Aircraft Detection Prior to the Invention of Radar
Prior to the invention of radar (RAdio Detection And Ranging), other means were needed to detect approaching aircraft during times of war. Human spotters were often posted in open fields, tall building rooftops, shorelines, and hills in order to provide a measure of warning against approaching enemy aircraft (acoustic defense). Effectiveness was dependent on many parameters like quality of eyesight, hearing and alertness of the observers, atmosphere transparency (visibility), light level, aircraft size, configuration, color and noise level, etc. Even under ideal conditions detected aircraft would be no more than a few minutes away from the observer, so that did not leave a lot of time to prepare a response. Observer networks were set up as far in advance of key targets as possible through a radio relay network, but it left a lot to be desired.
That's where the strange, often Dr. Seuss-looking listening devices shown below were designed and employed to aid the observers. These photos are all over the Internet, so I do not know their origins. I will be glad to credit the true owner(s) if contacted, and will provide a hyperlink to the originals. Thanks to Bob D. for letting me know about them.
The douglas-self.com web page link was sent to me after posting these pictures. It has more information on these pictures. You'll probably enjoy perusing some of the other topics he has as well.
Here is a page on early radar development.
See the Homemade Plane Detector article from the May 1942 Popular Science magazine.