raindownthunda
100+ Head-Fier
First time I tried this, I was shocked at how loud 100db is. If anything it will help put you at ease that you're not destroying your ear drums. Or, conversely, that you may need to dial it down a bit.
I tried a few different ways to seal the phones mic in there, and each gave similar peak readings.
I can recommend the free NIOSH Sound Meter app on iOS. Surprisingly good functional and usability for a free app, especially coming from a government agency. I tried half a dozen or so free & paid db meter apps and the results were all pretty similar in measurements.