I used a DC on turbo-props, and on the Twin Otter I used ear plugs under them, as described by Reddo. It worked a treat. On the 733 I used a non-cancelling Sennheiser, but found I was getting very hoarse from shouting, and very tired just from the high ambient noise coupled with the full volume required for the 'ear-off' technique of head set use that was championed by my earstwhile employers. I bought a Sennheiser noise cancelling headset, and I estimate that I was 20% less tired after a domestic rotation followed by a Med rotation (4 sectors). There was a sort of 'underground' movement at the airline that some people began to use the intercom, and 2 ears on which worked well, until the airline imposed a totalitarian ban on using the intercom. The noise cancelling headset was I believe a real contribution to reducing fatigue, and therefore a significant contribution to safety, and cockpit communication.