In this blog article I am going to try and explain the difference between RMS and LUFS, from a Mastering Engineers point of view. Here at Sigil Of Brass, we use LUFS. LUFS is the modern measurement tool of stand alone loudness. Now here is the thing to think about:
LUFS means “Loudness Unit Full Scale”
RMS means “Root Mean Square”
Both measure the so called “average signal strength”, that some call “loudness”, but both work fundamentally different. At least if we talk “pure measurement” then LUFS would be LKFS (Loudness, K-Weighted). In usability, they’re indeed “similar” (if we ignore the age of the metering tools and their general tasks – they can always be fine tuned, and used for “more” than just their basic function – if you know how).
- RMS is measured “unweighted”, meaning: flat frequency response.
- LUFS uses a “weighting filter”, which filters out the extreme low frequencies and puts an emphasis (boost) on everything above 1kHz. Therefore if you send in a Pink Noise signal, it should be offset by a couple of dB. Since RMS meters respond stronger to bass heavy material, and the LUFS meter does not (compensated!), the readout is a bit lower on a static signal.
- RMS uses “common known” time frames (it’s a really complicated topic that even I don’t fully understand) of 300 or even 600ms (the Dorrough meters first coined the term “RMS meter”).
- LUFS uses 400ms (called Momentary) and 3s (called Short Term) for the realtime meters, and an average value (declared every 2-5s) from the whole measured stream so far (really complicated process).
Another important thing:
- RMS measurement exists in two “flavours”: RMS, and RMS+3 (AES-17 compensation). These days, the AES-17 compensation recommendation is in use. So a Sine Wave at – xyz dBFS reads out both the same vale on max digital peak, and RMS. Else the RMS value would be 3dB lower. Also, each channel responds independent.
- LUFS “sums” the signals (which automatically covers the +3dB AES-17 compensation), which on the long run is easier for loudness measurement for multi-channel program streams (read: surround)
What The Hell You On About?
Okay, now that we got this out of the door, we can look at the meters themselves, and what is roughly what equivalent.
Let us assume that you shoot for your desired -8dB RMS average, meaning about -5dB RMS absolute max, which tends to be the limit for MOST modern productions these days – and also results in a “squarewave.”
Summing Up – How We Help Your Music At Sigil Of Brass
We’re not into the Loudness War so we use tools to Normalise to get your audio to the equivalent of -8RMS (for the club) and -14 LKFS for streaming (give or take a lot of high-tech wizardry). This means you will sound great in Clubs and in balanced listening conditions. You will have a copy to submit to Streaming services and you will have a copy at retail CD volume.