Before radar: Giant WW1 and early WW2 "sound mirrors" detected approaching aircraft acoustically

Many sound mirrors were built in England, and later scrapped when radar became available; some are being excavated and restored now.



Story: http://www.independent.co.uk/news/science/archaeology/sound-mirrors--the-earliest-form-of-air-raid-warning--unearthed-on-south-coast-9821837.html

Other, often bizarre-looking, early acoustic aircraft detectors:










If nothing else, perhaps they'd make the enemy die laughing. :)