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.
Beware generalization. One could easily come up with a scenario in which sound propagates between two points with more attenuation in one direction. Imagine a New York City deli owner closes up for the night. He pulls down the sheet metal gate and retreats to the back of his store. A friend comes up, puts his face six inches from the sheet metal barrier and says something to the owner inside store, not yelling, just raising his voice somewhat above conversational level. Having his mouth right up against the gate causes the sheet metal to vibrate and transmits the sounds inside the store. In the quiet of the store owner can hear every syllable. The store owner, standing in the cat food and paper goods aisle 16 feet from the sheet metal gate, responds. As humans are wont to do when speaking, he uses the same tone of voice. Of course, by the time the sound of his voice reaches the sheet metal surface sixteen feet away it softens by a factor of a thousand compared to the power other guy's voice hit the metal with. Not enough to overcome the damping of the hinges between the segments of the sheetmetal gate. His friend can hardly hear his voice, let alone make out the words. Also, consider that the deli owner's friend is standing out in a street surrounded by noises: traffic sounds, breezes blowing, people talking. The whisper of a sound coming through the metal gate into the noisy environment has such a low SNR that the friend standing outside is lucky to hear anything at all.
It's still a crapshoot when highly trained architects and engineers design a modern concert hall whether it will have any good acoustics at all. Some of the best halls were designed generations ago using a seat-of-the pants, intuitive approach. Acoustics are notoriously unpredictable.
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.
How well does "I see you, you see me" hold for a telescope? The optical system is reversible, but that doesn't mean that both ends are equal.
In your example, it sounds like reflection from the next building and the open windows. Your noise that reflects downwards from the open windows will make it to street level, but since your room is likely to be quieter than street level, you're probably talking more quietly than the people on the street.
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Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
The Sante Fe, NM Childrens Museum has a wonderful outdoor example of this. Two parabolic dishes, doubtlessly C-band castoffs, face each other across an open space of maybe fifty feet, centers at a young person's head height.
In front of each dish is a post-mounted yoke, into which you place your chin. Back-to-back conversation, outdoors, at about fifty feet and normal conversational volume - it's big fun.
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