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Old 09-21-2004, 11:56 PM   #80
Mee-n-Mac
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Talking Voices carry (apologies to Ms Mann)

Quote:
Originally Posted by Rob
So if I understand you correctly, this has a lot to do with how far sound carries at night, compared to midday. Part of it is the lack of background noise (other boats, cars, power tools, even wind), but another part of the equation is the flat water, which allows sound waves to carry.
Yes but there are 2 points here. First is the well known phenomenon of how well voices (sound) will carry over the calm lake at night*. Most science articles will tell you that it's due to refraction (bending) of the sound waves as they travel. You'd expect that sound not "aimed" at your ear would pass by you unheard but due to refraction some of the sound initially aimed overhead is bent downward and is effectively now re-aimed and impinges upon your ear. This indirect path can add to the direct path sound and increase the level you hear. I won't bore you with the physics behind the refraction (besides there's some debate as to what effect is actually dominant, air density difference due to temperature inversion or air density difference due to humidity).

The other point is that sound will emanate from it's source in widely varying directions. As I said above you hear only the sound waves that hit your ear but these can come from different directions (other than the most direct, straight line path) as the sound reflects off different surfaces, one being the water itself. Look at the Sun reflecting off the water and now imagine it's sound waves rather than light waves. A calm lake gets you (more or less) a single patch of reflection while a wavy lake gets you multiple, small, time varying reflectors. Other reflectors might be the glass windows of the cabin behind you or from the shore across the way or from ... you get the idea. Just like multiple waves intersecting on the lake can create places where the wave crests are higher (and troughs lower) than any single wave, so can the various sound waves add or subtract to create higher or lower SPLs. How much difference this makes in the real world I don't know but it's interesting that in order to get reproducable results the test procedures call for short distances (50') btw boat and meter and large distance from other potential reflecting items.

Quote:
Originally Posted by Rob
That's a great idea. (informal SPL measurements) I wouldn't mind swinging by, as I want to try out some new clamp-on muffler tips. They clamp onto the outside of the thru-hulls, so we can do a test with just my internal muffler system, and then try the tips to see if they reduce the dB level by as much as the manufacturer claims.

I could even call the MP and ask if they would let us use their official dB testing site over by Timber Island.

I have some friends who got tested (and passed), if they are willing and available, we could use their scores to calibrate your meter (or to adjust our test methodology).
Rob
Might be a fun experiment. Could make a few passes at varying distances to see how well the real world adheres to the anechoic, free space 6dB rule as well. Only problem would be the MP getting antsy about > headway speed passes

*This is one good use for CL's idea of allowing switchable exhaust. One reasonably quiet for daytime and one really quiet for night-time. No doubt too much hassle to enforce though ....

I do have 1 question for LRSLA How did ME arrive at the 75 dBA limit ? This is more restrictive than NH's 82 dBA. How well do older OB engines do with that limit ? (OK, 2 questions )
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Last edited by Mee-n-Mac; 09-22-2004 at 04:52 AM. Reason: spellin
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