I came across a picture of a Lockheed YO-3 'Quiet Star', and it got me thinking about what things can be done at design time to make an aeroplane quiet. Picture here: (http://home.comcast.net/~bzee1a/Edwards09/DZZ_2066.jpg from http://home.comcast.net/~bzee1a/Edwards09/Edwards09.html) To my mind why you'd want to build quiet is fairly easy - these days urban developments are getting closer and closer to airfields, and the less noise the planes make the less complaining the residents will do. So what can be done to make a plane quiet, without affecting the perfomance too much? I've heard of tuned exhaust muffler systems before, but I havn't seen anything like the prop on the YO-3. How much of the noise actually comes from the Prop?