As a long time amateur artist and sound engineer I still lack many principles and experience. In particular I want to know what professional engineers would consider acceptable dB levels for input signal and noise, why, and what I should realistically aim for.
 
Using:
    SM7B (Large windscreen, HPF)  
    Focusrite ISA One (@50dB, HPF)
    Return port of my Fast Track Pro (bypassing preamps)
 
I read peak levels of no more than -80dB (-90db down to -100db RMS) in my DAW across the spectrum. This seems to be a very low level of noise. The only audible noise is amp hiss. No room noise.
 
When singing at medium level, 3 inches from windscreen, I peak around -15dB to -10db in my DAW.
 
I am in the process of producing an acoustic "unplugged" album with just my voice and acoustic guitar. I want to make sure I am on the right track before I commit my process to the whole album. Do these input levels meet professional scrutiny? 
 
Please do educate me.

can't really answer your question with the information at hand, but I can give you some ideas...
 
Back in the mid 1990s (I think) Louis Fielder, one of the engineers at Dolby Labs, wrote a landmark paper titled something like Dynamic Range in Digital Systems. In this paper he suggested that the proper reproduction of a musical performance could require as much as 120 dB of dynamic range. That's a BIG number, a ratio of 1,000,000,000,000:1 (yes, that's 10^12 - feel free to check my math!)
 
That is theoretically possible, and while some chips claim that dynamic range, it is more of a marketing number than a real number.
 
Let's think about that for a minute... a really quiet recording space would be built to have a noise floor on the order of NC20, or about 20 dB-SPL broadband, average. And it is generally accepted that the threshold of pain occurs somewhere around 130 dB-SPL (depending on age, physical condition, and the content of the noise.) So it would seem that we are already in trouble<G>!

So what's practical? Or realistic?
 
You have to start with the room! How quiet is it in your room. You'll need to borrow (or buy) a real sound pressure meter if you want to know the answer, but you can get by with your recording gear if all you really care about is the S/N ratio in your recording.
 
But whatever you end up with is, well, what you end up with. The noise in the room is the noise floor, it's only going to get worse as you pass it through transducers and electronics, and especially modern (read low power) A/D converters. For sake of argument, lets say that the average SPL in your recording space, across the entire audio band, is around 30 dB-SPL. So even if you could hit (and survive?) 130 dB-SPL you'd still only have 100 dB of dynamic range... IF none of the subsequent stages distorted or created noise of their own... and they all add noise!
 
Practical... gotta remember that word!
 
I usually monitor so that the wide band, average level (slow response) is around 85 dB-SPL. Remember, this is what is delivered to my ears, this still has nothing to do with the electronics, that's a whole 'nother matter! And we, by which I mean "I" keep throwing that word average in...
 
It turns out that the human ear is sensitive to both average and peak levels. And that is one of the places where people tend to misunderstand poor Louis's assertion. He was thinking about the whole kettle of fish, and especially peaks. Peaks take on a couple of forms in music. There is a peak when you strike a snare drum that is very short in duration, but depending on the drummer and the drum, it can be REALLY LOUD for a fraction of a second.
 
And that' what Fielder was trying to preserve. Well that, and the natural "crest factor" of complex waveforms. And that's what you need to consider as you set up your system.
 
We really don't know much about the energy in the noise floor you are measuring in Sonar. We don't know who the contributors are, or what they contribute, but since you probably can't do much about it, and 90 dB down for an average noise level is quite decent really, let's forget about it, and worry instead about the upper end.
 
You probably know that you can not exceed 0 dBFS - which is the maximum level that the hardware and/or software can represent. What you may not know is that level is going to be a different physical level at different places in the system, and that can be a problem. You should also keep in mind that ALL dB measurements represent power ratios, even though we use them to represent sound pressure levels and voltages and whatever. And this means that they are, by definition, average or RMS (root-mean-squared) measurements. They are not peak measurements, and they can't be peak measurements.
 
