RTA is out Transmission Loss test option. This is found on the Setup page of RTA, and is available as an in-app purchase. This is a slightly older version, and has the previous iOS 6 user interface. RTA uses filters (rather than an FFT) conforming to ANSI and IEC specifications. It can be calibrated, and uses either the built-in mic, the headset mic, or the Studio Six Digital iTestMic or iAudioInterface2. RTA works by literally running 10 ANSI octave band filters, or 30 ANSI one-third octave band filters on the input signal in real time. This means that even if there is just one sine wave at 1000 Hz at 85dB, the 1000 Hz octave or 1/3 octave band will show 85dB. Every other sine wave that fits in that band could be at 84.9dB, or even 85dB, and the bar would not be any higher.

That's not so great when you are trying to EQ a room with a tone generator, but when pink noise or program material (music) is playing, it's really quite useful. And, there are many cases where various acoustical tests and standard are looking for 1/3 or octave band measurements. FFT on the other hand has much finer resolution, just a few Hz in some cases. Depending on the smoothing setting in FFT, you can see all of the individual frequency components.