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Date: Sun, 15 Nov 2009 21:25:03 -0600
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Phononic bandgap muffler[edit]
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Charlie England wrote:
> Ernest Christley 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.
>>
>>
>> ------------------------------------------------------------------------
>>
>
> Hi Earnest,
>
> 10-15dB is a very respectable reduction. A 10 dB change is a 10X power 
> change and a perceived doubling or halving in volume to the human ear 
> (dB's are a logarithmic power measurement).
>
> However, I doubt that working on the 8k-12k range is going to have 
> much effect on the perceived noise from a rotary, for a couple of 
> reasons. I can't find the emails & info from Monty Roberts' testing a 
> few years ago, but IIRC, the problem areas are much lower in 
> frequency. Look at the ~1k-2khz area & the

edit here: should read that the 3k-4khz stuff and above, not just those 
freqs above 4khz, were due to ringing of the muffler itself
> ~3.5k-4khz areas. Those areas are much stronger, and they are in the 
> 'zone' where human hearing is most sensitive. When Monty did his 
> testing, the stuff above 4khz was harmonics radiating off the muffler 
> itself (the muffler 'rings' like a bell). The muffler wasn't enclosed 
> in a cowling.
>
> So here's what I think. The 8khz & above stuff is barely audible to 
> people over 40, or who have rock concerts in their history, or fly 
> airplanes for any length of time. Additionally, stuff in that 
> frequency range hates to turn corners and is easily absorbed by 
> relatively low mass materials. When you take those two factors into 
> consideration together, something as simple as enclosing the muffler 
> in a housing that won't sustain a resonance (think fiberglass cowling) 
> will kill just about all of it. The stuff under 4khz, though, is a bit 
> harder to deal with, and that spot down below 2khz is really tough. As 
> I mentioned earlier, those are much more likely to sound offensive, too.
>
> If you can suppress the 1k-2khz range, the higher freqs will likely 
> take care of themselves because they are harmonics of the lower freq 
> stuff.
>
> Remember the discussion about Paul Conner's engine with the stock 
> exhaust manifold? The cast iron was massive enough to absorb all those 
> higher order harmonics and the FG cowl finished the work, even with 
> the exhaust being dumped out the cowl with just a little 8-10" stub 
> pipe off the manifold. It wasn't 'quiet', but it was very pleasing to 
> listen to, like a small block V-8. Of course, the downside for a 
> non-renesis 13B is the loss of power because you don't get a tuning 
> boost from a header. The only other pleasant-sounding rotary plane 
> that I've personally listened to (other than turbo'd engines) is 
> Dennis Haverlah's Renesis, and it uses the stock Renesis exhaust 
> manifold. If you look at the way it's constructed, it's made of 3 
> layers of material, apparently for noise suppression and heat 
> shielding in one package.
>
> If Monty reads this, maybe he can repost his test results & expand on 
> or correct what I've written here.
>
> It might be helpful to load a tone generator program & listen to 
> various frequencies to get an idea of what each frequency sounds like. 
> If you use Audacity or a similar sound editing program, it has a tone 
> generator built in.
>
> Charlie