At 16:29 2005-08-02, you wrote:
>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.
Fog is present under air pressure inversion. Then sound is reflected by a
"ceiling" of thicker air a bit above ground.
Klas.
>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/
>
>
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
>
Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
email:
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