Hi again dear friends!
Another puzzle for summertime. And again, I won't propose my explanation immediately, just in case it's not watertight...
All of you have probably noticed that distant sounds can be heard better if you're leeward
(hope it's the right English word for "downstream in the wind"). This effect is especially noticeable with distant sounds, like a train passing a couple of km away.
At first thought, it looks natural: sound carried by moving air has less distance to travel leeward. But at second thought, this explanation doesn't hold: with a good wind of 10m/s (already too noisy by itself), the air distance reduces by 3%, aiding the sound pressure by only 0.25dB - far too little
, as we need 10dB to perceive a significant difference, and rather 20dB change to notice something we haven't been made aware of.
So we need a better explanation
Perhaps is something similar to a flowing fluid through a pipe.
If you have water flowing through a pipe, the molecules most in contact with the pipe will move slower, while molecules near the center will move faster. Molecules that, in the open, would spread out, instead bounce off of the inside surface of the pipe, back into the stream.
Now if we visualize the moving air in this fashion, as well as the propagating sound waves, it is possible that the sound waves hit a boundary with the non-moving air (or slower moving air) and are reflected back into the moving air, thus causing the sound waves to remain concentrated instead of spreading, This could act as a "megaphone", channeling a larger percentage of the original sound energy in the direction of the listener, not just getting it there faster.
Slow Air or moist air
Slow Air or ground
The sound waves could also be rebounding off the bottom of clouds, and thus also concentrating "more" sound waves on your ear than normal circumstances.
Basically, the idea here is you are hearing a tunnel effect, only it is a tunnel of air, instead of a man made pipe.
21st July 2008 - 06:00 AM
QUOTE (kjw+Jul 21 2008, 03:42 AM)
what about this explanation here (under xi wind and waves)
Ooh, I like that explanation; it means that I might have been close!
21st July 2008 - 07:08 PM
You are collectively too strong!
I believe the wind shear is indeed the right answer. Nice page at the link, by the way.
That is: wind is stronger at altitude than near to the ground, and increases or decreases the speed of sound. So in leeward propagation, the resulting speed is smaller near to the ground, and this guides soundwaves near to the ground just as a fibre guides light. Windward, sound is dispersed towards the sky.
Well done everybody!
21st July 2008 - 07:11 PM
Sound travels differently at higher altitudes because the medium is less dense, not (only) because of the winds.
21st July 2008 - 09:28 PM
bm1957 Posted on Yesterday at 4:00 PM Ooh, I like that explanation; it means that I might have been close!
as for my lack of attention to information in front of my nose... not at all well done
21st July 2008 - 09:54 PM
All that have replied to this thread so far would be very welcome at my place!
This is the sort of loose, but reasoned and polite discussion that could blow the 'tumbleweeds' out of the Joint
There is so much humor in such a talk, as well as good acoustical engineering...
PF is wonderful, but they don't have recipes.
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