The trick then, is to find a spot somewhere below 0 dBFS where you can operate safely, and by safely I mean:
1) you never exceed 0 dBFS for more than a couple of samples
2) you don't clip analog stages before or after the computer.
 
Some folks think that you should treat a digital system the same way we treated analog tape. That's not a good idea. With analog tape we had a huge noise contribution from the tape, and probably the tape electronics. So we tried to operate as close to 0 VU (an entirely different measurement system) as we could. We don't need to do that any more. Digital storage and transmission do not add noise to the signal, only the channel, and the channel gets tossed away!
 
So unless you are recording the song of the Egyptian Fruit Fly (and since you mentioned amplifier hiss you probably aren't), the key here is to find a level in the digital domain that works for you. You need some headroom to accommodate peaks, and you want to stay as far away from the noise floor as possible... without exceeding 0 dBFS or clipping.
 
Sensing a pattern here<G>...
 
I tossed a fair bit of information that you may or may not need, but I hope it demonstrates the futility of sweating S/N ratios, or even dynamic range, in a modern recording environment. The bottom line is that the noise that is inherent to your recording chain is going to be there. If you record your tracks at a level significantly higher than the noise floor then the noise will still be there, but listeners probably won't hear it.
 
One last thought - a lot of this is pschological, or psycho-acoustic. When I was still working with tape we would use a trick called spot-erasing, where we would erase the noise in between the notes - ok, seldom got that silly, but if the guitar amp was humming a bit we'd erase the tape if the guitar player wasn't playing. It tricked the mind into thinking that the dynamic range of the PERFORMANCE was greater than it really was. (When inexpensive automation became popular we'd automate the mutes to do the same thing.)
 
Have fun... and don't spend too much time worrying about S/N ratio... or reading this post for that matter!
 
 

Much appreciated, Bill. 
 
I'm very much with you on the psycho-acoustic issue. Because I don't know what is an "acceptable" noise floor, I am constantly overcompensating to remove it when I notice it. This causes me to go overboard with compression/expansion plugins resulting in a harsh mix.
 
I should find a target recording to emulate. It's just me and an acoustic guitar but I'm not familiar with many artists like that. My songs range from soft singing and finger picking to loud singing and hard strumming. Any suggestions of artists and recordings to review? Anything is fine. I want to get ideas and discover sonic qualities I like.

If it makes you feel better I don't think you are alone in overcompensating for noise. When I had a "real" studio it was so much easier, but for now I record in the basement, which isn't awful, but it sure isn't quiet either. I try to focus on the fact that the room itself sounds nice when I'm feeling positive, and I try to use microphone selection and placement to minimize the noise problems.
 
Anyway, if I do wander off into that dark spiral of noise reduction after the fact I tend to end up with a mix that sounds awful... usually better to either leave well enough alone, or place the microphone close to the source!!!
 
Suggestions... geez, guitar and vocal... well Richard Thompson immediately springs to mind, his solo stuff is brilliant - all his stuff is brilliant really. Leo Kottke would be another, especially his really early stuff, there is noise there, but you won't notice it. Paul Simon, James Taylor, Dan Fogelberg, Jackson Brown, Lou Reed (don't laugh, he did some stellar acoustic guitar and vocal tracks), Warren Zevon, Townes Van Zandt, Rodney Crowell, Guy Clark, Steve Earl, Jimmy Dale Gilmore, John Hiatt, Lyle Lovett, Vince Gill, Frank Christian, Joe Ely, Robert Earl Keen, James McMurtry, and Tom Waits (his recordings of his demos are often guitar and vocal, they are a bit rough as they were never intended to be published, but still quite cool.)
 
And that's just the male singer songwriters that did solo guitar/vocal tracks that I can think of off the top of my head... there are lots more. And they cover the gamut of styles...
 
An acceptable noise floor is very much a function of the setting. My last studio was down in the NC30 range, which is barely good enough for a commercial facility, and yet now I'd love to be able to get there<G>! So it's also relative I guess...
 
