wst3
(just remember, you asked!)
 
The decibel is a power ratio! By definition it is 10 times the log of the ratio of two powers, and it is always RMS.
 
So right there we just shot down using dB for voltages, let alone the number of bits that are turned on.
 
Or did we?
 
Engineers are a curious lot, and once they figured out that we hear things on a logarithmic scale (this applies to apparent loudness as well as frequency - pretty cool eh?) they wanted to apply it to everything!

It turns out that we can do a little algebraic magic and use that same dB to describe the ratio of two voltages - we just have to use 20 instead of 10 as our multiplier. (one of these days I am going to write this up nicely, with equations and all that stuff, for now you'll just have to take my word for it!)

But we still dealing with ratios - how can we use the dB to describe levels?

Instead of measuring input and output we can replace the denominator with a known value, and then our dB measurements will provide us with something we can hang our hat on.
 
This is where it starts to get really messy, and since I'm lazy I'm going to just blow through this  next part.

The original reference was 1 mW across 600 Ohms which turns out to be approximately 0.7746 Vrms. That 1 mW reference was known as 0 dBm, or sometimes 0 dBm0, and it is a power measurement.
 
But wait, if we know that's the same as 0.7746 Vrms we can use that, right?
 
Yup, but you know we have to mess things up a bit first! Some folks starting using 0 dBm as a voltage measurement, with or without the impedance specified. It was ok, but it wasn't completely accurate.
 
So we ended up with a new reference, 0 dBU, which some folks claim stood for unterminated, I've never been able to prove that. But it was better. Then Teac threw a wrench in the works and presented yet another reference, and if you think a little it makes a lot of sense, they proposed 0 dBV = 1Vrms. Makes the math a lot easier. For a while we had all those, and more, but these days we can safely say we have only two references:
- the "professional" reference is 0.7746 Vrms
- the "consumer" reference is 1 Vrms
Keep in mind that each of those voltages is the 0 dB mark for that scale.
 
Which brings us to nominal level or normal operating level!
- in the professional world we use +4 dBu which is equal to 1.228 Vrms
- in the consumer world we use -10 dBV which is equal to 0.3162 Vrms.
And this is where we can finally state the difference between 0 dBu and 0 dBV is about 7 dB and the difference between -10 dBV and +4 dBu is about 11.7 dB,
 
Are you confused yet? Good, I'll spare you the VU scale then, and we'll just carry on.

This is where it gets a bit ugly, since the dB was never meant to be used to describe peak values, but as soon as we enter the digital realm that's all  we have, so we've invented the dBFS, as stated earlier, that's the ratio with respect to "all ones" - the highest level we can represent.
 
There is no standard! None!! Zilch!!! Nada!!!!! Zip!!!!!

Yeah, that annoys me just a little bit.

What we need to do is send a sine wave (mid-band is fine) out through our D/A converter, and measure the voltage.
 
For sake of argument lets assume we have a +/-15VDC power supply, and somehow our circuit will operate from one rail to the other, so the maximum voltage we can generate is 30V peak-to-peak, or about 10.6 Vrms or about 22 dBu or 20 dBV.  That won't happen in the real world of course, if it did we'd have 18 dB of headroom (+22 dBu - +4 dBu = 18 db).
 
Which, as it turns out, is how I've set up my studio, but that's really conservative, and you may wish to use 12 dB down from 0 dBFS as your nominal operating level.
 
To do so you send out your -12 dBFS sine wave and measure the analog output. You'll want to attenuate the output externally till -12 dBFS measures 1.288 Vrms or +4 dBu.
 
I've just re-read this, and I'm tempted to delete it, but what the heck - did this help at all? Perhaps this will generate some questions that I can answer more succinctly!

One other very important note - thus far I've limited the discussion to electrical measurements. When we start dealing with sound pressure level it gets even more complex. And then there is the "K System", but I'm stopping here!

Hey Bill thank you for going through the trouble of explaining some of this stuff. It's much appreciated but I had to take two aspirins after reading it and I don't think I want anymore info about it, I'll just use my ears. Lol.

As far interfaces go I was curious about mine and based on the info I got from Motu I wanted to know what is the best recording level to get the best out of it. They provided the following:

Motu 828MKII Hybrid Max line input and output is +20 dBu. Therefore analog reference is 0dB VU = +4dBu = -16 dBu on the inputs and outputs.

Not knowing what that really means I posted the question here. I hope you can clarify for me since you understand this stuff.
 
Thank you

There is a very simple answer.  If an interface can put out a maximum level of +20 dBu then it means at + 4 dBu it has 16 dB of headroom.  Use a VU meter plugin and set it for -16 ref level.  So your initial observation is correct.  You can work at -16 dB FS as an rms ref level.
 
Except you don't have to though.  You can work at any ref level in reality. e.g. -20.  It just means your interface is putting out 0 dBu at that ref.  Or at -18. (your output level will be + 2dBu in this case)  You do not have to work at the digital ref level that puts out +4dBu. 
 
There will always be plenty of level in order to feed your powered monitors.
 
Some USB powered interfaces only put out + 4dBu max level so when you are working down at say -18 for example it means they are only putting out -14 dBu. (Not a lot)  That is what you get when you try to run an interface off a 5V power supply. Any decent interface will have an internal power supply with higher supply rails e.g. +/- 15 V for example.
 
There is a basic standard and it is 0 dBu = 0.775V. +4 dBu = 1.23V It used to be into 600 ohms but now the impedance value has gone and the impedance can be and is usually high.  dBu can be used to measure voltages.  I have been doing it for years! 
 
Be careful making -12 FS your DAW rms reference. You can clip more easily. I work a lot at -14 but only because my digital mixer is calibrated there and then everything matches nice between the DAW and the mixer. -20 is a great ref for the best headroom. The industry has accepted -18 as a common ref as well. Higher ref levels can be used inside your DAW but mainly in the mastering stages. When I master I recalibrate by VU meters so they are always showing 0 dB VU. But what varies is what level is doing that. 
 
Think more about the internal DAW ref level, not so much what the output from your interface happens to be. At least you have chosen a decent interface and it will have plenty of headroom.
 
K System has introduced 3 other ref levels compared to the basic industry one (-18) They are -12, -14 and -20. K System also requires you monitor your SPL levels in your control room at the same level no matter what ref level you are using.  85 dB SPL comes to mind. Some say its too loud but I think it is perfect.