Known in Japanese as “large air sound detector ninety formula,” the “War Tuba,” as it is popularly known, was a Japanese military aircraft acoustic locator. The photo above dates from the 1930s and shows the Japanese emperor Shōwa (Hirohito) on an inspection tour. Acoustic locators were used from mid-World War I to the early years of World War II to listen for the sound of approaching enemy aircraft, which could be heard before they could be seen, providing more advance warning. Large acoustic horns, like an oversize “ear trumpet,” were attached to stethoscope-like earphones. The technology was rendered obsolete during World War II by the introduction of radar. Below, an American version in use at Bolling Field, Washington, D.C., in 1921.