What’s the difference between polarity and phase?
Although some might use the terms polarity and phase interchangeably, there are differences. Polarity is a function of positive and negative voltage or sound pressure, while phase is a function of time.
In this video, you’ll learn the difference between polarity and phase and why it is important to differentiate these terms.
What Are Polarity & Phase
While it’s very common to confuse polarity and phase, it’s important to understand how they are different.
It will help to have a basic understanding of how sound works. I wrote this article to help you quickly learn the basics of how sound works.
As you may know, a sound wave is a cycle of positive and negative pressure changes. A microphone converts those pressure changes into a cycle of positive and negative voltages in a wire.
As you can see, this signal begins with a positive voltage and then continues through a negative voltage.
Let’s flip the polarity of this signal. Now the signal begins with a negative voltage.
Most mixing consoles, DAWs, DSPs offer the ability to flip the polarity of a signal like this. It’s the equivalent of swapping the positive and negative wires in a microphone cable or a speaker cable.
I wrote an article about speaker polarity. It will help you understand what is happening when you reverse the positive and negative wires from a speaker to an amplifier.
To better understand the difference between polarity and phase, let’s imagine identical audio signals travelling through two cables.
These two copies of the signal are currently in the same polarity. They will add together at the destination. This is called constructive interference.
Flipping the positive and negative wires on one of the cables will put these signals into opposite polarity. When they are mixed together at the destination, they completely cancel out. This is called destructive interference.
While polarity is a function of the positive and negative, phase is a function of time.
To help you visualize this, I’ll put a delay in the second cable path.
At the source, the signals are synchronized. They begin and end at the same time. However, one of the signals is delayed.
By the time they reach the destination, they are out of sync. This causes a phase shift.
Remember – audio waves are cycles. The cycle of this wave begins at zero, goes through a positive phase, returns to zero, and then progresses through a negative phase.
Just like a circle, the points along the phase of a sound wave can be charted from 0-degrees to 360-degrees.
In our example, the second copy of the wave arrives as the first copy is halfway through one cycle. This means the signals are 180-degrees out of phase.
The first wave is negative as the second wave is positive. This will cause destructive interference, because now the waves work against one another.
A phase shift doesn’t always result in a perfect summation or a perfect cancellation.
Most often, the signals will be slightly shifted. This results in a partial cancelation.
Polarity vs Phase with Multiple Frequencies
Up until this point, we’ve used signal examples of the same frequency. However, most audio signals contain many frequencies. This highlights an important practical difference between polarity and phase.
In this example, we have two identical signals, each containing the same three frequencies.
Inverting the polarity of one of these signals will cause a complete cancellation when they mix at the destination, just like the example with only one frequency.
However, a phase shift affects each frequency differently.
Some frequencies will sum together, some frequencies will partially cancel, and some will perfectly cancel.
Although some equipment might use the terms polarity and phase interchangeably, there are differences.
Polarity is a function of positive and negative wiring, while phase is a function of time.