A hollow tongue with some restricted outlet does not do much for noise as the fellow have already stated but the Dubro Tongue muffler uses a removable baffle plate near the outlet and has a hole plug near one side on the end and this has a long coil spring inside that is removable.

The idea is to shorten or remove the spring to taylor the back pressure and/or noise, when the full length spring is used there is noticible quitening of the exhaust note.

I have used one on a fox stunt in more recent years but not currently as I only use throttled engines now. Also the dubro tongue muffler provides useful pressure for those of use who use bubbless tanks rather than a more conventional uniflow set up.

I no longer use my dubros since all my controlline engines now are throttled RC engines and I use mostly stock mufflers.

John