-----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
>



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