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I'll have to do some digging for my notes and such. That was a couple computers ago. As I recall, the problem was the high frequency content of the rotary exhaust. This is present in all ported engines. Think dirt bike: didinnnnggggdinnngggdinnnnnng. The muffler I made was according to some NACA papers on silencer design. It was effective at muting the problem frequencies. Unfortunately the muffler casing did "ring" at the fundamental frequency the engine hit at about 6000-6500 rpm. I don't think the thing would have lasted very long ringing like that. It would have fatigued and disintegrated in short order. The resonator can work, but the casing will have to be designed not to ring anywhere in the operating range. The tests were promising, I honestly haven't had a chance to set down and look at modifying the muffler to eliminate that problem. The base frequency of the rotary in cruise is comparable to a Merlin or a V-8 in their respective operating ranges (IIRC 200 Hz or so). I don't think that it is necessary to muffle the low frequencies. It would require a large heavy muffler to do so anyway. It is just the 1500-2000 kHZ that grates on everybody (chainsaw). The higher stuff is as Charlie said, harmonics of the 1500-2000 kHZ noise. You can't kill the higher frequencies effectively without an absorption type silencer. We all know how long they last behind the rotary.....
So concentrate on the 1500-2000 kHZ range. It is possible to make a resonance/band type filter in this range, and taking care of the fundamental Freq. should take care of the higher harmonics also. The problem is mostly making something that will stand the thermal and mechanical abuse.
Of course a simple expansion chamber with a restriction on the exhaust will make it ultra quiet. You will pay a power penalty.
Dennis has a combination of things working in his favor. One is the Renesis' cooler exhaust, the very heavily engineered stock manifold, a 90 degree turn to kill a lot of the high frequency stuff, and an absorption type muffler to clean up the rest.
I'll do some digging and see what is on my old computer.
Monty
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 ~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
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