Actually, Excel sort of choke on it {:>)

 

Ed

 

 

Ed,
You ought a be able to load that into Excel and crank out muffler dimensions for any airplane {:>) That is what it does ant it???
All this is well over my head...
Bob.

-------------- Original message from "Ed Anderson" <eanderson@carolina.rr.com>: --------------


> Ernest, keep in mind that the results you get with your sound recording may
> not be indicative of what real exhaust pulse would give you. The reason
> -the exhaust pulse is a Finite Amplitude Wave - and they do behave
> differently in reflecting and refracting than standard sound waves.
>
> The math is not too intimidating {:>)
>
> http://www.wikiwaves.org/index.php/Finite_Amplitude_Wave_Theory
>
>
>
> Ed Anderson
>
> Rv-6A N494BW Rotary Powered
>
> Matthews, NC
>
> eanderson@carolina.rr.com
>
> http://www.andersonee.com
>
> http://members.cox.net/rogersda/rotary/configs.htm#N494BW
>
> http://www.dmack.net/mazda/index.html
>
> -----Original Message-----
> From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
> Behalf Of Ernest Christley
> Sent: Wednesday, January 07, 2009 4:07 PM
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Re: Muffler design (was 20B isssues . . )
>
> 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-band
> gap.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.
>
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