Audio Geek Zine

Phase is a constant concern for recording and mixing engineers. Problems with phase can ruin your music, they can be easily avoided or corrected, but first you need understand how these problems occur.
This guide will attempt to explain almost everything there is to know about phase, what it is, how it happens, what it can sound like and some techniques to deal with it.

What is phase?
I’m going to consult my Engineering school textbook Audio In Media for this.

The time relationship between two or more sounds reaching a microphone or signals in a circuit. When this time relationship is coincident, the sounds or signals are in phase and their amplitudes are additive. When this time relationship is not coincident, the sounds or signals are out of phase and their amplitudes are subtractive.

No wonder people are confused about phase. Even I got confused at that, looking up other entries on phase in the book were even worse. I guess I shouldn’t read books.

I’ll try to break it down more simply.



Phase VS Polarity

OK, so lets define things a bit more starting with Phase and Polarity. These two terms are often used interchangeably but they ARE different.

• Phase is an acoustic concept that affects your microphone placement. Acoustical phase is the time relationship between two or more sound waves at a given point in their cycle. Phase is measured in degrees. When two identical sounds are combined that are 180° out of phase the result is silence, any degree between results in comb filtering.

• Polarity is an electrical concept relating to the value of a voltage, whether it is positive or negative. Part of the confusion of these concepts, besides equipment manufacturers mislabeling their products, is that inverting the polarity of a signal, changing it from plus to minus is the basically the same as making the sound 180° out of phase.

In case you missed that: Phase is the difference in waveform cycles between two or more sounds. Polarity is either positive or negative.

In and out of phase

When two sounds are exactly in phase (a 0° phase difference) and have the same frequency, shape, and peak amplitude, the resulting combined waveform will be twice the original peak amplitude. In other words, two sounds exactly the same and perfectly in phase will be louder when combined.

Two waves combined that are exactly the same but have a 180° phase difference will cancel out completely. Silent output. These conditions rarely happen in real world recording, more likely the two signals will either be slightly different, like two different mics on the same source, or the phase difference will be anything other than 180° out of phase.

In cases where the signals are not 0° or 180° or the signals are somehow different, you get constructive and destructive interference or comb filtering. The peaks and nulls of the waveforms don’t all line up perfectly and some will be louder and some will be quieter. This is the key to combining mics on a single source.

Examples

Here are some examples using sine waves
this is a 250Hz sine wave with a peak amplitude of -20dB: LISTEN

If I add another the exact same and combine them (in mono) the output is the same but louder, a combined peak amplitude of -14dB. They have a 0° phase difference, amplitudes are additive.  LISTEN

Now if I change the phase, the time and waveform relationship of these by having the second track start 2 milliseconds later it’s like this: LISTEN
Silence, this is 180° out of phase.

Here is the same kind of thing with some white noise, -20db, the same audio file is copied to another track and combined. Louder same as before -14dB combined.

Now I’ll use the invert function on the second track and since these sounds are exactly the same it the completely cancel out.

I think everyone understands that now, so here’s something slightly more interesting. I’ve taken the first 1 second of the white noise clip and repeated it 9 more times. The second track is the same, but on each repeat I’ve delayed it by an additional 1ms. Now you can get an idea of the constructive and destructive interference and the comb filtering. If you look at the frequency spectrum in an analyzer you will actually see notches cut out like the teeth of a comb. LISTEN

Real world examples
Here is a bass guitar going into a DI box, the signal splits and goes to an amp and to the audio interface. The amp is miked and the mic is going into the interface too. This is a very common way to record bass, but you may run into phase problems when the fast as light signal from the DI box is combined with the air pushing out of a speaker into a mic some distance away signal.

Here is the bass DI Signal: LISTEN
Here is the Microphone signal: LISTEN
Combined they sound a bit funny, definitely some hollowness going on: LISTEN

Correction By Inverting Polarity

The first thing I would try to troubleshoot this is to invert the polarity of one of the tracks and see if that’s better or worse. I know that because there is a time delay with these tracks that inverting alone won’t fix everything.
I can invert the polarity in two different ways, I can either use an offline process and invert the whole wav file or I can insert a plugin on the track. Some DAWs have a polarity reverse button on each channel of the mixer that will do the same thing.

I’m just going to play the tracks and invert the polarity a few times so you can hear the difference: LISTEN

Correction By Time Adjustment

I’m going to keep it the way it was and then go to the next strategy. That is moving the tracks around in time. The microphone track is delayed compared to the DI track just slightly. So I can either nudge the microphone track earlier or delay the DI track a little.
I’m going to delay the DI track just slightly. To do this I’m going to insert a delay plugin that works in samples, such as the Time Adjuster plugin in Pro Tools.

I’m going to invert the phase, then scroll through the delay value 1 sample at a time until I achieve the most cancellation, then switch the phase back to normal. I found that 152 Samples did the trick. You can also zoom in really close on your waveforms and nudge a track until it lines up. LISTEN

Processing delay

Another common cause of Phase cancellation is when doing parallel processing with delay causing effects or external gear. If the delay is long enough you will hear it as a discrete echo, if it’s short then you will get the comb filtering problem. The way to get around this is to delay all the other tracks by the same amount so they all reach the master bus at the same time. See HERE for more on handling DAW delay compensation.

Multiple mics on a single source

When using two mics on a speaker cabinet you need to be aware of the phase relationship. You can never get the mics to perfectly cancel out, but you find a place where the two mics work together really well. Start by positioning the first mic in any way you like. Put on headphones and start moving the second mic around, you’ll hear all kinds of phase cancellation but there will be at least one placement that sounds really good. It helps to invert the polarity of the second mic while doing the listening in headphones find the place where you get the most cancellation then flip the polarity again for a nice big sound.

Phase issues with a single mic?

Believe it or not you can also get phase issues when using just a single mic. Reflected sound from nearby surfaces like the floor or walls can get into your mics and cause partial phase cancellation. There is only one way to deal with this, and that is at the source. Put down carpet, sound damping materials, lift the amp off the floor whatever you have to do to get rid of the problem reflections. This is one of the only things you can’t fix after it’s recorded. Never time adjust when using multiple microphones, especially for things like drum mics, get it right with your mic placement.

That’s just the beginning

Wow, this article got really long, congrats if you followed along to the end. It was exhausting preparing this article and I feel like I’ve only covered about half of what I should. I didn’t talk about drum miking, stereo, M/S or tricks using phase.

I hope this has been a helpful guide to phase and the importance of the concept has been understood. Comments, praise and concerns are always welcome. Thanks for reading.