It is safe to assume here as an audio engineer you want to take full advantage of the stereo field in your mixes. It is also somewhat of a mystery amongst aspiring audio engineers why a good stereo mix can be a little tricky.
Employing Stereo Panning does not guarantee your project will sound wide. Nor does it ensure if played back in mono it will retain all of your tracks. Even phase can be a major detriment to a stereo mix. So let’s get a basic understanding of Stereo before we go into how to achieve a superior stereo mix.
LCR (STEREO) PANNING
Panning in a two channel or left/right mix seems pretty simple at first. You have basically a 180 degree sound field that is broken into three different segments: Left, Center, or Right (LCR). Panning a guitar hard left essentially removes it completely from the right channel. Leaving it dead center leaves 50% in both the left and right channels.
What you are essentially doing when stereo panning is fooling the ears such that it perceives the sound sources are coming from multiple locations. Scary movies do this well in theaters with surround sound; the crack of a twig under the foot of an approaching axe murderer behind you is no different than a cymbal panned hard right. Stereo gives us depth and width, and most closely represents how we hear in real life.
But don’t think your stereo mix is going to be played in stereo always. If you are “lucky”, it may be played over a Mono system in a grocery store, at a bar, or even at an outdoor festival. So even your Stereo mix needs to sound good in Mono.
MONO MIXING
On the other side of reality, ever heard a good stereo mix on an elevator? Probably not. This is because most public broadcast systems work in Mono.
The first time I recognized a Mono signal, I was listening to Black Dog by Led Zeppelin over a loud speaker at a store in the early 80’s. In the intro, Robert Plant’s vocal is panned hard left for one part and hard right for another. When the left channel vocal was being played, you couldn’t hear it at all.
My first reaction was something happened to the speaker. Little did I know stereo panning and the resulting playback of only the right channel were actually at play here.
But Mono isn’t simply playing the left or right channel, it is the sum of Left and Right channels. In fact the calculation for gain in Mono = (Left + Right) / 2. You don’t gain loudness in Mono, because the sum of the two are divided by two, but there are some tricks to panning and mono that you should understand.
THE 6dB RULE
If you take a single track and duplicate it in the mix, you will add ~6dB to the Master:
This is not only a fundamental rule of gain staging, it is also a critical first step to understanding Stereo/Mono Mixing.
NOTE: Hard panning duplicated tracks only adds gain, and won’t make it sound more “stereo” because they simply sum to Center.
Now pan each signal hard left and right, you will see the 6dB boost on your Stereo Out drop by ~3dB, but notice each of your tracks are now 3dB louder than when they were panned Center:
What is happening here is call the Pan Rule which states: when a signal is panned hard left or right, the perceived loudness must be maintained, therefore it is automatically increased by 3dB.
Pan Rule may be different from DAW to DAW or console to console. SSL actually has a 4.5dB rule on some of its consoles, while some DAWs allow you to set the Pan Law at a number of different increments. All the same, it is meant to maintain the loudness of the track equally no matter where it resides in the stereo spectrum.
What have we proven so far:
1.) Duplicating a Centered track adds ~6dB to the Master
2.) Hard panning the duplicated tracks reduces the Master by ~3dB
3.) Hard panning tracks will increase by ~3dB of gain
Simply put, a well panned mix will result in less overall gain on your Master, but you need to be careful when panning tracks because the 3dB boost may actually cause clipping on some tracks or upset your balance. This is why most mixers will pan early in their mixes. Fiddling with panning after levels have been set can be a recipe for frustration.
THE STEREO / MONO RULE
So now we know Stereo panning will impact overall gain, but does Mono result in the same odd phenomena?
Change your Master, with two duplicated tracks panned hard Left and Right, to Mono. Notice the Stereo Out will only increase slightly:
But now pan both tracks back to Center, and in Mono, the Master returns to its original peak level when you duplicated the tracks (i.e., reversing the Pan Rule):
So regardless if you are in Stereo or Mono, overall gain is essentially unchanged, only panning creates changes in gain.
CENTER PANNING
So why not just mix everything centered, with no panning? This seems to solve the problem with the 6dB and Pan Rule right? Well, yes and no. Granted if you are writing Muzak, stereo panning is not even an option. But mono music is not only lifeless, it makes even a simple mix nearly impossible to deliver with any level of quality.
For example, say you have a project that has two guitars, a vocal, bass, and drums. Stacking the vocals on top of the guitars in the center, you will find it is very difficult for either instrument to shine, punch through the mix. Now add Bass and Drums to your “mono” mix. It sounds like a mess right?
