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Well, folks, got the new muffler design installed
today - and initially results are encouraging. I designed the muffler
based on what I believe I have learned concerning Finite Amplitude Wave theory
and applied it to this muffler design.
The mufflers consider of two 2" dia tubes 4' long
with sound attenuating discs inserted. Each muffler hangs off of my two
header pipes and extend back under the fuselage with the exhaust exit dumping
behind the cockpit area.
Subjectively, two hanger bums standing by the wing
tips told me that the exhaust was no louder at WOT than at idle and that the
sound of the prop drowned out the exhaust at WOT. I also
noticed that while sitting in the cockpit with the radio on I heard
ignition static over the radio receiver for the first time!
I first took off the old exhaust and recorded the
following sound levels from the two header pipes at 2000 rpm. (unfortunately at
that point I had no one to do the sound measurement and did not feel I
could run it faster without someone in the cockpit (that big prop does cause it
to sway and bounce) - so did not get any sound readings at WOT (5600
rpm). However, previously while at Tracy's it was measured at 125 DB (max
scale reading - so it could have been higher) 10 feet to the
rear.
Anyhow, today Without any mufflers - headers
only. Using a Radio Shack Sound meter I measured:
At left tip of Horz Stabilizer 113 DB at C
weighting Fast response (same for all measurements)
10 feet from Horz Stab on the same angle
(approx 30 deg off center) 105 DB
20 feet from Horz Stab on the same angle 97
DB
With new Mufflers
At left tip of Horz Stabilizer 98 DB
10 Feet from Horz Stab 94
DB
20 feet from Horz Stab 88
DB
These measurements were all at 2000 rpm,
Normally I would have expected those readings to increase considerably at WOT -
but according to the two witnesses the WOT exhaust sound was not louder and the
prop noise drowned it out. So it looks like I get around a 10 db decrease
in sound based on the above.
For you sound experts, I presume if a 3DB increase
represents a perceived doubling of loudness does that mean a 10 DB decrease
represents an approx 3 fold decrease in perceived loudness??
If true, this would seem to indicate that I
can possibly reduce the number of sound deadening discs and while my Static rpm
appeared unchanged, I can't help but feel I must be losing some power if the
sound is being deadened that much.
Now need to go flying with it.
Ed
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