Audiotools is a good start, you can run the RT60 test w a handclap but a popped ballon is better. Audiotools runs on iphone or ipad. I would not trust the iphone microphone below 120 hz. Audio Control ( Lynnwood, WA make a decent affordable calibrated microphone that can be trusted ). Of course you can get there….eventually..just by listening….you can also wander in the desert randomly looking for water…
Jim Smith’s book - Get Better Sound may also be of use.
Acoustics is part science, part art and some luck….but my mentors put together great rooms you can hear…in the reference recordings they help create….
Best to you
Jim

