NOISE!!!

That got your attention didn’t it ??

A Noise reduction article by Mike Sotomayer

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This intent of this article is to give you some information and methods for lowering the sound footprint of our aircraft without the need to invest in the expense of canister style exhaust systems .

The best part of this article is that it allows you to use the less expensive stock style mufflers that you may have on hand .
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First , lets examine the stock style exhaust mufflers and determine why they are considered marginal for silencing and what we need to do to make them work.

As you can see the standard mufflers are more of an exhaust diverter.

And when used with the larger engines they are marginal at best for quieting our big engines.
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With this in mind I set about to see how to modify my current stock mufflers to quiet the exhaust noise from my 3W70 Twin.

The first conclusion I came to was the need to increase the volume of the muffler .

To that end I purchased an assortment of Aluminum tubing in ¾ , 7/8 and 1 7/8 dia. In two foot lengths .

I elected not to use the ¾" tubing and went with the 7/8 and 1 7/8 tubing
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With the engine mounted to the airplane and the lower cowl removed I cut the tubing to the size I felt would fit into the cowl and still give me enough volume to make the ,mods effective.

Due to engine box limitations I determined that the tubing needed to be mounted to the sides of the stock mufflers giving me a left and right side .With the new tubing welded to the stock units they appeared so:

 
Mvc-0008.jpg (23353 bytes) The old exhaust dump tube was cut off and the hole filled in with Al. Weld .

The 7/8 tubing was then cut to length and I then proceeded to drill 38 holes 9/32 dia. holes along the length of the tube stopping short of the length that extended past the mufflers ends .

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After finishing the muffler welding and grinding off the excess I mounted the mufflers to the engine and started it up .

I ran a 24 – 10 Mejlik prop and Tached it at 6680 RPMs

I then used a Radio Shack analog sound meter and the engine DBd at 94 at 9 .
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For a 70cc engine these numbers are great.

I then did some further in air testing and found the in air noise level to be very quiet .

After some more work I have settled at less 98 at 9 …I can get it quieter at the expense of throttle response from idle ..The little higher sound level is a small price to pay for the restored throttle response .
 
   
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