Inspired by this, and a recent visit to Baltimore’s Inner Harbor with a set that only projected across 20 feet, I was wondering how far apart sound-reflecting dishes could be and still send an intelligible voice across to the other side assuming:
– The sender is allowed to talk in a loud, but not screaming, voice. Semi-yells are permitted.
– Height is not a problem (in case the answer involves the curvature of the earth!)
– Size of the reflectors is not a problem
I’m wondering if the Napoleonic telegraph system wouldn’t have been better implemented with human voices, as they transmit at a higher bandwidth than visual telegraphy (if more error-prone and not as high an initial rate of speed due to the speed of sound.)
Without anything extraordinary, a couple hundred feet is not hard to accomplish. The Museum of Science and Industry in Chicago has (had? The exhibit’s probably been rebuilt since I was there) a pair of reflectors that would allow two people to carry on a conversation in normal voices - no yelling required - amid the ambient cacophany of the museum. IIRC, these were at least 100 feet apart.
Not really relevant to your question, but sufficiently interesting and conceptually related that I know some dopers will find them interesting: http://en.wikipedia.org/wiki/Denge
Did you see the set outside at the Science Center? They were about 150-200 feet apart if I recall, and they worked at normal speaking volume. I checked Google Earth and can still see one of the dishes, but I’m not positive that they are still there.
There would be a problem because sound does not travel horizontally. It curves - forget if it’s upward or downward, but it doesn’t travel in a straight line. This is because of the pressure gradient in the atmosphere. I think this is the limiting factor for sounds traveling long distances (though I read it in the context of loud sounds, not sounds focused with dishes).
Atmospheric conditions can play heck with your scheme. There is frequently a significant temperature gradient at low altitudes. Because the speed of sound is temperature dependant, this tends to refract sound upward or downward.
One frequently observed manifestation of this is the “annoying fishermen” phenomenon. Often in the early morning the air over a lake tends to refract sound downward. If the surface of the lake is very calm, it will reflect the sound back upward. This creates a ducting effect, and allows the fishermen in the boat speaking in normal tones to be clearly heard a mile or so away by the campers on the shore trying to sleep late.
Hang glider pilots can make themselves easily understood by those on the ground from over 1000’ away with no focusing device at all.
>Hang glider pilots can make themselves easily understood by those on the ground from over 1000’ away with no focusing device at all.
How is that possible? I could have six of them standing at arms length talking as loud and as fast as they could, and I’d still never understand it. Authorities say hang gliders should never go higher than they feel like falling. What are they thinking?!
Ask them to speak one at a time, and more slowly. That’ll help.
Seriously, I expect the hang glider pilots in question are talking more or less horizontally downwind to people on the ridge, not necessarily 1000 feet down to people below them. (Which is not to say both parties can’t be 1000 feet high or more.)