On Thu, 27 Dec 2007 11:51:39 -0500, "Ethan Winer" ethanw at
ethanwiner dot com wrote:
Bob,
If one reflector was all it took to get 100% reciprocal operation, then
they wouldn't use two reflectors.
I don't think that follows. With only one reflector, the gain would be
insufficient for EITHER person to hear the other. Again, I'm not arguing too
strongly because I don't really know for sure. But so far I can't see why
it's not reciprocal.
The guy without a reflector can aim his beam at the distant reflector,
which might have an area of (say) 10 square feet, and anywhere he aims
within that area the beam will be captured and received more-or-less 100%
at the focal point.
Yes, but a person talking is not a beam.
The reflector provides gain for the listener, by concentrating the
sound from a large area down to a small area at his ear. Let's
say that the reflector is 10 square feet, so all sound impinging
on that area is concentrated down to his ear due to the nature
of the parabolic shape. The gain of this operation could easily
be 100:1 if the ear catches only 0.1 square foot.
Now when we turn around and speak into the reflector
it *spreads* the sound from the small source (mouth) over
that same 10 square feet. Is there gain here? Yes, but
not the same as the prior case: Here the only gain (as far
as the guy at the other end without a reflector is concerned) is
due to the fact that without the reflector the sound would be
a spherical source from the mouth, falling off with the square
of distance. With the reflector, assuming everything was just
right, the best you could hope for would be sound that was
collimated into a plane wave that did not fall off with distance.
But the sound would still be spread over 10 square feet in
the ideal case, and much more in reality. The ear of the listener on
the other end only catches a tiny percent of that without a
reflector. If his ear only catches 0.1 square foot, there is a *loss*
of 100:1 compared to having a reflector.
Best regards,
Bob Masta
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