A good silencer is going to reduce the noise intensity by about 30 decibels or a factor of 1000. This is eight time less loudness. It should be safe to shoot without ear plugs even with supersonic ammo. I would guess that it would be somewhere between a 9mm and 45 in noise level. But you would be better off with the opinion of an owner of a 40 cal can. There might be a better opinion on http://www.silencertalk.com or http://www.subguns.com . Sure sure ain't going to be quiet though, that is a fact.
Ranb
Ranb