A diffuser is a device fitted over an underwater breathing set's exhaust outlet to break up the exhaled gas into bubbles small enough not to be seen above the surface the water, and make less noise (see acoustic signature). They are used in:
Designing an adequate diffuser for a rebreather is much easier than for open-circuit scuba.
A system was shown on a Discovery Channel documentary about diving with hammerhead sharks at Cocos Island, Costa Rica. Eddie Paul designed—and in the early 1990s underwater photographers John McKenney and Marty Snyderman tested—a system they called the "scuba muffler", for open-circuit scuba. This system had two large filter stones on the back of the tank with a hose connected to the exhaust ports of the second stage regulator. The two filter stones were mounted on a hinged arm to float 1 to 2 feet (30 to 60 cm) above the diver, to set up a depth-pressure-differential suction effect to counteract the extra exhalation pressure needed to breathe out through the diffuser. The scuba muffler cut the exhalation noise by 90%. Closed circuit rebreathers proved more useful in letting divers get near sharks.