https://rarehistoricalphotos.c....om/aircraft-detectio

Aircraft detection before radar, 1917-1940 - Rare Historical Photos
rarehistoricalphotos.com

Aircraft detection before radar, 1917-1940 - Rare Historical Photos

Acoustic location was used from mid-WW1 to the early years of WW2 for the passive detection of aircraft by picking up the noise of the engines.