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From: "rijakits" <rijakits@cwpanama.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-563436@logan.com>
Subject: Re: [FlyRotary] Re: exhaust design question
Date: Sat, 11 Dec 2004 14:51:12 -0500
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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
>