An RTA shows the frequency response of an electronic or acoustic system in real time. The following screen captures are the various scaling options for Spectra Foo Complete’s “Spectra Graph”, their take on an RTA. As you split up the scale, you get finer & finer resolution. 1/3 scale being the industry standard since it matches the industry standard 1/3 graphic EQ.

1 octave scale
1/2 octave scale
1/3 octave scale
1/6 octave scale
1/12 octave scale
1/24 octave scale

Initially RTA devices was a stand alone units but modern FFT measurement software packages include an RTA function so stand alone units are some what obsolete now. There are many RTA apps for Ios devices. An RTA is useful for recognizing feedback frequencies but is not the right tool for PA tuning. The right tool for tuning a PA is the Transfer Function because you can see impulse, phase & frequency response again a known value.

Friends don’t let friend’s tune PA’s with an RTA.