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From: "Al Gietzen" <ALVentures@cox.net>
To: "'Rotary motors in aircraft'" <flyrotary@lancaironline.net>
Subject: Muffler design (was 20B isssues . . )
Date: Tue, 6 Jan 2009 06:42:50 -0800
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-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Ernest Christley
Sent: Tuesday, January 06, 2009 6:02 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: 20B install issues overcome

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.

----------------------
Unless I did the math wrong; the maximum filling fraction you can get
 with these rods is about .8.  So I think there would be a problem there.

Al G