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"Amplifiers at Bolling Field, 1921." Two giant horns with ear tubes, evidently designed to listen for approaching aircraft. View full size. National Photo Co.

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Bolling Field

The building in the background is the Army War College at Fort McNair.

 

Can you hear me now?

Verizon, the early years.

 

Acoustic Amplifiers

There still exist some of the specially constructed concrete "amplifiers" along the coast of England that were used to listen for approaching aircraft in WW2.

http://en.wikipedia.org/wiki/Acoustic_mirror

 

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