Well, well, well spectrum analyzer, eh?
Here's the free RTA I use and recommend, Voxengo SPAN:
http://www.voxengo.com/product/SPAN/
Unfortunately for you it's VST which Pro Tools can't use (I don't believe).
There's another one by Elemental Audio, now Roger Nichols owns it, for Pro Tools (not free - big surprise?):
http://www.rogernicholsdigital.com/inspectorxl.htm
I've got
a Behringer DEQ2496 that has an RTA in it but even though it's 1/6 octave (specs) in reality I think the FFT size used is a little smaller than 1/6 octave so the resolution isn't what you would expect or need IMO.
What folks here are trying to tell you is get your skills and equipment together since you might seem a little overly-confident about the RTA fixing a lot of issues. I don't know what mixing/mastering equipment or monitoring or skills you're using to push your mixes into the "rap zone" that your using for reference. You're up against some pretty ferocious equipment though! Good luck.
Besides all that, back to the RTA. I use one daily to help judge sub, bass, lo-mids, hi-mids, treble when I can't hear it. I'm in an apartment so there are certain hours where I can only do setups and fine-tune at some other time when I can turn it up (still not too loud here, < 74dbSPL I'm sure). Then it's out to the F150 truck stereo for the final judgement. You do what you gotta do. Also there's at least 2 times a year when my eustachian tube blocks and my hearing isn't up to par - more info than you needed?
So if you had the RTA, what are you looking at? Using Voxengo SPAN as an example you're looking at upto 8K bands (16K FFT) of spectrum info and readouts, some of it dynamic, some of it static between 5Hz and 30KHz.
The RMS/PRMS readouts tell you where the average audio envelope sits in db relative to 0dbfs. This average envelope RMS db readout is meant to represent the average loudness of the audio you hear thru the air referenced as dbSPL. The PRMS is just the maximum or "peak" rms during the reference window - this accumulates over time until you click the readout to reset it and the average can build again. It's good to reset the readouts if you're looping over a passage fine tuning something.
Next the spectrum display can be set in a way that will either average the frequency display very slowly or allow it to respond very quickly and dance with the music. In slow mode the frequency display will build over time and you will see what shape the general spectrum holds. This can show you a general tilt such as 3db, 4db, 5db per octave or whatever your reference target is. This can show you where to set a lo/hi shelf, HP/LP or wide peaking EQ. Also you can find a resonance if you're not too good at sweeping - or if you're using a digital EQ with a cut but it doesn't seem to be doing anything (many can't cut gain accurately past a certain Q). Speeding up the spectrum allows you to easily see where peaks and transients are for an EQ cut or MB compressor if it's poking you in the eye (or ear!).
Lastly there's a peak hold where you can see the transient peak shape of your reference and your own mix to see how it stacks up. Peak hold can be infinite or you can let it fall periodically during different sections of a bar, measure, section or whatever you're monitoring.
The difference between RMS and Peak (not PRMS but Peak - you'll need peak metering for this like Roger Nichols now has...) is the dynamic range everyone is so interested in. It's the range of the average loudness to the fastest loudest transient. Kind of like the groove and the excitement found in the song but in terms of loudness. This range is the headroom of the music (the full scale digital headroom is something else but is always "usually" 0dbFS). The actual headroom of the music is what everyone from you and me to Bob Ludwig are slammin with our L2 Limiters (or whatever he's got out there).
But anyway, I'm real sure that your mixes are not going to match the commercial rap that has been mastered and crushed to about 4db of dynamic headroom. I'll bet the headroom on your mixes are 10-15db (?) This reference difference will probably throw off your mix decisions and certainly will look funny in an RTA (haha - it took me a while to say this). But after a long while you will begin to notice what a spectrum in an RTA means - if you want to go that way.
And that's all I have to say about that!
