I didn't state that it was supposed to be accurate, just so he could have an idea, bare in mind i did this on a whim as i was already in the process of testing my gun (new hop up and all) so didn't have to do it for him but did. My phone only registered 80dB right next to my blank firing 8mm Glock 17 pistol and i can tell you compared to the polarstar, your ears ring when that shoots.
In my view my Polarstar M4 when shooting, the sound level is about the same as a large claw hammer hitting a block of soft pine timber but again that's subjective.
My phone was sat about 15cm away from my Chronograph which is mounted in an ammo box.
Again this wasn't a scientific test, just to get an idea.