What is the difference between RTA and Spectrum Analysis? In a sense, the graphs are similar yet different. I guess more to the point is, what uses would RTA have over spectrum analysis and vise versa?
Spectrum analyzers are normally associated with an examination of your spectrum and it's distortion products.
With REW, there are really three ways to use it. You can play an internally generated Periodic White noise and examine the spectrum (the RTA requires Periodic Pink noise), or you can play an internally generated Sine Wave and examine the THD and THD+N, or you can simply examine the spectrum with no signal at all.
The latter is interesting because it lets you examine the room for noise energy that you may not be aware of.
Below is a pic of my office with the Spectrum analyzer running with no signal being generated.
I am using an FFT length of 65536, with an averaging of 8, and using a Hann window (standard setup).
Everything is set up as if I am going to do a measure and all the levels are set to go, but instead I simply start the Spectrum analyzer and listen to the room.
You can see my furnace is introducing some low frequency noise. If I shut it off, the noise is eliminated.
You can see I have the typical poor PC computer 60Hz hum and its related harmonics at 120Hz, 180Hz, 220Hz etc.
You can also see (amazingly), my NTSC RPTV CRT's horizontal oscillator frequency of 15.750Khz present in the room.
The TV is several rooms away and happened to be turned on, but the oscillator noise is present in my office (not that I can hear it, but there it is). Shut the TV off and it goes away.
The Spectrum analyzer can tell you a lot about your room and system before you even do a measure.
Sometimes you'll see a waterfall with a strange signal that rings out for many hundred of milliseconds. If you looked at the spectrum, you might see the source, rather than incorrectly deciding it comes from a modal resonance.... My furnace is a case in point on that issue.
You can certainly play a Sine Wave and look at the distortion products and distortion + noise results.
Here's a pic of a 1000Hz fundamental and its associated harmonics. REW gives % THD and % THD+N readouts of the fundamental plus all harmonics..
Personally, I feel it's wise to do a 1KHz simple loopback Spectrum with THD and THD+N reading of your soundcard and PC alone to establish a baseline for yourself. Then add in the system and see how much greater it is.
The RTA as you've seen is quite useful. Here's a response measure and then a quick RTA. Once this is running, I can then move the microphone around to see what different listening positions look like (all in real time). It is revealing to see the difference a few feet make when you move the mic.
The RTA also allows you to change filters and watch the RTA screen for the real time changes. It's also really useful to adjust phase on a subwoofer for the best crossover response. You simply watch the RTA screen as you dial the phase control. A lot better than taking a bunch of measures to accomplish this task.
Here's a pic of RTA and a response measurement on the same screen (measurements are selectable to be placed on the RTA screen for comparison).
I was sure not to move the mic when I took the measure and then started the RTA.
Then there's a feature that you may have missed that is rather cool.
Instead of using a soundcard calibration file, you can use a loopback cable on the left channel and use that as the soundcard calibration.
One of the things this new reference allows is speaker distance readout (called System Delay feature).
I can hook up a single speaker at a time and do a full range sweep and REW will tell you the distance the microphone is from the speaker tested (in msec, meters, and feet). It doesn't really apply to subwoofers given the low bandwidth of a sub, but it is quite useful for the other five or six speakers in your system. You would of course require a full range microphone such as an ECM8000 or Galaxy CM-140 to use this feature.
This is useful if you think that your simple measuring tape method to establish speaker delay trim values for your receiver isn't that accurate.
I have tested this feature and it does seem to jive with my measuring tape. That's fine, but systems may have strange delays in them that you can't account for with a tape measure. It's a useful feature.
Below is the results from a single mains speaker in my office that is exactly as indicated by REW at 10.8 feet away from the microphone.
I did a simple sweep up to 20KHz and REW shows the system delay. I could easily substitute any center or rear speaker by moving the speaker cable around and establish the acoustic distance for every speaker in my system. Now that's cool.....