-------------- Original message from "Ed Anderson" <eanderson@carolina.rr.com>: --------------

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