Some years back I tried to assess some ear plugs and earmuffs for shooting. I had 6 dB and 15 dB foam earplugs, 25 dB plastic earplugs, and 35 dB earmuffs. I ran a fairly subjective test by using pink noise and sitting close (~1 m) from one of my main speakers. I listened and noted the SPL using a cheap RS meter, C-weighted, then put in the various earplugs and tried to adjust the volume to where it sounded the same, recording the SPL to get a relative difference. One thing I overlooked is how sensitivity declines upon repeated exposure to loud noises, but the idea was to set the level to just above where I could reliably pull it from the noise floor. I was not going for great accuracy, just a ballpark of where things ended up. Repeated trials with better controls would provide better data; this was just a quick test one evening before an outing and I was curious. It is by no means rigorous. Add disclaimers
ad nauseum -- this was just for fun and to satisfy a bit of curiosity.
Results from memory: The 6 dB foam plugs provided almost exactly 6 dB, and the 15 dB very nearly 15 dB at best, but had to be inserted very carefully. The 6 dB plugs were pretty soft and easy, but the 15 dB foam was harder and you had to "roll" it a bit and insert very carefully or isolation quickly dropped to 6 dB or so. The hard plastic 25 dB plugs had a "tube" to help them seat reliably, but I only got about 20~23 dB so of isolation. The 35 dB earmuffs gave me about 30 dB, very dependent upon how I placed them and how tightly they pressed on my head. Most of the time I was getting ~25 dB or so unless I fiddled with them a bit. If I pressed on them a little I could get 30+ dB (narrow head, not much between the ears
). Nothing unexpected, but there's some anecdotal results. I am sure ear canal and head size and shape will cause results to vary.
My somewhat limited experience with active earmuffs (we use them at work and I have tried 2-3 of them) is that they do well at suppressing low-frequency periodic noise but by the upper midrange or so the active part gives up and you get whatever "native" isolation they provide. I have not seen curves for them (not in the box, anyway). The ones I am using currently are Sony WH-1000XM2 FWIW. Again, If I press hard on them, things get a lot quieter.
One of the fancier shooting earmuff companies offers different pads to help adjust the isolation. That seems like a nice idea. I adjusted the (metal) band on one pair to apply more pressure and that did indeed increase isolation, but also made them more fatiguing to wear over several hours just from the physical pressure.
Didn't measure the SINAD...