WHISPER DISHES: FOCUSING AND TRANSMITTING SOUND

In the last section you investigated the focusing properties of a parabolic dish.  When waves of parallel light hit the dish they are focused to a point.  The opposite is true as well.  If a point light source were placed at the focus of the dish, the light that hit the dish would emerge in parallel beams.

Investigation: Sound is also a wave (though quite a bit difference from a light wave) and can also be focused using the parabolic dishes.  Set up the dishes so that they are facing each other, are level with each other, but are at opposite ends of the room.  Locate the focus of each dish.  One person should then place their ear at the focus of one dish while another person whispers very softly with their mouth right at the focus of the other dish.  Experiment with different distances and volumes of whisper.

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Questions: Explain carefully why you are able to hear whispers across a room.  Would this work if the whisperer did not use the dish, and simply faced you?  How important is it to make the dishes level with each other?  How much obstruction can there be between the listener and speaker before the communication becomes inaudible?

Finally, explain why radio receivers and transmitters can make use of the same parabolic dish (called an active antenna).

In 1974, using the ultimate "whisper dish" (where whispers are really faint radiation), the 1000 meter radio dish in Arecibo, Puerto Rico (shown below), people working for SETI (Search for Extra-Terrestrial Intelligence) sent a "whisper" into space.  The message was the coding of the image to the left.  For more information on this image go here.  The target was M13, the great globular star cluster in the constellation Hercules, a distance of 25,000 light years from earth!

The Arecibo dish can be used to receive whispers from space (in the form of incoming radio waves).  SETI has placed a small receiver that continues to monitor radio sources from space.  You can help analyze the radio data that SETI receives using SETI at home.

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