flyrotary@lancaironline.net Mailing List Archive

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. -- Homepage: http://www.flyrotary.com/ Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/List.html __________ Information from ESET NOD32 Antivirus, version of virus signature database 3267 (20080714) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com __________ Information from ESET NOD32 Antivirus, version of virus signature database 3267 (20080714) __________ The message was checked by ESET NOD32 Antivirus. http://www.eset.com