Sure, there are lots of things that can cause hearing damage. Heck, driving on the highway with your windows down can cause hearing damage. My point here is simply that the silencer industry’s claim of silencers being “hearing safe” is misleading and is medically incorrect.
Yes and no. 140 dB
is the established level for instant damage from an impulse noise. But there are many other variables with acoustics. For one, frequency makes a difference; high SPL at a low frequency is less damaging, and less offensive to most people than a sharper tone at the same pressure. Another really important consideration is the attenuation; dBs attenuate by 6 for every doubling of distance. The waves may also be vectored, which is why guns with muzzle brakes sound so much louder to our ears than those with bare muzzles, even though the "uncorking" noise is the same (this is why my suppressors have a countersunk front cap). And, of course, reflection, refraction, diffraction. It's complicated, and unlike my uncle, I don't have a college education in acoustics to better explain it. What I
do have is a very expensive sound meter that doesn't lie or have biased perception, and there are many noises which are higher than one would think, especially if the distance from source is closer than "normal".
When it comes to metering suppressors, I do it at mil spec position for a comparative baseline, but the only number I really care about is at-ear levels. This is the part which is misleading in suppressor marketing. As you mentioned, it's often done under the most ideal conditions with tailored hosts and ammunition to achieve the lowest possible figure. SilencerCo is famous for this, and one should add about 5 dB to any figure they advertise for real-world results. At ear levels are
really important with autoloaders, particularly gas operated or blowback. ARs are one of the qiuetest autoloading hosts, but even a perfectly tuned "system" is gonna produce at-ear levels between 135-140 dBA. Most gas operated rifles will have shooter's ear levels over 140 regardless of can, and straight blowback centerfire guns (PCCs, generally) can easily be over 140 at-ear, even if they're sub-130 at muzzle; my ultralight 9mm folding rifle, with a very lightweight bolt (high bolt velocity) meters 12 dB higher at ear than milspec with supersonic 115 gr. ball loads. In terms of the logarithmic dB, that's almost 20 times louder.
On shooting indoors or otherwise near reflective surfaces, let your ears be the judge. Reflection tends to make a noise louder to our ears, even if the meter says it's not. .22 LR suppressed is one of the better examples, as the low pressure & low gas volume results in virtually no dBA change between open outdoors shooting and firing in pretty confined indoor spaces, but they seem much louder to us inside. When you move into larger rounds, there actually will be a measurable difference, which increases as the overpressure condition in the room increases. A 9mm may only pick up 1-2 dBA in a 12x12, 8' ceiling room, while a .338 Lapua could be 6-8 dBA louder in that space.
My ears are generally offended by suppressed guns metering over about 135 dBA, though some of the cans with softer tones are still comfortable at a higher levels (suppressed .45 ACP metering ~136 bothers my ears, while my .375 RUM @ 137 dBA through my Accipiter does not). I let my ears tell me what's acceptable and what's not. If it's uncomfortable, I don't do it.
Ultimately, yes, it's still best to use ear pro even with suppressors. But that's also true of everyday noises for many of us, so we have to make the call on whether or not it's worth the use of protective gear in a given situation.