Thanks for the info about the app. Got it and already used it yesterday night in App Arena (86-88 decibel). I used earplugs, and when Thai who invited me to their table asked why I use earplugs, I not only told them that the music is too loud and hurts my ears, I showed them the app!

I am well aware that this is not a scientific measurement (that's why this idea has not occurred to me), but can serve for comparison between bars.