What is Phase?

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What is Phase?

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What is Phase?

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http://theproaudiofiles.com // A video breaking down the concept of phase.

Transcript:

I’m working on an Intro to Mixing tutorial, and in that tutorial, I’m going to be going in depth into a concept called “phase.”

Phase is a foundational idea in engineering. It’s something that is easily misunderstood. It can be quite confusing, but it’s something that’s really, really important to get down and understand in order to really move forward in this field.
So, I wanted to talk a little bit about phase to define some terminology, because I think that’s one of the places where people get lost. They either don’t have the terminology, or don’t really understand the terminology, and that leads to further misunderstanding.

So we’re going to talk about phase. Here’s what’s happening when sound is produced.

Energy hits a set of air molecules. That’s going to be my hand here. Once those air molecules become excited, they move toward the next air molecules, creating pressure. As a response, the next set of air molecules moves forward, and the original set moves back, creating a vacuum. It creates a cycle that kind of looks like this.

That cycle moves forward in space. Alright?

So air molecules create positive pressure on the next set of air molecules, return to their normal state, and then experience negative pressure, which is a vacuum. When we see it on a graph, it makes a lot of sense, right? We’ve all seen that sine wave graph where we have the little line in the middle, and the sound wave goes up, then it goes down back to the line, and then it crosses under the line and goes down underneath.

What we have is positive amplitude when it’s above the line, that’s pressure. We call that “compression.” Now, that’s not to be confused with dynamic processing compression. Same word, different meaning. So positive amplitude is compression.

When it hits that line, that’s called a node. That’s where you have equal pressure, both positive pressure and negative pressure. When it goes under the line, that is the vacuum state of the molecules. That’s called rarefaction. When the molecules are in rarefaction, they are creating a vacuum that’s negative amplitude.

So while amplitude in total is a absolute force, we can think of it in terms of both positive and negative. Now, let’s discuss the word “phase cancellation.”When we have something that is at a point of positive pressure, and another sound that’s at a point of negative pressure, we can think of it like a thread. One sound is forcing things this way, while the other sound is trying to force them back this way.

That’s phase cancellation, as it results in a diminished amount of energy, and if it happens to be the exact same sound with the exact same energy at the exact same time, we get perfect cancellation, which is called a null, meaning we get no amplitude at all. This is why if you take a sound, copy it, flip the polarity, and play them at the same time, you get no sound coming out of your speakers.

It’s because the sounds are canceling each other out. Very important.

Now, in the process of this sine wave, we start at a node, we get to our highest point of compression, we go back to a node, we get to our deepest point of rarefaction, and we come back to a node. A zero point. Then the cycle starts again.

That one process is called a period. That’s sometimes called a cycle as well, but if something is a 10Hz sine wave, that means that ten cycles occur over the course of a second. So that’s the length of — that gives us the length of the wave, it gives us the tone of the sound and how our ear detects it. So that’s our period.

When we talk about phase angle, what we’re really saying is where along that cycle are we? So, if it’s at 90 degrees, and we’re talking about a sine wave, we’re talking about it’s highest point of positive amplitude, or it’s maximum point of compression. We’re talking about 180 degrees through it’s period, we’re at a node. If we’re talking about 270, we’re at it’s lowest point of rarefaction, and if we’re talking about 360, we’ve completed the entire cycle. We’ve gone around. Right?The other thing to be aware of is that all of this change in pressure occurs through time. So, when things are out of time, they also change in terms of phase angle, and therefore, they will affect each other differently. So I’m going to leave it at that. There’s a lot of technical jargon in this video. This isn’t going to be one of those videos that gets a whole lot of click hits and stuff like that, but this is core, important knowledge and terminology that you need to know if you want to move forward in the field of audio engineering.

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What is Phase?

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What is Phase?

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