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From: "Joe Hull" <JoeH@PilgrimTech.com>
To: "'Rotary motors in aircraft'" <flyrotary@lancaironline.net>
Subject: RE: [FlyRotary] Re: exhaust design question
Date: Sat, 11 Dec 2004 21:35:01 -0800
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Some styles of silencers (pistol) are made with a series of disks
inside a perforated tube. If one has enough space outside the cowl a
section of perforated tube would help dissipate the exhaust pulses.

Joe Hull
Bellevue (Seattle), Washington
Cozy MkIV #991 (working on Engine & Electrical & Finishing)
http://www.maddyhome.com/canardpages/pages/JoeHull/index.html
 
> -----Original Message-----
> From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
> On Behalf Of rijakits
> Sent: Saturday, December 11, 2004 11:51 AM
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Re: exhaust design question
> 
> A wild idea/question: How is the sound emitted from something like a
> silencer?
> Remember any pics of sound proof chambers - like the car
> manufacturers have
> to check source and dynamics of sound coming of their cars. These
> chambers
> are all full of triangular foam (or so it seems) to eat up the
> sound, as not
> to reflect anything and falsify the data.
> 
> Would it work to weld triangular shaped pieces on the OUTSIDE of a
> muffler?
> .... or better has anyone any idea or tried that already?
> I have no reason to believe that this works, but I think is worth a
> thought!
> Any sound engineers on the list?
> 
> I remember in the old LP (music) times the very highend turntables
> would
> come with an accessory: a heavy glassplate that was to be supported
> by
> inverse ceramic cones or glasscones that in turn where supposed to
> be placed
> on ceramic or glass again (point down). Supposedly to not absorb any
> frequency from the floor/suroundings and to not loose any sound to
> the floor
> either....
> 
> I am not enough of a music conoseur to ever have found a difference,
> but
> that doesn't mean it didn't work:))
> 
> Any ideas?
> 
> Thomas J.
> 
> 
> 
> 
> ----- Original Message -----
> From: "Charlie England" <ceengland@bellsouth.net>
> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> Sent: Saturday, December 11, 2004 2:30 PM
> Subject: [FlyRotary] Re: exhaust design question
> 
> 
> > I was hoping the same thing, but at the rotary roundup, Tracy's
> engine
> > still had a lot of that sharp edged bite as the rpm came up. Not
> as
> > strong as the old 13B, but still there. Standing to the side of
> the
> > plane, it almost sounded like the muffler tube (it's exposed under
> the
> > fuselage) was resonating at the upper midrange pitch that's
> irritating.
> >
> > Higher frequencies are usually easier to control & damp out than
> low
> > freqs, so I wonder if the sound would be as bad if the muffler was
> > inside the fiberglas cowling. That way, the cowl itself could
> become
> > part of the silencing system by damping the sound radiated by the
> muffler.
> >
> > Charlie
> >
> >
> > Dennis Haverlah wrote:
> >
> > > I would expect the Renesis engine with the split/side exhaust is
> not
> > > as irritating.  What is the experience?  I have not heard either
> a 13B
> > > or Renesis takeoff or fly-by.
> > > Dennis H.
> > >
> > > Charlie England wrote:
> > >
> > >> Russell Duffy wrote:
> > >>
> > >>> So it appears its as much the nature of the Rotary "bark" as
> it is
> > >>> the sound level.
> > >>>
> > >>>
> > >>> I completely agree, and I've tried to get people around the
> airport
> > >>> to understand this.  Since the engine doesn't sound like a
> "normal"
> > >>> engine, everyone takes note.  Since they noticed it, they
> assume
> > >>> it's primarily because it must be louder than other engines.
> This
> > >>> isn't necessarily true of course.  Glad you got your power
> back.
> > >>> Rusty (yawn)
> > >>
> > >>
> > >>
> > >>
> > >> This phenomenon is widely understood by audiophiles & (ex)sound
> > >> engineers. Certain frequencies, and certain harmonic structures
> added
> > >> to any sound, are much more irritating to the ear than others.
> The
> > >> 'bark' or 'edge' in the sound of a 2stroke or rotary is the
> harmonic
> > >> structure of the sound, not the actual level.  I suspect that
> if you
> > >> fed a microphone into an oscilloscope, you'd see something like
> a
> > >> sine or triangle wave from a 4stroke piston engine & something
> > >> approaching a square wave from a 2stroke or rotary. We  can run
> that
> > >> experiment here at Slobovia the next time we have a visit from
> a
> rotary.
> > >>
> > >> I suspect that's why Paul Conner's exhaust is actually a
> pleasing
> > >> sound while most rotaries 'hurt' your ears. His iron manifold
> is
> > >> probably filtering out the irritating harmonics & 'rounding
> off' the
> > >> waveform.
> > >>
> > >> Charlie
> > >
> >
> >
> > >>  Homepage:  http://www.flyrotary.com/
> > >>  Archive:   http://lancaironline.net/lists/flyrotary/List.html
> >
> 
> 
> 
> >>  Homepage:  http://www.flyrotary.com/
> >>  Archive:   http://lancaironline.net/lists/flyrotary/List.html