acoustics
RichD wrote:
A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level. Their words came in, clear as a
bell, almost like being in the same room. I was surprised,
I thought the power would attenuate a lot faster.
Then I wondered if it also works the other way - could they
hear our conversations, just as crisp?
I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.
Does that also hold for acoustics?
I think it does, but only if the radiation pattern is also
reversed. For example, if someone stands at the focus of a
parabolic reflector, it very efficiently captures the radial
sound waves from their voice and produces a nearly plane
wave radiation pattern that travels long distances with
little dispersion. In effect, the parabola converts the
near center radial wave pattern to a far center radial
pattern (the waves act as if their center of radiation is a
lot further away than the speaker actually is) so the square
law attenuation rule still applies, but getting to twice the
distance to have wave strength fall to 1/4 means getting a
lot further away.
Now, think of this acting in reverse. Your voice radiates
in a spherical wave, so falls by square law, from your
mouth. Only a small fraction of that sphere is collected by
the parabola to reach its focus. The non-reversibility is
not the fault of the parabola, but your fault for not
radiating a reverse spherical wave pattern similar to what
you received. That kind of wave front would return to the
parabola and focus almost perfectly reversibly to the one
you received.
Now, if you add another parabola at your location, you will
send almost plane waves (spherical waves as if the center
were far from you), almost the reverse of what you received.
Replace both parabola with ellipsoids and the reversibility
is even better.
--
Regards,
John Popelish
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