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John,
My reason for the 4' length was two fold. One, it put the end of the
exhaust exit behind the cockpit (always helps in sound reduction {:>)) and
two, I could then used that length to try and fit as many sound trap as many
different harmonics of the engine rpm that I could fit into that length My
rough calculations indicates my traps should reduce sound intensity in the
range from 50 Hz to around 2400 Hz. However, if you are focus on say the
fundamental frequency (or rather a 1/4 wave length of that frequency )and
perhaps its 2nd harmonic (twice the fundamental freq.) which is where most
of the exhaust sound energy is located, you could probably get away with 24"
or so.
Now, I use two separate tubes which means at 6000 rpm each tube is dealing
with a fundamental frequency of 100 Hz. Now if you put both exhausts into
the same tube then the fundamental exhaust pulse frequency is 200HZ. That
reduces the length of the tube requirement somewhat, it should halve the
length required.
I believe (have not done the calculations) that if you put both exhaust into
the same tube that you may find 24" is sufficient to get some good noise
reduction. However, in order to handle double the exhaust gas flow that
putting both exhaust ports into one tube, you might find that a 2"DIA tube
might be on the small size. I would probably think about at least a 2 1/2"
perhaps 3" dia tube. Unfortunately, I had little success in finding 3" dia
SS discs at least .062 thick. So I did not go down the 3" path.
I did a number of calculations of wave length of the fundamental frequency
from 1500 rpm to 6000 rpm and from that calculated the 1/4 wave lengths of
the sound wave (which I calculated the wave was traveling at 2400 feet/sec
due to the very high temps in the exhaust). I also calculated the lengths
for the 2nd through 6 harmonic for each rpm. I then looked at what lengths
occurred most frequently in those calculations. I picked those lengths that
occurred most frequently (in some cases for example a 2nd harmonic length at
one rpm was the same as a 4th harmonic length at another rpm). That way my
hope was I would have the few trap lengths I could fit in that 48" capable
of providing some attenuation at more than one frequency (and some cases as
many 4).
Sorry, if this is as clear as mud. It may be that the dimension between
discs are not all that critical - but that is the way I went about it and
the reasons why. My theory could be all wet - but the results are
encouraging in any case.
Hope it helps.
Ed
----- Original Message -----
From: "John Downing" <frmrjohn@netonecom.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Tuesday, November 30, 2004 10:31 PM
Subject: [FlyRotary] Muffler design
> Ed; In lay terms, is there some reason for the 4' of length? I only have
> 22" of metal on the bottom of the boot cowl before the fabric starts on
the
> Tailwind. I'm looking to reduce the noise, but I don't necessary need a
> flaming turkey. From the farm. JohnD
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