One way to look at it, the way I look at it these days anyway, is that if the listener is bothered by the noise then I goofed. Either the performance is not sufficiently good to keep their attention, or there is a real noise problem (or both I guess.) Once you enter the D/A converter there is really very little opportunity for contamination, and it stays that way until you exit the D/A converter. And from D/A to ears is HUGELY variable and beyond your control.
 
That means you just need to focus on the room, the microphone selection and placement, the instrument (which includes amplifiers) the preamplifier, any processing prior to the A/D, and of course gain staging. Of all of this gain staging is probably the most critical, followed by microphone placement. The rest really should not play into the equation, unless the room is really noisy - at which point we repeat the bit about microphone placement<G>!
 
Does that help a little?

Thanks for the advice, Bill. I'll check out those artists and see if I can emulate something I like using my gear. 
 
Like wine tasting I wish there was a "tasting" studio for audio hardware. It's hard to get meaningful information on forums for preamps and mics. Most advice is too subjective. I was hoping that there would be some guideline numbers I could use to keep me from overcompensating with the gear I have. If I new that my ambient noise (amp hiss in this case) was considered to be at a "good" level for mixing by a professional, I would stop worrying so much about my hardware and focus on getting better at mixing and mastering.
 
Here are my ambient and singing levels for my SM7B + Focusrite ISA One @50dB in my recording space:

(Levels are shown using slow RMS + peak level markers)
 
I realize a snapshot of my vocals is inconclusive, but it stays near these levels the whole track. It's not a very dynamic song.

The SM-7 is an industry standard, but needs a lot of gain.  The ISA should supply that, but that is covered in your other post.  Realistically, both your  level seems a bit low, unless you have a quiet room so you can add gain in the digital realm.  The ISA has more gain available.  I'd try that, +5 or 10 dB.  For such a simple (kinda like dumb questions - there are no dumb question but simple can devolve into complex quickly) project, you should have lots of headroom mixing a vocal and guitar at those levels.  I've found it easiest to lower gain than try to add it after it goes digital, as long as the recording is good.
 
@

Your 70-80dB SNR is entirely acceptable. 
 
Possible exceptions:
1. There are very quiet passages where you can actually hear the noise, in which case a little creative editing, volume automation or noise gating usually takes care of it.
2. You are aggressively compressing the vocal to the point of pumping, periodically raising the noise floor. Solution: don't do that.

In my mix I am keeping my vocal track level at 0 gain and mixing around that. I use an expander to bring the noise floor down a bit while maintaining the same output level as the source.
 
I bring the acoustic guitar part way down until the vocals sit on top and I adjust EQ and stereo spreads to give the vocals and guitar their own space in the mix.
 
On my master bus is some light compression to bring the overall level up a bit but this tends to undo the noise reduction from the expander in my vocal track. I try to balance all these things until I get something acceptable, and I think I have, from a technical standpoint, now I am working on getting it to sound "pleasant". 
 
Does this sound like a good start?
 
(edited first paragraph for clarity)

Try lengthening the release time on your bus compressor to avoid bringing noise levels up noticeably, and of course keep ratios and makeup gain low.
 
You should be able to pull levels up at the end of the chain with a limiter, just being careful to let the limiter only catch occasional excessive peaks rather than being engaged all the time. Since your tracks are reasonably hot at -10 to -15 dB you should be able to preserve that 70-80 dB SNR right through to the end.
 
Now, this may be sacrilege and totally off the mark, but I'll suggest it anyhow...you might consider adding some subtle pads under the guitar. This will go a long way toward masking noise. It can be very subtle, barely above the noise floor.
 
Listen to "New Favorite" by Alison Krausse and Union Station. It's one of the most beautifully subtle, delicate and clean acoustical recordings ever - but if you put on some good headphones and listen very closely, there are - I am not making this up! - barely-audible synth pads in there!
 
Of course, a pad needn't be from a synthesizer. It can be any steady or long-release filler-type tone, including a second guitar lightly strummed with fingers. Organ or strings work well.
 

Thank you for all your help. It has been very informative.