Good to see the NIOSH app. I compared it using the SoundLog app on an iPhone 5S and the standard internal microphone. The NIOSH app seemed to be consistently low by about 8dB compared to an inexpensive sound level meter, a UNI-T UT353.The SoundLog results were within +/- 1 dB of the UNI-T.
Sound levels were provided by a 1000Hz tone produced by a loudspeaker.