OSS Helix 556 is 140db at the muzzle 138db at the ear. As stated, short barrel's are bad for suppression purposes, unless its .300BLK. I recommend the Surefire Socom 556 RC2-SB as its best suited for SBR use as it has a better blast baffle to deal with the shorter barrel. The regular SOCOM RC2 is rated down to 11.5 if I remember correctly, but I would take advantage of the better blast baffle in the SB model. I would look into the TBAC Dominus SR also.
Hood886
has one and I think he said it was fine at the ear. As far as blowback is concerned, the OSS Helix will be your friend in that department. If you plan on running mostly suppressed, I'd run a better suited size gas port to help with better performance or run an AGB. If you can find one, Gemtech made a adjustable bolt carrier group that could be switched to suppressed mode to reduce blowback. Or if you're really bad, you can acquire a LANTAC E-BCG and really stand out. I also bought a Gas Buster charging handle that helps divert blowback. Be prepared to spend some coins...