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For the time being I've decided to deal with it by
using a better headset and making the rest of the world live with it. But I'm
anxiously waiting for the time when somebody takes all of the theoretical stuff,
builds a real muffler, and bolts it onto a screaming rotary to test it against a
known benchmark. My plate is full just getting my Phase 1 testing done and I
dont want to sidetrack that with a bunch of testing/experimenting. But I'd be
willing to provide my DNA muffler as the benchmark if someone is willing to step
up and do this testing.
Off topic. Did a little data gathering yesterday to
verify performance of my airspeed/static system. Flew the 4 leg course described
by Doug Grey in a recent Kitplanes article and plugged the numbers into the
spreadsheet provided in that article. Looks like my airspeed indicator and
static system are performing well, at least at the 75% cruise condition. At
8500' and full throttle I'm getting 191 MPH TAS. This lines up pretty well with
Van's book number for a 160HP Lyc and definitely exceeds the performance of my
old RV-6A.
Mike Wills
RV-4 N144MW
----- Original Message -----
Sent: Sunday, November 15, 2009 5:46
AM
Subject: [Norton AntiSpam] [FlyRotary]
Re: Phononic bandgap muffler
Ernest,
This is very interesting and applicable to every rotary
installation. Sooner or later everyone must deal with the exhaust noise
one way or another. For me, I'm waiting to hear more (no pun intended)
about your experiments.
Mark S.
On Sat, Nov 14, 2009 at 11:37 PM, Ernest Christley
<echristley@nc.rr.com>
wrote:
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
TechnicalTakedown,
LLC
www.TechnicalTakedown.com
101 Steep Bank Dr.
Cary,
NC 27518
(919) 741-9397
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