Jim the software may not be as important as the phone you are putting it on. A couple of phones ago I loaded up an SPL app only to find that the mic on the phone topped out at not much over 70db, which when you think about it it is more than enough level for what a phone is really meant to do, but not enough for measuring the sound levels we may encounter in audio playback.
I got a cheap SPL meter on Amazon a few years back for about $20. I have never had the opportunity to check it for accuracy, but I don' use it to measure absolute levels as much as I use it to match level. So even if it is off by a few db, that error would occur across the board so if I match levels to a certain reading the meter is doing what I need to accomplish even if the value of the reading is not exactly correct.