What is RMS?

Earlier I was reminded by a Facebook friend of 60’s/70’s blues rockers Canned Heat’s old “RMS is Truth!” bumper stickers. Someone in the thread asked for a translation and the original poster recounted the Heat’s advocacy of RMS amplifier power output ratings vis a vis other, more ‘marketable’ spec standards.

Root-mean-square. It’s a power averaging method. As noted, it was used by electronics manufacturers with some integrity, while many others used ‘BS’ ratings like ‘peak power.’ 

One place where you may see it increasingly is in ‘smart levelling’ systems for multimedia playback. 

We’ve all likely had the experience of listening to some nice, classic Billie Holiday at a comfortable level in our play shuffle and then some Skrillex comes on and pins your eyeballs into the back of your skull and your ears into the next dimension. 

The deal is that digital formats have a ‘maximum peak loudness’ (0 dB FS [full scale]) that essentially is the loudest the device can put out. Some material is loud all the way through (or mostly) like Skrillex — the producers have intentionally used extreme settings on one or more compressor/limiters to make all the quiet bits as loud or nearly as loud as the quiet bits. (Now, Skrillex has ‘dynamics’ in the sense that it’s almost full loudness through most of the song but has big level ‘drops’ inserted strategically through the course of the song, presumably to keep attention in focus, since the music is otherwise so mind-numbing. J/K.)

So, even if that Billie Holiday record’s loudest bits are every bit as loud as the Skrillex, on average, the level is much much lower.

How much? Such average levels are typically measured in dB RMS. A difference of 6 dB can be said to be equivalent to a doubling of volume.

That Billie Holiday record might have an RMS average level of, let’s say, -18 dB RMS (particularly if it was mastered for CD before the ‘loudness wars’ era of the late 90s on; Skrillex’s big hit (something about Sprites I think) has an RMS average of around – 6 dB RMS. 

That means that the Skrillex record will seem to you to be about FOUR TIMES as loud as the Billie Holiday. (12 dB difference; each 6 dB is about equivalent to doubling perceived loudness.)

Awesome, eh?