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I finally got the recordings from our experiments
loaded up onto my computer from my PDA. I actually got some good data before the
mic saturated.
Turns out, to my great surprise, the muffler did
just what it was supposed to do. It eliminated the harmonics above about 750 hz.
The wave form was clipped and the output much closer to a sine wave. The problem
was the amplitude of the sound it let through. The fundamental
200hz amplitude was very high. Most people (including me) who were standing
close to the test stand probably had saturated eardrums and were not able to
detect the change in the sound quality. There was just too much of it (first row
at a Stones concert sounds pretty crappy, back off a bit and things get better).
Those standing farther away could hear the difference and thought it sounded
good. I didn't think this was the case until I could listen to the sound files
on a good sound system where I could turn the volume down.
So the next plan is to keep doing what I did
before, but put a resonator to attenuate the fundamental frequency down to
something bearable. The bad news is this frequency is between 100 and 200 hz
depending on what RPM the engine is at. That means a longer heavier muffler.
Initial numbers show it being about the same diameter and 15 in
long.
I also had a chance to ring the muffler and see
what the resonant frequency of the structure is. As fate would have it, the
fundamental is 2400 hz. This is a multiple of the 1200 hz resonant
frequency of the cavity of the muffler. So it will self excite when the muffler
is really working hard. This explains the horrible screech at 6000-6500 rpm.
This is also bad for fatigue life. I should be able to weld a couple of
strips of metal to it and fix that problem.
Monty
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