In another discussion entitled "Prop Noise Comparisons" James Pistorino asks about the prop size vs. noise factor.  This got me curious about something else, to which the answer seems intuitive, but I was wondering if anyone had real-world confirmation of this...

For an RC plane/drone flying at a few hundred feet (let's say 200'), using a four-stroke 28cc engine (so as to quantify the data), is there a substantial noise difference (as measured from the ground) between the muffler port exhausting (all assuming horizontal flight) upward, toward the back, or downward?

Does anyone have any real-time experience with this, as opposed to hypothetical extrapolation based on intuitive SWAG with some hyperbole thrown in to sound more credibile?

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Easy way for you to test, lay your model on its side and walk around it 200 feet away and see where the noise is the least. But a good muffuler will do more than port direction. This I can show a 60% reduction in engine noise in a bell 47 when we added the muffuler. We use them for crop dusting which is why we care about the noise
"lay your model on its side and walk around it 200 feet away"

Apples to oranges. Testing it at ground level, on its side to simulate 200' of height, and walking around it is vastly different than 200' up in the air, due to ground reflections. I guess I'm going to have to get a decent dB meter and actually engage in quantitative tests. I agree that a tuned muffler (ported for the engines RPM zone and equipped with an expansion chamber) does a fantastic job of changing the resonance as well as smoothing the output gas.
Muffler point upward and low RPM, sub 4000rpm will do the trick just fine - I've done it.
Thanks, guys. I'll keep the RPM range in mind, too.

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