The Japanese war tuba (up image) is a colloquial name sometimes applied to Imperial Japanese Army acoustic locators due to their visual resemblance to the musical tuba. The name derived from a misidentification, probably in jest, of a historical photo from the 1930s featuring the Japanese emperor Hirohito inspecting the acoustic locators with anti-aircraft guns in the background.
Acoustic location was used from mid-World War I to the early years of World War II for the passive detection of aircraft by picking up the noise of the engines. It was rendered obsolete before and during World War II by the introduction of radar, which was far more effective.

Many examples of acoustic location mirrors can still be found on the seacoast of Britain to this day (below).

[eng. text from wikipedia]