An excellent sticky for anyone wondering about sound pressure level & hearing damage.
To break it down, make it a tad more concise and cover some things absent in HSO's thread for the OP:
*140 dBA (Decibels, A weighted measurement) is the maximum peak impulse noise allowable without hearing protection.
*Mil Std. metering of gunshots is done 1 meter left of muzzle, 1.6 meters off the ground. Sound pressure at the shooter's ear with an unsuppressed firearm will almost always be lower than mil spec position, though not by much.
*The most powerful firearm which reliably has a muzzle report below that is a .22 LR rifle with a 20" or longer barrel. Even 16" rifles can hit the shooter's ear with >140 dBA. .22 pistols are well into the 150s
*Centerfire handguns & rifles generally exceed 160 dBA. That's 100 times the sound intensity of the max allowable. And yes, 160 dBA is more damaging than 145 dBA. Much more.
*Magnum rifles with muzzle brakes can peg a meter at over 170 dBA. That's
1,000 times the max impulse noise our ears can handle.
*Properly used quality hearing protection will generally offer 25-30 dBA reduction. Technically not enough in many instances, especially indoors shooting with lots of reflected sound pressure, but there's also a deflection component to the ear protection, muffs more than plugs. Before you decide to only use one or the other rather than both, know your firearm and your environment. A 9mm pistol outdoors is a whole different animal from a 5.56mm SBR with a birdcage being fired between stall walls and under a roof indoors.
*Some suppressors can offer similar or a little better protection on rifles than just muffs or just plugs. Lots of quality cans will meter below 140 at the mil spec position with full power rifles, and usually a few dB lower at shooter's ear on manual action rifles. Semi-Automatics are often louder at the ear than mil spec position due to noise coming out of the ejection port or when the gas piston vents.
*Except for subsonic loads in good suppressors which come in under 125 dBA at shooter's ear, it is still a very good idea to wear hearing protection even with a suppressed firearm, especially for prolonged use. My Hunting model suppressor, which meters 135.8 dB @ shooter's ear on my .25-06 firing indoors, gives enough reduction for one or two rounds in the field without ears, but a whole bunch of 136 dB impulses playing at the range can still damage hearing.
Like HSO points out in the linked thread, hearing damage is irreparable. His explanation is a little different, but basically high pressure sounds will break of the cilia, the little hairs in the inner ear, which detect vibration. They don't grow back and cannot be replaced surgically. See HSO's thread for futher details.