Mike Wills wrote: 
> At one point a few months ago I added a SuperTrapp to my existing > exhaust just to see if it would tame it a little. Cost me about 500RPM > on the top end with little or no improvement in the sound output. 
> > Mike Wills 
> 
> ----- Original Message ----- 
> *From:* wrjjrs@aol.com <mailto:wrjjrs@aol.com> 
> *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net> 
> *Sent:* Monday, January 05, 2009 11:49 AM 
> *Subject:* [FlyRotary] Re: 20B install issues overcome 
> 
> Earnest, 
> I believe that the SuperTrapp muffler is an example of a band gap 
> device and it worked well, if properly tuned. 
> I think the large block idea has merit too, but you will have to 
> allow enough thickness to prevent the tubes that see the exhaust 
> first to fight the harmonic vibration and heat. Different size 
> diamonds in the exhaust stream might also help fight harmonics. 
> Bill Jepson 
> 
Looking at their website, the SuperTrapp isn't a phononic filter. In fact, it looks to me to be little more than an adjustable exhaust restriction.  
The secret to the phononic devices is that they use a crystal structure to maintain destructive interference of the sound waves within a narrow band of wavelengths. Increasing the size of the "crystal" will not only reduce restriction, it will improve the filtering/muffling properties, in direct contrast to what the SuperTrapp is doing. 
 
The design problem is to first identify the frequencies that need to be filtered. I'm pretty sure, from work done early by Monty Roberts, that we're mostly concerned with frequencies from 0 to 12kHz. Referring to the second link below, it appears that a chamber of 1/8" stainless rods, arranged in a hexagonal matrix with a .907 filling fraction would result in a 60dB noise reduction. The inlet to the muffler would look like the diffusing duct to spread out exhaust heat and energy over as much area as needed to get good airflow. 
 
http://www.springerlink.com/index/m633428032462347.pdf 
http://arxiv.org/pdf/0708.3669 
http://web.mit.edu/course/3/3.042/team1_08f/documents/complete-phononic-bandgap.pdf 
 
Ernest,
I understood what you were after, interactive self-cancellation of the sound waves by using the proper interference pattern. A good idea. I hope it can work and doesn't get too heavy. I understand Ernest the Trapps weren't a harmonically tuned device. If I understand correctly the muffler will need to be tuned to the proper frequency to make the system work best.
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