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Date: Tue, 19 May 2009 10:50:43 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Phonic Band gap filter
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George Lendich wrote:
> Al,
> I would off- set the tubes, the idea is to not have a direct path for 
> the sound waves to travel in a direct line, front to back (path of 
> least resistance) make them stainless and of a reasonable thickness.
> George (down under)
George, we're skinning a different cat here.  We can actually have a 
completely see-through muffler.  The idea is to control interference 
patterns so that certain frequencies are canceled.  The dimensions are 
critical to determining what frequencies are canceled, and the depth and 
width of the band gap is heavily influenced by the difference in Young's 
Modulus between the two materials.  The greater the difference, the more 
pronounced the effect.

My ultimate goal is a stainless box using .050" material.  It takes 
about 10ft of tube to make what you saw in the pictures.  I think the 
technique will be to make a box, then drill the hole pattern all the way 
through.  Push a tube into each hole, and then weld the tube to the box 
on both sides.  With this constructions, air can flow through the tubes 
to keep the steel strong and help cool the exhaust.

I did just think of something.  My experiment wasn't as valid as it 
could have been.  I should have run a test with just the filter box in 
place.  That is, the wooden box with the filter elements removed.  It 
could have been the box had more to do with the results than the filter 
elements.  I'll update you when I've run another test.

-- 

http://www.ernest.isa-geek.org