If you’ve got any kind of relatively advanced audio diagnostic plugins which are used in mixing and mastering for determining levels, you may have seen volume represented as an RMS value. Let’s identify what is RMS in audio and why it’s important in accurately appraising the volume of your song either in mix or master form.
What is RMS in Audio
Before we identify what is RMS in audio, let’s identify what RMS stands for in the context of audio.
RMS is an acronym for “Root Mean Square” and without getting too mathematical in explaining what is RMS in audio, suffice to say that it’s a measurement of the AVERAGE level of music or sound.
When we think of a level in audio, we typically think decibels. After all, audio level is most often quantified in our digital audio workstations like Ableton Live, Pro Tools, etc. as dBs, or decibels.
Decibels are merely a sample to sample measurement of the volume’s peak in that moment.
Decibels are useful to know when we’re setting the threshold for a compressor, for instance, because compressors are all about taming and bringing down peaks to achieve a more consistent average level with less dynamic range.
Outside of guaranteeing our track, bus, or mix aren’t clipping, meaning passing the 0dB ceiling and distorting, decibels DON’T give us an accurate picture of the overall volume of a song or even a section of a song.
This is where RMS comes in – it’s a quantified measurement of the average level because it takes into account the average dynamic range of your audio, or in other words the difference between the quietest and loudest moments.
How is RMS in Audio Calculated
This is where things get a little mathy, as you might expect with an acronym which translates to Root Mean Square.
RMS in audio is calculated by squaring each sample of your audio, taking the mean of those square values, then finally taking the square root of that mean value.
This is then represented as a number in plugins which measure it for you, as is the case with Izotope’s Ozone mastering suite:
Measured in negative numbers, the thing to remember with RMS is that a “larger/higher” figure suggests a greater amount of dynamic range (and vice versa for a “small/lower” figure).
Furthermore and as you already know if you saw my overview on dynamic range, less dynamic range translates to a louder average volume.
In other words an RMS of -14 suggests more dynamic range and a quieter average volume than an RMS of -10 which suggests less dynamic range and a louder average volume.
What RMS Should Audio Be
This brings us to the final and obvious question of what RMS level should you aim for?
This is a loaded question and the answer will vary from genre to genre, not to mention track to track (or better said, master to master). For instance, EDM generally has far less dynamic range and thus those masters will have a lower/louder RMS level than orchestral music which is all about the dynamics.
This question also brings into focus the ongoing struggle between the importance of dynamic range and masters needing to be louder to be competitive with their contemporaries (see my overview on the Loudness War).
By definition, dynamic range is a casualty of achieving louder mixes and masters because dynamic range is sacrificed to make a song consistently louder on average.
Of course, MOST music listeners consume their music through audio streaming services like Spotify and Apple Music, and MOST of their users use the normalize feature which is on by default. In other words, every song plays at virtually the same level by design through the service so listeners don’t have to adjust their volumes between tracks.
Rather than recommending specific RMS levels for submissions, Spotify and Apple Music recommend target LUFS (see LUFS vs RMS) as this is a better representation of PERCEIVED volume, making it a more practical way to gauge average/overall volume.
With that in mind, check out my overview on what LUFS to master to as I show both what LUFS each streaming service recommends (they’re all very close) and give a free plugin you can use to measure your mix or master’s LUFS.
As a final word (and I know it sounds like a cop-out), don’t worry about a specific target for RMS but keep as much of your music’s dynamic range intact for a more natural, better sounding, and more ENGAGING song for your listener which will keep them locked in as the volume climbs and falls rather than serving up a fat audio sausage which will lose their attention (and you a listener).
