All guns are far too loud for your hearing. Yes, some are louder than others but NONE are quiet enough to NOT do damage.
The sound is creating by the pressure in the barrel, strongly affected by muzzle pressure, so longer barrels and lower pressures are quieter. The bullet noise itself is really not a factor.
I would not choose a SD cartridge based on noise level. I would make an effort (in the home) to have electronic hearing protection available should the situation and timing allow for its use.
Electronic protection will enhance your hearing and help you in a defensive situation. However, it's obviously not life critical and I'd rather be alive and deaf than dead.
Permanent damage can be and has been done by one single shot. I've done it myself. I have tinnitus in my right ear from one single shot of a very loud gun. I had plugs in but it hadn't sealed yet. Trouble is, it's unpredictable. You might shoot for years outside and never have a problem and then one shot and your ear never stops ringing. Shooting indoors is MUCH, much worse.
In any case, never, ever practice without protection and make electronic protection available along with your defensive gun. Don't get yourself or your family shot while you try to don electronic muffs but if possible, use them.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.