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
10dB of muffling worth?

I've expanded on my experiments with phononic bandgap filters, and I
think I've fairly conclusively demonstrated up to 15dB of attenutation
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
was a little more methodical than last time.

For those that don't remember, a phononic bandgap filter is created by
geometric arrangement of dissimillar materials, with the goal of
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.

I set up my simulated muffler...a wooden shell with PVC filter
elements.   I stuck a couple speakers in the inlet side, and a
microphone on the outlet side.  In this setup, there is going to be all
sorts of losses from the conversion from an electronic signal to sound
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
with one less row of filter elements than the last, until I was
measuring an empty box.  The empty box measurement is the control that
lets me know if I'm doing anything with the array of tubes.  What I
found was that each row of tubes provide a significant attenuation in
the frequencies between 8kHz and 12kHz.  The ones that give us the
weed-wacker sound.  I determined this by taking a time sample across
each of the 5 recordings and having Audacity plot the spectrum.

I have an xcf file created by The Gimp ( a free image editing
software).  The xcf format allows you to keep an image in layers.  Each
of the plots is in a separate layer, and by playing with the opacity
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
interested, I'll be happy to send it direct.  I'll do a writeup with
links to the audio files "real soon now".

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
frequencies where the  4 row would be louder....around  4kHz and 7kHz,
but I think the attenuation of those frequencies around 10kHz would be
most beneficial.

--
Ernest Christley, President
Ernest@TechnicalTakedown.com

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