Loudness FAQ: Target level
You recommend a target level of -23 LUFS. How does it relate to -24 LKFS?
It is closer than you may think. The EBU recommends to normalise audio according to EBU R 128 , which specifies a target level of -23 LUFS. In the USA the ATSC recommends -24 LKFS. Besides this 1 'dB' difference and the use of LKFS instead of LUFS (which is only a difference in naming), the idea is quite similar. But only comparing the target levels is not the full story, as the specs also differ in the tolerances and the maximum True Peak Levels which are allowed. One could argue the EBU spec allows for a more predictable approach, as it has a smaller tolerance window.
What is the difference between LUFS and LU?
LUFS is the unit that is used to express loudness levels on an absolute scale, while LU is the unit for differences between loudness levels, in other words, loudness levels on a relative scale. So, a programme that has a loudness level of -23 LUFS is 2 LU quieter than a programme that has a loudness level -21 LUFS. LU can also be used as the units for loudness levels relative to the target level. It is anticipated that common usage could be, "You are 2 LU low," rather than "You are at -25 LUFS."
If you are familiar with the decibel (dB) as a unit, you will know that a dB is an expression of the ratio of two levels - the level to be described, and a reference level. The postfix to the dB tells you the reference level, for example, dBm is referenced to 1 milliwatt, dBu to 0.775Vrms. LUFS is a measurement on a decibel scale and is relative to the loudness level of stereo (front left and front right) 1kHz tone peaking at 0dBFS.