Uncovering Pan Law: The Gain Structure Behind Panning (And Why It Can Mess Up Your Multitrack Playback)

Pan law is something you may have heard about from a textbook or maybe from a video online talking about it. Generally speaking, it is not the most exciting audio topic; however, it is incredibly important when it comes to proper gain structure.

So what exactly is pan law, and why did you decide to write about it?

First, let’s cover what panning is, just in case. Panning is the ability to control the volume of a signal between the left and right sides of a stereo destination; no matter if that is a stereo aux, group, speaker system, headphones, etc. Starting in the center, as you pan a signal to one side, the opposite side signal is reduced until it becomes completely off. Panning never inherently adds volume, it only removes it from the opposite side.

Pan law is the amount of volume compensation that happens when panning a mono signal to the center of a stereo destination. When you have a centered mono source that is being reproduced through a stereo system, that signal is effectively being summed from two channels or speakers. This goes for any mono-to-stereo point in the signal flow of a mixer where a mono source becomes stereo. By the nature of something being played back from two sources instead of just one, we can assume that the reproduced signal is inherently louder. This is where pan law comes into play.

Pan law is measured in dB by way of how many decibels the mixer will reduce the volume as the mono source is panned from one side to the other. There are three main pan laws that I have generally come across with one interesting addition which we will cover at the end. These settings are typically 0dB, -3dB, and -6dB. This means that when a mono signal is panned center, it will be reduced by the corresponding amount. It is also worth noting that this compensation is variable as the panning is changed from side to side where the max compensation happens in the center based on the pan law set. Now, let’s go one by one and explore each of the pan law settings.

0dB

First up is a pan law of 0dB. This setting in practicality is not that useful, but it is the easiest to comprehend. When a mono signal is in the center at a pan law of 0dB, neither side is compensated for the volume jump caused by the same signal being recreated in both the left and right side. Therefore, the signal will sound much louder despite no gain actually being added. It is only the result of going from a mono to a stereo signal.

-6dB

Second is a -6dB pan law. Based on the name, we can correctly assume that a -6dB pan law would reduce the centered mono signal by 6dB. This value comes from the rule of doubling amplitude, where adding two identical signals results in about +6dB in volume increase. The -6dB pan law provides you will an equal volume of a mono signal across the entirety of the stereo spectrum. This sounds like the ideal choice, but you have to think about signals that are not mono. A reduction of 6dB for correlated but not identical sounds such a stereo keys sounds would suffer from a drop in center information because they are not identical from left to right.

-3dB

This is where the third option comes in. -3dB pan law is a result of the rule in doubling power, where doubling two independent noise sources results in a 3dB volume increase. This rule considers that not every source will be mono and that there will be some variation. As you pan a mono signal from one side to the other, there will only be a drop of 3dB. When it comes to stereo sources with somewhat correlated information, the center image will feel more present. This is also a pan law that is considered a compromise for situations where the playback system is not tuned properly. This leads us into the outlier pan law I referenced before.

-4.5dB

One console manufacturer decided that they did not want to adhere to any of these options. Solid State Logic (SSL) decided that since the odds were that their consoles would primarily be used in well treated studios, they made a further compromise in pan law using a value of -4.5dB, a split between equal power and equal amplitude.

Great, so thanks for that Wikipedia explanation, but I could have googled that.

Despite how simple this concept is, we as engineers do not usually think about pan law as a point of gain structure. If you read the title and have gotten this far into this post, you may be able to infer what I am about to say next…

As a Reaper user for the last few years for all of my multitracking needs, I am slowly learning all of its quirks. Most recently, I was using a new console that I was not as familiar with. I set up virtual sound check and at some point during this process, I realized that my playback was louder than when the band was playing. Things just seemed off. Then it dawned on me, I was using a new installation of reaper on a new machine. I had forgotten to change the default setting of the pan law.

Wait so you’re telling me the pan law of Reaper made your playback louder? How does that even make sense?

Reaper handled channels much differently than other DAWs. A “track” in Reaper can be anything. It can be mono, stereo, multichannel, or even a MIDI track. Despite setting up my multitrack recording to be mono channels with mono inputs and outputs, Reaper has two defaults that cause an issue with mono in to mono out playback. The first is that the pan law is set to 0dB by default.

This can be changed by going to File>Project Settings>Advanced>Change the Pan Law to -6dB and click “Save as default project settings”.

Second, the reason this is an issue is because of the multipurpose track architecture, both hardware and software sends can be picked off in one of three places: pre-fader (pre-fx), pre-fader (post-fx), or post-pan. The default is post-pan, which would therefore include any gain structure that is a result of pan law.

If you want to operate a different pan law, but still want mono outputs, you can choose either of the pre-fader modes to be the default under Options>Settings>Track/Send Defaults and change the default to whichever you prefer.

I say all of this as a precaution to those who use Reaper. I feel quite silly I did not realize this sooner. Also, I do not blame Reaper for this because much like everything else in their software, it is all customizable, you just have to know where to look. Let me know if this is something you have experienced or if I am the only ignorant Reaper user.