At 09:19 AM 8/2/2005, flawn wrote:
> >"Getting close is in fact even more effective than that. Signal strength
> >is inversely proportional to the square of the distance. So that half the
> >distance means four times the signal strength."
>
>In a free field, sound emanates out from a source in three dimensions. So I
>would think the drop in power is in a sphere relationship, not a square. In
>that example, halving the distance to the source would achieve eight times
>greater signal, not four. In the real world, with the ground as a barrier
>and
>trees etc providing scatter, the actual difference would be somewhere
>between the
>squared and the cubed ratios.
In real life, I think, it is far more variable. We commonly find that when
there is a fog, distant sounds seem subjectively to be louder. They
actually may be.
Loss with distance is never as bad as a cube relation, however, since sound
intensity has the dimension of power per unit area, not per unit
volume. Power is of course energy per second, as in ergs per
second. Double the radius of a sphere, the AREA quadruples, so the worst
loss is a factor of four.
When the bottom 1000 ft of the atmosphere are fairly uniform, but quiet,
sound does radiate in three dimensions, so doubling the distance makes the
sound 1/4 the power per unit area, a loss of 6 dB.
When there is an inversion, or any thermal sudden change in the lowest
hundred feet or so, sound is confined to a ring of air, roughly given by
the circumference. Here, on a foggy still morning, doubling the distance
doubles the size of the ring, now vertically confined. So we find the power
to be 1/2, or a loss of 3 dB. Distant sounds ARE louder under such
conditions that sound is trapped. This is somewhat analogous to the SOFAR
layer in the ocean, where water density/temperature/salinity afects trap
sound for miles in a ring of a certain depth range. Roger Payne often said
to me he thought whales might be able to hear one another, in an aboriginal
quiet ocean, halfway around the world.
We live 1500 ft from a (newly moved, but legally grandfathered in NH) skeet
range, often exceeding at 6 PM 30 shots per minute. Boy does it get
lively, Twelve gauge shotguns register 84 dBA at our doorstep. Recordings
available ad nauseum on request. ;^)
-- best regards, Marty Michener
MIST Software Assoc. Inc., P. O. Box 269, Hollis, NH 03049
http://www.enjoybirds.com/
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