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Date: Sat, 11 Dec 2004 10:27:44 -0600
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: exhaust design question
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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