Etymotic High Fidelity Earplugs, ER20XS Standard Fit, Polybag Packaging, 1 pair https://www.amazon.com/dp/B00RM6Q9XW/ref=cm_sw_r_cp_apap_OecYrlZ1jUAdo
I used these for freestyle and they worked wonders. I ruptured my ear drum river diving, and the bass caused me to feel the scar tissue, which hurt a bit. Using these, you can hear the music and everything else clearly while protecting your ears with no pain from all that wonderful bass
If you don't know what size will fit (I have small ear canals so normal fit me), I recommend buying the $25 pack that gives 3 diff sized plugs (6 total, 2 are foam)
I like earplugs like these etymotic ones.
When I'm at a social where the music is really loud, then I will use those 34 decibel foam ones that you can buy a large container of at pharmacies. I have a container on my keychain which I carry those etymotic plugs around in just in case I forget to put the foam ones on.
You're very smart to be concerned about caring for your ears. If you're at a festival for several hours then you are definitely causing hearing damage. Some peopel say I worry too much about it, but I think I'll be happy I did when I turn 80.
Here are my 2 cents... phone is a decent way to get a reading below 90dB. What exactly are you doing that you're concerned about? The easiest thing to do is just to get flat response ear plugs - it's just going to turn down the volume on everything, but you'll still hear like normal. I wear these to all of the shows I go to that are hitting in the 90s and above: https://www.amazon.com/Etymotic-High-Fidelity-Earplugs-Standard-Packaging/dp/B00RM6Q9XW/
Use your phone to get a general baseline, but I get erring on the side of caution. Figure out about what sounds like it's not good for you, pop in your ear plugs, don't stress it.
If you're going to buy ER20's at least get the ones that don't make you look goofy.
Oh! We got these on Amazon, Etymotic Research ER20XS... https://www.amazon.com/dp/B00RM6Q9XW?ref=ppx_pop_mob_ap_share
https://www.amazon.com/Etymotic-High-Fidelity-Earplugs-Standard-Packaging/dp/B00RM6Q9XW
<$20 and worked great for me! get the universal sizing to get multiple different sized tips
I'm bringing these this time around they're by etymotic but a little more(+ $8) and they're supposed to be a little better and more discrete.
Also if comfort is more important to you than sound attenuation then the Earasers are pretty good from what I understand and after a while don't feel like anythings there
I'm gonna repeat what /u/Diagonalizer said below and recommend you buy this pair by the same company: https://smile.amazon.com/dp/B00RM6Q9XW/
They're a bit better sound reduction and actually fit flush with your ears. Those stubs sticking out of the blue ones hurt if I wore a mask/hugged someone too hard. :D