Aircraft detection before radar, 1917-1940

During World War I, aerial warfare became increasingly important. Zeppelin airships conducted bombing raids on the south coast of England, and winged aircraft were employed as bombers by both sides. 

If their approach was detected, fighter aircraft could head off the enemy or shoot them down.

Aircraft engines produced unprecedented sound, so in order to hear them at a distance, the war efforts developed listening devices. Some were small and portable, for the use of one person.

The operator listened using stethoscope-style headphones. 

But detecting engine noises at greater distances allowed more time to prepare a response. Large and elaborate detectors were experimental, and their efficiency varied widely.

 A number of designs were quite striking, such as the Japanese acoustic locators nicknamed "war tubas."

Their use continued until the early years of WWII, but increases in aircraft speeds and the development of radar rendered them obsolete.