Normally one has to go with the canister muffler setups to reduce the noise by quite a bit.
with a twin you need to run dual exhaust or fabricate a two into one exhaust with a canister muffler. Some creative folks use the souvenir aluminum beer or soda bottles for canisters too. You can get rear exhaust outlet or front exhaust outlet canister mufflers as well. But the problem with the canister mufflers is getting them to fit Ok in a airplane as well as weight too.

Now then I have had good success at reducing engine noise by using a small insert baffle into the muffler outlet. This actually works fairly well at helping to knock a few DB of noise off of a engine.

Next is the airplane itself can generate a lot of noise from the engine vibration. You can use those special vibration isolation mount setups to reduce the airplane noise.

Then of course the propeller is a big source of noise too. Using one of the special propellers that are aerodynamically designed to reduce noise are good to use for this.

Beer bottle mufflers http://www.rcuniverse.com/forum/m_73...tm.htm#7419797



Troy Built Models offers some of these baffle inserts for some gas engines too. ref http://www.troybuiltmodels.com/items...0SILENCER.html

Here is my homemade exhaust tip baffle that I made.