ORIGINAL: babflyer
Abel, I may be wrong on this but the "A" part you talk about, would that be the same thing as comparing a high pitched two stroke engine to a four stroke, plane, dirtbike, whatever? I guess is the "A" part supposed to be a way to compensate for a lets say a louder 4 cycle to become the equal of a quieter 2 stroke becuase the human ear finds the noise less offensive? Just asking. If I take the rest of your post right you are basically saying that you really cant control the noise because we have props making a lot of it no matter how they are driven. That might be true as I heard a 3w 4 cylinder turning a prop to where it made WAY more noise than any other plane I have ever heard. I thought it was great
.lol. <snipped>
Yeah babflyer, you got the essence of it straight. I wouldn't go so far as to say you can't control noise because prop noise is the limit, though. You just have to recognize that prop generated noise will be the limiting factor when you have done all that is feasible to control engine generated sounds, and control that even if it limits the amount of power you can supply to it.
Regarding exhaust extensions of silicone compounds and such, they can indeed work to reduce exhaust sound level. A caveat is that they can also detune the exhaust system and result in all sorts of mysterious engine problems. The usual expansion chamber muffler represents a capacitive reactance to the flow out of the exhaust port. We don't often think of stock mufflers as 'tuned' exhaust systems, but they are in fact just that - though due to their usual short length they are tuned to a harmonic frequency several times the fundamental exhaust port opening frequency. When properly 'tuned,' a low pressure front is reflected back to the exhaust port at the time it opens, promoting exhaust scavenging. A length of tubing attached to the muffler outlet is constrictive, and in electrical analogue terms represents an inductive reactance. This can change the the timing of exhaust port/wavefront timing dramatically, such that a reflected high pressure front coincides with the opening of the exhaust port, impeding the scavenging part of the cycle and significantly affecting performance. Just be aware of this and that an exhaust extension should be first on the list of suspects when the engine just won't run right. It's not always a negative impact - many owners of K&B .65 engines, for example, have found that it makes more power and produces even less noise (the stock muffler is itself one of the better supplied with engines) with an 18" length of automotive fuel/vapor tubing attached to the muffler outlet.
As for the public relations aspect, yes in spades! We should not forget that sound level is just one of many factors that contribute to noise annoyance, which is entirely subjective, and sound level is just a simple objective measure that happens to correlate well with it. For me, Wagner at 110 dB is music, a mosquito at 40 dB is noise. One's attitude toward the source of a sound can make all the difference in the world.
Abel
edited: format