A diffuser is a device fitted over an underwater breathing set's blowoff hole to break up the resulting bubbles small so they will not be seen from out of the water. They are mostly needed in frogman-type diving operations where secrecy is needed, and in marine biology to avoid scaring fish which are being studied. Designing an adequate diffuser on a rebreather is much easier than on open-circuit scuba. A system was shown on a discovery channel documentary about diving with hammerhead sharks in Cocos island. The divers had made and tested a system they called the "scuba muffler". This system had two large filter stones on the back of the tank with a hose connected to the exhaust ports of the 2nd stage regulator. The two filter stones were mounted on a hinged arm to float ~1-2ft above the diver, this was to set up a pressure differenctial to aid in reducing exhalation resistance. Little information has been found concerning the scuba muffler.
