-------------- Original message from Ernest Christley <echristley@nc.rr.com>: --------------


> 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
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> Cary, NC 27518
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>