Exhaust pipe noise reduction

You have a very fine tuned and expertly designed engine. Year of testing and millions spent to make it the best horsepower to weight ratio piston driven engine on the market. If you change anything that isn’t a Rotax approved part you will regret your decision. Leave it alone and enjoy the ride! 

Speculation:

With reference to the Rotax log style silencer/muffler.

I am sure that I have heard that European noise pollution laws are probably the strictest in the World and that this has impacted on aircraft engine and propeller design.

If the above is correct;

  Rotax will have designed their exhaust system accordingly. The result will be a compromise between performance & noise generation/control.

  Therefor there may be room for improved exhaust flow and therefor some gains to engine efficiency.

  This would be at the expense of increased noise (the opposite of Edmonds question).

The 912IS is already one of the quietest engines out there. I agree with the others it would be best to not mess with it.

However if you insist.....

1) Whom are you wanting to keep the noise away from ?. As an example if you are concerned about noise on the ground then simply shift the exhaust stub 180 degrees and point up. There are probably few people above you !

2) If it is cockpit noise then more barriers on or behind the firewall and more engine isolation. Some installations are far better than others.

3) Should you want to get really serious.....like you want to fly over the Ho Chi Minh trail at night and not be heard then copy the exhaust designed to do that. The Lockheed YO-3-A Quiet star, muffler extreme on show at the Hiller museum.

There are many sources of noise I have flown both the Pipistral 80 hp Rotax powered Alpha trainer and the electric motor powered one ! The electric is of course the quietest powered Aircraft I have flown...but it is not totally silent, prop and airframe noise is still there.

So I suppose my question is "what is it you want to achieve?"

Regards

Glenn

European Annual Registration costs are partially based on the aircraft noise signature.
The quieter the aircraft, the cheaper the annual Registration costs.

My German-made Gyro came with a "GlassPack" muffler on the output of the expansion muffler.
The Engine sounds more like a Turbo-Prop than a Piston engine.
The propeller is definitely the dominant sound.

My goal is to reduce noise inside the cockpit.  ANR headsets do only so much.  Sound waves can still damage hearing through continuous exposure, as well as transmissions through the skull and other bones.