
At this point, let’s take a quick look at a simple sine wave — a pure one with no distortion or overtones (fig.1).

Figure 1

As you’d expect given its name, it looks exactly like a wave in water…duh! And as you can see, said wave has positive components (the sections above the horizontal time axis) and negative components (the sections below the horizontal time axis). Let’s now label a few things (fig. 2).
Peak (or Crest) – The wave’s highest point.
Trough – The wave’s lowest point; the exact opposite of a crest.
Amplitude – The vertical height of a wave, measured from the horizontal time axis to the wave’s peak. A sound wave’s amplitude is a measure of its loudness or volume: the higher the wave is, the louder the note is. And vice versa: the lower the wave is, the quieter the note.
Wavelength – The distance between the peak of one sine wave and the peak of the next.

Figure 2

Next up is the definition of a term not indicated on the above diagram but still relevant to our discussion.
Frequency – Literally the number of times something occurs per unit of time. In the audio world, the frequency of sound vibrations is directly related to what we hear as pitch, though the relationship is NOT linear. It is also inversely related to wavelength….
The shorter the wavelength is, the higher the pitch of the note. And conversely, the longer the wavelength, the lower (deeper) the note is. So high note waves vibrate faster than low notes, which vibrate more slowly.
A sound wave’s frequency is measured as the number of cycles per second (i.e., from peak to peak as illustrated in figure 2), and the unit used is Hertz (Hz). One wave per second is called 1Hz. So a note that sounds at 50Hz has 50 waves per second.
To put this in musical or “guitar” terms, the open A string note is 110Hz, or 110 cycles per second, and the frequency of the open high E string is 329.63Hz.

A440
Question: So if the open A string note on a guitar is 110Hz, what on earth does the A = 440Hz used in so-called “standard tuning” mean?
Answer: 440Hz is “the American standard orchestral reference pitch” for A. The low A string on a guitar is two octaves lower than said A.
Trivia: Raising the pitch of a note by an octave doubles the frequency, so an octave above A440 is A880 (440 x 2 = 880). Conversely, halving the frequency of the note results in a note that is an octave lower, so the octave down from A440 is A220 (440 ÷ 2 = 220).

What Does “In Phase” and “Out of Phase” Mean?

Now that we understand the basics of a sine wave (plus octaves!), let’s look at in phase and out of phase.
In Phase
If two identical waves (both having the same frequency and amplitude) are “in phase,” then they line up exactly. This means that when they’re combined, they will form a single sound of the same note (the wavelength or frequency remains the same, as they’re identical) that’s louder (i.e., has more amplitude). This is sometimes referred to by tech-heads as constructive interference. Figure 3 presents a visual that demonstrates that Waveform A added to Waveform B results in Waveform C.

Figure 3

Sonic Trivia: While the combined wave’s amplitude (loudness) is doubled (remember, both single waves have the same exact amplitude), the audible “volume” increase here is +6dB. While this is louder than each separate wave, it is not perceived by the human ear as “twice as loud” — that would be around 10dB…or, in Nigel Tufnel’s case: 11dB!!
Out of Phase
Whenever two identical waves are not in phase as just illustrated above, they are out of phase. Consequently, as you can imagine, there are degrees of how out of phase two identical waves can be — from slightly to…
a) Completely out of phase (180 degrees)
As luck would have it, this just so happens to be the simplest, most easily understood form of out of phase. Here’s what this looks like (fig.4):

Figure 4

This is often referred to as “180 degrees out of phase,” and the result is the “sound of silence,” as the waves literally cancel each other out, resulting in no sound at all. This is called destructive interference, as shown in the bottom waveform (C) in figure 4.
b) Partially out of phase
As you might expect, there are varying degrees of how out of phase two identical sine waves can be.
When two waves are partially out of phase with each other, both constructive and destructive interference will occur as there will be points where the two will combine positively (adding with each other) or negatively (canceling each other). This is because the two waves are sometimes in phase with each other (hence the partially constructive interference “addition”) and sometimes out of phase with each other (hence the partially destructive “subtraction”).
That stated, that’s about as deep as I care to dive into the turbulent and murky waters of the exhaustive number of degrees phase has. And we haven’t even mentioned, let alone discussed, polarity.

Disclaimer
In the real world, we normally don’t listen to pure sine waves. Most of the music we hear and the instruments we use and record create pretty darned complex combinations of multiple waves and harmonics. Consequently, the results of phase pedals on said waves will be equally  complex — and cool!

Hopefully, this short(ish!) piece has given you a better basic understanding of phase, plus constructive and destructive interference. If you’re interested in digging a little deeper and unearthing the differences between phasing, chorusing, and flanging, then grab your shovel and click here.
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