If you take a look in your DAW settings, you’ll find something referred to as either pan depth or pan law. These two interchangeable terms refer to the same thing, but what exactly is pan depth and why is it of relevance?

In essence, pan depth is a setting which governs the amount of signal attenuation when a mono track is routed to a stereo output and panned centrally. This is important in order to maintain a consistent perceived level, whatever the pan position.

The graph below illustrates how pan depth works. Note that, in this example, the signal level is down by 3db in each speaker when panned centrally vs when it’s hard panned to either side.

Pan depth is expressed in decibels. In Pro Tools, there’s a choice of 4 different settings:

• -2.5dB

• -3dB

• -4.5dB

• -6dB

Of these, -3dB is the default, and this is the setting which will correctly compensate for the acoustic summing caused by a centrally panned sound playing through two speakers in most rooms.

If there were ever to be an acoustically ‘perfect’ room, the level of centrally panned signals would actually increase by 6dB due to the acoustic summing within the room. A -6dB pan depth setting would compensate for this unlikely eventuality. Some old broadcast mixers used a pan depth of -6dB, as this setting gives an even balance when stereo mixes are summed to mono. The -6dB setting is still there in Pro Tools and other DAWs should you ever need it. 

The -4.5dB setting is a compromise between the -3dB and -6dB options.

A pan depth of -2.5dB was used in some earlier versions of Pro Tools, in particular Pro Tools LE back in the day. These differences in stereo pan depth can mean that a mix might sound different as you move between systems if they’re set to a pan depth other than the one you used when mixing. A -2.5dB pan depth is still there as an option, in case you need to open old sessions and maintain the original balance.

As a general rule then, -3dB is the best setting to choose in most cases and is the default. In fact, when mixing in surround, the pan depth is actually fixed at -3dB. So why else is pan depth relevant if the -3dB setting works most of the time? The answer lies in metering.

Take the example of a mono dialogue track routed to a stereo output. If you want to meter this with any of the standard metering options available in your DAW, you have the option to do this on either the dialogue track itself or the master bus. For the sake of simplicity in this example, we’ll assume that the dialogue isn’t routed though any auxes along the way.

In the example here, we can see the metering from both a mono dialogue track and the stereo output to which it is routed. The metering type is set to Sample Peak. As you can see, the mono track reads around -10dBfs whereas the master reads about -13dBfs. This 3dB difference is due to the stereo pan depth. Also bear in mind that metering on an audio track in Pro Tools can be set to either pre or post fader, so that’s another potential point of confusion.

This type of difference, equivalent to the currently set pan depth value, will be seen across most metering types, certainly all of the default options available in Pro Tools.

Based on what we’ve said, you might expect that this same difference would also exist when using LUFS meters. Actually though, loudness meters of this type will read the same values on the mono dialogue track as on the stereo output. This is due to the fact that LUFS metering measures loudness as perceived by human ears.

In the example below, we can once again see the metering from both a mono dialogue track and the stereo output to which it is routed. The meter at the top is on the mono audio track and the one underneath is on the stereo master fader. As you can see, the Integrated, Short-term and Loudness Range (LRA) values are the same in both. The 3dB difference due to pan depth does, however, show up in the True Peak value at the top right, with the mono source track reading -1.3dBTP and the stereo master reading -4.3dBTP.

I’ve used Nugen VisLM here, but the same will hold true for any LUFS meter.

The thing to take away from this is that, because of the way pan pots work, and their associated pan depth setting, when using most metering types, a difference in measured level will occur on mono tracks vs the master, equal to the currently set pan depth or pan law setting for centrally panned sounds.

The one exception to this rule is metering based on the BS.1770 standard, including LUFS meters, which will only display a difference in the True Peak value, but not in actual loudness. This makes complete sense when you think about it because the whole point of having a pan depth setting is that the loudness of a sound should be perceived as remaining uniform across all pan positions. If it sounds the same, it reads the same, at least with these kinds of loudness meters. Be aware of what pan depth is and the implications it has on metering. That way, you can be sure that you’re always metering things accurately and correctly.