100% true. AR-style direct inpginment actions suffer from the gas tube ending a few inches from your face and can be impossible to suppress to a hearing safe level because of the gas discharge. Bolt actions can be made hearing safe - it's a function of distance. Sound works based on the inverse square law, meaning that each doubling of distance will reduce amplitude by more than half. MIL-STD-1474E is based on readings taken 60" perpendicular to end of bore, and 64.5" behind end of bore. If you take 150dB as a standard non-braked end of bore reading based on a 60" perpendicular reading , then the notional corresponding 60" behind end of bore reading would be the same. A suppressor only has to make a 10dB reduction to be hearing safe from 60" behind the end of the bore. Meaning any suppressor that reduces sound to 140dB or less in the side position will effectively be hearing safe from the shooter's position. That doesn't account for the many reasons why shooter's ear readings are consistently lower than perpendicular readings - there can be a 6-9dB decrease at shooter's ear over the side mic position based on the ejecta pattern and how sound propagates to the rear, which is why many "MIL-SPEC" (note - not actual MIL-STD) advertising numbers on cans will claim sub-140dB ratings without specifying the exact test parameters. They take the side versus ear variance and apply it to their side-taken rating without mentioning things like gas ejection from an AR.
Temporary threshold shifts in hearing can occur below the 140dB "immediate damage" threshold to the point that many people (myself included) will wear hearing protection in situations there the shooter's ear reading does not exceed 140dB. There are anatomical and phycological reasons behind it, but the ear and brain can literally ignore/shut down hearing in certain frequency ranges during continual exposure. As gun shots tend to have high frequency ranges then it's logical that difficulty with hearing in vocal ranges of sound can follow repeated exposure to suppressed gun shot noise. Basically even though your hearing isn't permanently damaged, it's easier to hear someone talk to you right after you pull off ear protectors versus right after being continually hit with high frequency range noise of suppressed shots.
The people who told you about cumulative hearing damage are correct, but I think that ignores the consideration that if you're shooting below 140dB shooting more doesn't really matter. If you're over, then yes it does. If your muzzle is two feet past the end of a slit window in a blind, not all of the sound is making it's way back to you, so you could marginally suppress a series of shots and ultimately be fine. You could also shoot something at exactly 140dB under a metal cover and leave your ears ringing. There are too many factors to conclusively say whether or not ALL situations are safe, you have to look at the factors of what you're going to be doing.
Sound is also a factor in perceived recoil. Wear double ear pro and shoot a suppressor sometime, because of the reduced concussion you will almost certainly perceive a reduction in recoil. It's crazy, but it's true.
TL;DR - you can get a bolt action rifle to sub-140dB at shooter's ear. If you're shooting near or under a cover or reflecting surface that can change based on your position relative to those surfaces. Wearing ear pro while shooting a suppressor is advisable for a number of reasons, the least of which is unless you have a sound pressure meter you can't be certain so why be dumb? But if the alternative is no ear pro (aka old school hunting) then a suppressor will never be worse than nothing.
(This is partially where I came from - a couple hunting shots a year won't make me any deafer than I already am fast enough to notice, but a suppressor really cuts down on the time my ears spending ringing after).
Impact sound is a very different concept than muzzle signature suppression. Animals will 100% hear and respond to bullet impacts - both solid meat hits, and the much worse ZZZZZZzzzzzzz of a bullet whizzing off after impacting/skipping from a hard target after a miss. Animals can be confused by the non-directionality of the suppressed muzzle sound of a clean miss or when shooting at various dispersed groups in a single field, but there's no reality behind shooting one animal and the one standing next to it is oblivious. The impact sound itself will propagate from where the bullet hit the animal, and will be loud enough to potentially startle any animal with a few dozen feet. Like when you smooch at a deer from inside a blind to make them lift their head - it's not so loud they immediately bolt but it's abnormal enough they pay attention to it. (That said, I shot two Whitetail thirty-ish minutes apart last year because one dude stood there while we were laughing and joking and gutting his friend - stupid is as stupid does
He definitely heard us though.)
An odd situation is you can get to a point where the at-the-speed-of-sound muzzle signature catches up to then passes a bullet that goes trans- and then ultimately sub-sonic. A 5.56/.223 Rem will somewhere just short of one mile get to the point where at the impact site you'll hear the muzzle pop immediately before impact. A suppressor diffuses that muzzle signature, which is what I mean when I say it causes directional confusion- animals hear reflected sound waves, but from too many sources to locate the origin point. And the reflected frequencies will be higher because those frequencies propagate better, so the bullet passing might not sound like something they've been exposed to and would recognize to bolt away from. The crack is essentially continual as it passes objects, so the higher frequency "crack" will reverberate and echo around and obfuscate the point of origin.
(The best part of this video is when you hear the guy yell "HIT" because he's so pumped the guy made a 1500 yard impact with a .223
)
This doesn't even broach the subject of brakes, and why no one should ever be within 10 feet of a brake without doubled up ear-pro
and eye pro. A suppressor won't instantly shred someone's ear drum like a brake can - if your bench is too close to someone and they're on line with your baffle angle then the sound pressure they're exposed to can be amplified several more loudness levels - including to the point that even double ear pro won't keep them below the threshold for hearing damage. Distance is the only way to be sure about sound pressure levels.
(There's a whole side bar here about ear pro ratings and why anything electronic probably isn't even safe to wear when shooting, much less when being around people shooting, and how ear plugs for double ear-pro don't protect nearly as well as you'd think from paper ratings.)
I'm not saying there's anything wrong with brakes, but I do view the "wear ear pro with supersonics" statement that you heard from the shop as being on the same level as "wear eye pro with brakes". You have just as many eyes as ears. And you're about as likely to be damaged from supersonics from a can as you are from shrapnel from a brake. Basically if you're dumb, it'll happen. If you aren't, take appropriate steps and you'll be safe.