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Originally Posted by Fat Jack
Woodsy,
Again, you insult the intelligence of those who disagree with you rather than debate the facts with civility.
The degradation of sound at the rates you describe relate only to sound emitted from a point source (like fireworks in the air), not from a directed source, like a gun or an exhaust pipe. This is why megaphones are shaped to focus sound waves and eliminate that degradation. The waves propegate from the point source in a growing sphere. Sound intensity, measured in dB's, is a measurement of power per unit area. Since the sphere is growing as it leaves the source, its area increases as the square of the distance from the source, while the power remains the same (in theory), hence the exponential decrease in intensity. If you have ever heard a loud cigarette boat approaching and passing you, you have noticed how much much louder it is going away from you, because you are then in its focused exhaust stream. The current noise testing procedure looks for the highest noise intensity as the boat approaches, passes and travels away from the dB meter. During the period where the boat is traveling away is ALWAYS when the tester realizes the highest intensity...when they are in that exhaust stream. I cannot say whether a boat that produces 88 dB (4 times louder than the 82 dB limit currently allows) at idle when measured behind the exhaust stream will be louder at WOT than the current laws allow, but it certainly deserves a close look....wouldn't you agree? If this change in the law will allow louder boats, would you want to know that? And would you still support it?
Do you disagree that 88 dB is four times louder than 82 dB?
BTW, I have boated on Lake George. It was a wonderful experience, but no, I do not want to move there. Since the speed limits were enacted, property values have gone through the roof...similar to those on our Squam, which also has a speed limit.
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While you've accurately described why the inverse square law holds true, I'd like to point out that even for megaphones or other "directed" sounds it still holds true. All a megaphone does is concentrate energy that might go out to the side and send it forward. On any radial line going out from the megaphone you will still measure a 6dB decrease in measured SPL for each doubling of distance with really just 2 caveats. First is that you need to be in the "far field" and 2'nd that you need to be in "free space". Let's take the latter first as it's the easiest to explain.
To be in "free space" means you don't have any other reflectors the will add echoes into the energy you're measuring. On the lake we have a big reflector, the lake water. When flat it can reflect a fair amount of energy that will decrease the amount of attenuation due to distance. The exact amount will depend on the sea state and sound frequency but the general rule of thumb that I've seen is a 5dB/doubling vs the 6dB above.
To be in the "far field" basically means that you can reasonably model the emitting source as a point source (more technically the phase front of the waveform should resemble that of a plane wave). Normally the distance per J2005 would be sufficient to be "far field" for any single pipe but when you add in multiple pipes and the reflections off the transom and water then I'd have to guess you're still in the "near field". Somewhere between 10 and 20 ft away I'd say you're "far feild", it'll vary a bit from boat to boat. This means that instead of the 5 db/doubling above I'd expect the attenuation to be more like 2.5 - 3.0 dB/doubling. Once in the "far field" the ~5 dB/doubling then applies.
As to when the SPLs are the loudest then as we all know it's louder at the back than at the front (megaphone effect if you will) but if you look at the levels as a boat passes by (like the NH 50' test) you'll see the inverse square law pretty much rules. The boat is loudest when it's closest. Look at these curves measured by another poster ...
http://www.winnipesaukee.com/forums/...=8713#poststop
As to 88 vs 82 dB, well it's pretty much all be said. Yes it's 4X the power level but we humans hear sound logarithmically it doesn't sound 4X as loud. As for damaging ... you have to be loud enough to be damaging in the first place. As already said OSHA places a 75 dBA, averaged equivalent exposure as the limit a person should be exposed to for 8 hrs/day, 40 hrs/week. Below this value 4X or 10X doesn't matter, there's no damage. Even way above this level, for the short duration of it's passage, I don't see the vast majority of noisy boats as damaging, just unacceptably noisy.
Lastly as to Lake George, how loud did you find it there ? They have a 86 dB limit at 3500 rpm and 50' for their law.
http://www.lgpc.state.ny.us/646_2.htm#646-2.8
ps - I'm no sound engineer either but I do have some background in wave propagation.