I hear you - iphone mics are NOT designed to capture 30Hz, and any absolute reading (this signal is x dB SPL) would be meaningless.As scientific as you made all seem. I am not to convinced about dB readings from an iphone.
But my readings are actually pretty solid because they are relative readings (what dBs are all about anyway). If X is how loud the compressor is when it’s on, and Y is how loud it is when it’s off, my iPhone couldn’t give you and accurate absolute measurement of how loud X or Y were. But because the microphone is essentially a linear device, it can pretty accurately tell you the difference between the two levels.
The other concern is that the mic could be overloaded...but if it was, you’d see harmonics at 60, 90, 120, 180Hz, etc., which we don’t (it’s also a pretty quiet signal, not likely to overload a mic in the first place).
Some I’m pretty confident about the 40-50dB relative measurements I reported.