English: A T3 aircraft sound locator used for air defense by the U.S. Army Signal Corps. Before radar was invented in World War II, passive acoustic aircraft location devices were used by a number of nations to detect incoming enemy aircraft by listening for the sound of their engines. This example consists of two pairs of horn sound collectors on a steerable mount, attached to stethoscope-type earphones worn by observers. The horizontal (right–left) pair was used to determine the azimuth of the aircraft, and the vertical (top–bottom) pair was used to determine its elevation. "AWC" may indicate the photo was taken at the US Army War College, then at w:Fort McNair, Washington D.C. "BOWEN" might refer to Admiral Harold Gardiner Bowen (1883–1965) who at that time was directing air defense research at the Naval Research Laboratory and later had a role in developing w:radar.