Paul wrote:

Taking that idea a step further can you think of a way of making the
resonators out of low temp low weight composites?

Frank van der Hulst wrote:

As I said, I'm no expert on exhaust system design.

Actually, my thinking on this came from information theory, which I
teach as part of data comms. It seems to tie in well with what Brian
Perry wrote.

Simplistically, what we want to do is to split the exhaust pulse into
two, then delay one of these pulses by half a wavelength, so that it
comes out the exhaust pipe exactly 180 degrees out of phase with the
other pulse. This way, the waves will cancel each other out and there
would be no sound at all!

However, there are numerous frequencies present, so delaying by half a
wavelength of one will double the amplitude of waves of 2x that
frequency. And won't do anything useful for waves of other frequencies.
(Another thought -- what about some kind of ANR on/in the exhaust pipe?
A heat-proof microphone connected via an ANR circuit and amplifier to a
heatproof speaker placed close to the exhaust. Or would this take an
exhorbitant amount of electricity to drive? How much energy is in the
exhaust sound anyway? Oops... we're back to needing absolute amplitude!)

Hence (I think) Brian's suggestion to basically aim to squash the peak
frequencies down to the same level as the rest of the noise, so that a
uniform 'white noise' would be heard. To do this, large pulses are
'chopped up' into separate small pulses travelling by different paths,
each of different length, so that a higher frequency but lower amplitude
sound is produced.

I think that's how the 'multiple holes in the exhaust tube' muffler
works... each hole provides a slightly different length path for part of
the pulse to take. I guess that ideally the holes should be spaced at
irregular intervals. (Incidentally, in my youth it was popular to have
extra loud exhausts on our cars and motorcycles. The authorities didn't
approve, so for our 6-monthly 'Warrant of Fitness' vehicle inspections,
we used to fill the muffler with fibreglass building insulation. I gue
woulin the same way).

I do wonder about how important the positioning of the holes is -- like
a flute, you could place holes to amplify or attenuate particular
frequencies.

Paul wrote:
The other thing to think about is how much is it engine exhaust noise
and how much is it prop noise.

Turn the prop at 1500RPM instead of 3000 and the prop noise should
reduce markedly. Is there any reason why we couldn't run a 4.34:1
redrive instead of 2.17:1? Would this require a prop with very long
blades with consequent ground clearance problems, or could you just
increase the pitch of a conventional-sized prop? How about a 4-blade
prop? 4 blades at 1500rpm should be about as efficient as 2 blades at
3000, I'd guess. Maybe it would be more efficient?

My guess is that props are made to turn at about 3000 RPM max because 50
years ago that was about the max that a Lycont could turn.

Frank.

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