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Date: Tue, 30 Nov 2004 18:49:07 -0600
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: New Muffler Design
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Bill Dube wrote:

> At 04:02 PM 11/30/2004 -0500, you wrote:
>
>> 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. 
>
>
>         Tell us more about Finite Amplitude Wave theory.
>
>>  
>> 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.
>
>
>         Pictures? Drawings? How many disks? What spacing?
>
>>  
>> 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!
>
>
>         Did you do the test in the open or in the hangar?
>
>>  
>> 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??
>
>
>         It is a log scale. 3 dB is indeed 1/2 the sound intensity. The 
> 9 dB is not 3 x 0.5 it is 0.5 x 0.5 x 0.5 Thus it is 1/8th the sound 
> intensity.
>         
>         I'm surprised you haven't connected the two pipes after they 
> leave the engine. In cars with dual exhaust, they put a cross-over 
> pipe just after the header (or manifold). The cross-over pipe is 
> typically smaller than the main pipes. The cross-over effectively 
> doubles the frequency and halves the amplitude of the sound pulses in 
> each side. The higher frequency is more strongly attenuated by the 
> low-pass filters (mufflers). The effect of adding a cross-over pipe is 
> dramatic. The exhaust note goes from painful to pleasant.
>
>         If the cross-over pipe placed properly, there is no loss of 
> power. (Sometimes you get a small increase.) You want the header to 
> scavenge the combustion chamber properly, so you have to leave enough 
> header to do this.  On a V-8 trying to make good low-end torque, this 
> is about 18" or so. I have no clue what header a rotary would like to 
> make max HP at high RPM. 


And from a guy who did pro sound work in a former life: the 3dB drop is 
indeed 1/2 the *power* but is typically just noticeable as a change to 
the average ear. The 10dB decrease is 1/10 the power, but is perceived 
as being 1/2 the sound level by an average ear. (This is worth 
remembering when the stereo sales guy tries to convince you that 
100watts is twice as good as 50watts.)

Bottom line, if you got a 10dB decrease, your plane should seem 1/2 as 
loud to the listener, an impressive accomplishment.

Way to go Ed!

Charlie

BTW, which weighting are you using? If memory serves, the 'A' weighting 
is an almost worthless measurement for working on protecting your 
hearing. (details if anyone is interested)