Hey guys, I finally found some time to create a video tutorial: how to turn your old an unused iPhone/iPad into a nice external spectrum analyzer. Cheaper than buying a second monitor (but only if you already have an iOS device of course).
With the addition of Audio Hijack app is also possible to run every audio played by your computer through the analyzer without having to import it in a sequencer.
An extra thing not mentioned in the video is that you can add your favorite eq plugin in the chain created with Audio Hijack (it must be an AU, though, not VST or AAX) and do a custom room correction or even creating custom room correction presets if you have more than one set of speakers.