An acoustic aircraft locator developed by the US Army in 1925 to locate incoming enemy aircraft by listening for the sound of their engines. Between World War 1 and 2, before the development of radar, many nations used passive acoustic locators like this for air defense. It consists of 4 exponential horns which concentrate the faint sound of the aircraft, which is channeled through rubber hoses to stethoscope-type earphones worn by two observers. The horns are mounted on an altazimuth mount and the stereo earphones of the observers allow them to pinpoint the location of the aircraft. The horns are used in pairs; the vertical pair are attached to the earphones of the righthand observer, who adjusts the elevation wheel; the horizontal pair are attached to the earphones no the lefthand observer, who controls the azimuth (horizontal direction) wheel. They adjust the device until it is pointing at the plane, then report the bearings by phone to an air defence center.