Now just hard pan the guitars left and right. All of a sudden your vocal is pretty much all on its own in the center and the guitars can be heard with clarity. They aren’t stomping on each other in the center, clamoring for the same frequencies and share of the speaker.
In it’s simplest form, Stereo Panning is powerful to a mix, but it’s way more than just LCR, it is a delicate balance of gain staging a full Stereo field that also works in Mono.
WHY CARE ABOUT MONO?
There are some industry experts who give Mono mixes very little attention. Apart from phase issues that can result from spreading tracks in Stereo, that are then nearly lost in Mono, most engineers will mix a project to sound best in Stereo, and ensure the mix is acceptable in Mono. But if you mix exclusively in Stereo and do not validate your Mono mix is OK, you may also encounter some very disappointed customers when they hear their song over a mono phone or Bluetooth speaker which surprisingly are still very popular today.
Phase Cancellation occurs between two tracks that have similar content but the waveforms are not in sync.
Double miking a guitar cab and setting the capsules at non-equal distance from the speaker cone is one of the most frequent occurrences of Phase Cancellation (see Phase Cancellation section in Recording Electric Guitars: Part II for a deeper explanation).
The same occurs when comparing drum overheads to a snare mic. The distances between them can cause phase issues which can be quite obvious.
Simply put, Phase Cancellation occurs when two similar waveforms are conflicting with each other. When played in solo, each sounds strong, but when played together, a noticeable - and sometimes not so noticeable - loss in signal occurs.
When approaching a Stereo mix, even if all your tracks are perfectly in Phase, it is easy to accidentally create Phase Cancellation. Which is why using Mono or another tool like a Correlation Meter to check your mix is so important.
For example, say you take a guitar lead panned center, just a single track, and apply a heavy reverb. When the reverb persists over the top of the original signal, it can cause Phase Cancellation. This is why a lot of Reverbs come with modulation that modifies the waveform of the reverb signal to keep it from going out of phase with the original signal.
Phase Cancellation can also occur using a Stereo Spreader like the Waves Doubler which provides the ability to add a delay to one or both sides. Remember I said above simply doubling a track in stereo does not make it more stereo? Adding a delay to one side between 5-40ms will achieve what is called the Hass Effect, which very much sounds stereo, but there are limitations.
Under 40ms, and the listener perceives the two signals as coming from the same source. Beyond 40ms, and the listener will consider the second source as a slap-back echo, not a true stereo image.
Other stereo imagers employ EQ, mid-side balancing, and even pitch shifting to achieve a wider stereo sound. But again there are limitations to their effectiveness.
If you start delaying one side of a stereo signal or modifying the waveform too much, you are likely going to start creating Phase Cancellation. Push it too far and the cancellation will be detrimental.
Long story short, Phase Cancellation is very easy to create, but the good thing is it is also pretty easy to detect and correct.
FIXING PHASE ISSUES
First off, even if you can’t hear Phase Cancellation, you can detect it with a Correlation Meter:
Simply placing this meter on your Stereo Out - it is also found in the Logic Multimeter - and soloing the tracks in question will allow you to check for Phase issues.
There are basically two sides to a Correlation Meter:
In-Phase (Green) - When the dial is between 0 and +1, the left and right channels correlate positively and your signals are in phase.
Out-of-Phase (Red) - When the dial is less than 0, this indicates Phase Cancellation is occurring.
So if you are detecting Phase Cancellation, what can you do about it?
Multiple Tracks - Well first, if you are referencing two completely different tracks, like drum overheads and a snare mic, I would examine the waveforms themselves and make adjustments until they no longer are out of phase (see Phase Cancellation section in Recording Electric Guitars: Part II for a deeper explanation).
If, however the Phase Cancellation is the result of a single track you spread stereo, or an effect like reverb, you may need to adjust some settings.
Single Track / Spreader - When I am dealing with Phase Cancellation as a result of a stereo spreader, I will usually try to eliminate any delay in the signal, and then slowly move up the delay, watching the correlation meter, until the Haas effect is noticeable and I am still on the green side of my Correlation Meter.
Single Track / Effect - If the cancellation is the result of an effect, you may want to add modulation to it, which modifies the waveform and will likely remediate most of the issue.
STEREO SPREADERS ON A MIX BUS
If you talk to most engineers, they are very hesitant to use a Stereo Spreader on a mix bus. There are many reasons for this but the primary one is the degradation of the center versus the enhancement of the sides. It’s called “Losing your Center”.
Most accomplished engineers will tell you if you cannot achieve a wide mix without a Stereo Spreader, you likely need to go back to basics.
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