US Army Signal Corps aircraft sound locator equipment, 1935.  Before radar was developed in World War 2, acoustic horns like this were used in air defence to detect incoming enemy aircraft at a distance by the sound of their engines.  Stereo horns, one attached to each ear, allowed the observer to judge the direction of the aircraft.     First Army Maneuvers, Pine Camp, New York, August 1935.  Signal Corps Photo 103030.
Unknown author<span style="display: none;">Unknown author</span>
1935-08
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