Tracy Crook wrote:

Agreed, this is definitely a new concept and I hope it works.

The only thing that bothers me is that, as you pointed out, the
bandwidth needed for attenuation is about 0 to 12 Khz.  That
represents MANY octaves even if we bump the lower limit to a realistic
number ( only a closed pipe will work at zero :-).  This bandwidth is
mutually exclusive with the term "tuned".  And if by tuned they mean
low pass, then it is not a new concept since that is covered by
conventional mufflers.  There may be something to this new concept but
until this contradiction is explained, I would hesitate to start
cutting parts.

The first parts I'm going to cut are wood sides and PVC pipe baffles.
That'll be good enough to experiment with.  I've got a sound sample of a
frequency sweep.  I can build a test muffler, play the frequency sweep
in one end, and record it on the other.  Displaying both samples in a
sound editor will tell me what, if any, frequencies are attenuated, and
by how much.  Losses in the playback speakers and microphone can be
documented by running a test without the intervening muffler.  I may
have to spend money on good speakers and a good microphone (Oh,
bummer!).  If that shows promise, I replace the PVC with stainless
tubes.  If that works, I replace the wood with stainless sheet.

Which frequencies need to be attenuated?  Remember that Monty Roberts
did some work a while back showing a sonagram of a rotary recording?
The sound energies are much higher around 6 and 12kHz.  With that in
mind, look at the graphs on the top of page 4 of this report
http://web.mit.edu/course/3/3.042/team1_08f/documents/complete-phononic-bandgap.pdf

It shows very significant attenuation in frequencies from 4 to 8kHz and
then 10 to 14kHz.  This is with 13mm cylinders set in a 30mm square
array.  Hollow cylinders showed similar efficacy.  Close enough to 1/8"
pipe (mcmaster-carr part# 44635K422 ... cheap!) to make a prototype
almost mandatory.  So, my current plan of action is to produce some
sonagrams of rotary exhaust noise, and verify the target frequencies for
attenuation.  With the experimental jig, optimize for bandgaps around
those frequencies while using the smallest filling fraction possible.
At that point, I will build a matching prototype.  Two sections of steal
sheet, match drilled with 1/2" holes (or whatever size was found to be
optimum).  Fill the holes with 3" sections of tube.  A bunch of welding
later, a 8.5" wide muffler with a .500 filling fraction will render
about the same flow area as a 4" diameter exhaust pipe.

Monty built a resonator a couple of years ago and brought it to Shady Bend. Tracy ran it on his test stand. If I remember correctly, it took some of the higher frquencies out, which seemed softening the engine sound some. After that test I welded a piece of 1 1/4 "  pipe to the end of the resonator, just as I have it on the single rotor engine. The result of this narrowing the exit down was, at least subjective, a tremendous reduction in exhaust noise. The power loss was, according to Tracy, 200RPM at 6000. Obviously, there is no free ride.

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