It gives a live read-out of the data in the ‘noise’ app on the Apple Watch. Not sure if that functionality exists without the watch, though!

Edit: not the most scientific measurement, so apologies if my original comment was misleading

I know it’s way more expensive, but the last gig I went to, I used my AirPods Pro in transparency mode, and it reduced the sound down from an insane ~110db to peaks of 90! Definitely worth protecting your ears.