Why -19 LUFS

As you can see in the Standard, we have specified
-16 LUFS for stereo and
-19 LUFS for mono and Split Track.

Why that difference?

The brain cumulates all the auditory perceptions your ears perceive and translates it into a "noise level information".
Loudness calculation does the same thing, it adds the energy from all the channels to compute the Loudness.

When you playback a stereo file, the left channel feeds the left loudspeaker and, you guessed it, the right channel feeds the right loudspeaker.
The accumulation of all the "noise" gives you the impression of audio level.

If we want to reach a level of -16 LUFS with two loudspeaker, each loudspeaker will contribute for a part.
That part is -19 LUFS! (because -19 dB + -19 dB = -16 dB!)

Now what about mono?
Most of the time, mono files are played back on stereo systems which feed the same signal to the left as well as to the right, effectively doubling the energy.
In order to achieve the same result as with a stereo file, the mono signal should be a - 19 LUFS.

That is all!

If you don't quite understand,try it for yourself.
Take a -16 LUFS stereo file (not a split track!) and analyse both channels separately...

Why -19 LUFS for Split Track?
Split Track is actually a single-channel content spread on two channel. Typically to allow extracting the main speech that is on one channel and remove the (copyrighted) music and moderation  that should be on the other channel.
A Split Track file is not meant to be used as such. The two channels need to be combined or extracted before final distribution. And that is where it comes down to mono again.

Would it be a problem to normalise a mono file to -16 LUFS?
Technically it is possible but that allows for 3 dB less dynamic, or Peak-to-Loudness-Ratio (PLR).
A -19 LUFS mono file with peaks at -1 dBTP has a PLR of 18 dB. That is the same for each of the two channels of a stereo file.
A -16 LUFS mono file with peaks at -1 dBTP has a PLR of 15 dB!

But there is a twist...
Depending on the DAW (audio editing software) you are using, it might be that mono tracks are handled in a stereo environment. That means they are played "centered". In other words they play on the left channel as well as on the right channel, effectively doubling the energy.
If that is the case, your Loudness reading might be 3 dB (actually 3 LU) too high!
For example, a mono file that has -16 LUFS (not TWR Loudness compliant!), will be measured having -13 LUFS.
Please go to the Part 2 or the Loudness webinar for more details.