I have a fox 40 micro tethered to the cord on my badge holder around my neck. It is advertised at 122 db, and I think I can squeeze a little more out of it. Our security guards here are trained to respond to it when I blow it continuously and repeatedly (unlike my dog back home, who comes back on one two short blasts). The sound will carry farther than my voice, is more recognizable, and will last longer than my vocal chords.

The down side is it lets the bad guys pretty much know where I am as well. Once the screeching starts, they are keen to make it stop before the guards come. Not something you use indiscriminately.