Some of the phone apps are pretty accurate compared to an actual meter but some are way off and it depends on the phone it's installed on too.
My hope is to get 2 “standard noise levels” on the cheap:
1. Some automobile horn measured so many feet away for “loud” standard sound. (Approximately 95 dBA)
Second choice would be one those “Lost in the Wilderness mouth whistles”
2. The noise level of some automobile interior traveling at 70 mph over smooth blacktop pavement for quiet standard sound. ( approximately 65 dBA)
Second choice would be a common telephone ring tone from a common cell phone with the volume set middle position.
This is like checking a thermometer in a stirred ice bath, then in boiling water corrected for altitude.