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From: "Al Gietzen" <ALVentures@cox.net>
To: "'Rotary motors in aircraft'" <flyrotary@lancaironline.net>
Subject: RE: [FlyRotary] Phononic bandgap muffler
Date: Sun, 15 Nov 2009 09:05:37 -0800
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In-Reply-To: <list-3976637@logan.com>

Ernest;

Interesting stuff.  Just a guess; but it seems to me that, while 15 db =
is an
appreciable reduction, having that reduction only over the 8K to 12K =
maybe
isn't all that productive.  It would reduce the 'harshness', but isn't =
the
greatest amplitude at lower frequency?  Also, do you envision that this =
can
be made into a compact, workable package for aircraft application?

Best,

Al

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Ernest Christley
Sent: Saturday, November 14, 2009 9:38 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Phononic bandgap muffler

Here's a question for you audiophiles on the list.  How much effort is=20
10dB of muffling worth?

I've expanded on my experiments with phononic bandgap filters, and I=20
think I've fairly conclusively demonstrated up to 15dB of attenutation=20
in the frequency ranges between 8kHz and 12kHz.  I used the run-up video =

that Mike Wills provided for us a few weeks back (thanks Mike), and I=20
was a little more methodical than last time.

For those that don't remember, a phononic bandgap filter is created by=20
geometric arrangement of dissimillar materials, with the goal of=20
filtering specific vibration frequencies.  It appears that 1/2" diameter =

tubes, arranged in a grid with 1" centers does a nice job of attenuating =

frequencies in the 8kH to 12kHz range.=20

I set up my simulated muffler...a wooden shell with PVC filter=20
elements.   I stuck a couple speakers in the inlet side, and a=20
microphone on the outlet side.  In this setup, there is going to be all=20
sorts of losses from the conversion from an electronic signal to sound=20
in the speaker, then conversion of sound to an electronics signal in the =

microphone.  Comparing the original to the recorded muffled sound, which =

I did in the first experiment, is not valid.

What I did this time instead, is to compare several recordings.  Each=20
with one less row of filter elements than the last, until I was=20
measuring an empty box.  The empty box measurement is the control that=20
lets me know if I'm doing anything with the array of tubes.  What I=20
found was that each row of tubes provide a significant attenuation in=20
the frequencies between 8kHz and 12kHz.  The ones that give us the=20
weed-wacker sound.  I determined this by taking a time sample across=20
each of the 5 recordings and having Audacity plot the spectrum.

I have an xcf file created by The Gimp ( a free image editing=20
software).  The xcf format allows you to keep an image in layers.  Each=20
of the plots is in a separate layer, and by playing with the opacity=20
individual layers, you can easily see where the attenuation occurs.  The =

file is 465kB so I can't post it to the list, but if anyone is=20
interested, I'll be happy to send it direct.  I'll do a writeup with=20
links to the audio files "real soon now".=20

I have attached a flatten image showing the 0 row run, against the 4 row =

run.  The 0 run is the lighter purple shade.  There are some some=20
frequencies where the  4 row would be louder....around  4kHz and 7kHz,=20
but I think the attenuation of those frequencies around 10kHz would be=20
most beneficial.

--=20
Ernest Christley, President
Ernest@TechnicalTakedown.com

TechnicalTakedown, LLC
www.TechnicalTakedown.com
101 Steep Bank Dr.
Cary, NC  27518
(919) 741-9397