Look up “ANSI standards for hearing protection”. You can find how devices are rated and what is required.
Also, 3M lost a recent lawsuit on their hearing protection used by members of the US military, and the documents you can find that were used in that suit can be found pretty easily, and are interesting. I did this a while back and the whole field struck me as unscientific. A lot of the regs were developed for chronic noise such as factory work, and are not well adapted for protecting from gunfire noise.
Yes. Hence, this thread.