Before Radar "Acoustic amplifiers were in widespread use to attempt to detect unseen incoming enemy aircraft.
The Scientific Eggheads from all the advanced nations developed what was then the latest technology to listen out for approaching engines via a now bizarre looking array of devices designed to aid the ear of specially trained operators.
Most had a range of about 25km in calm weather.
While most were fairly mobile units, England in the late 1920 -30s, constructed a series of "acoustic locator walls" and concrete antennas along the coast, some of which can still be found today.
Presumably if you listen out for them...
1928 10 metre Concrete Antenna ( England )
1930 70 metre Locator wall. Reputed to "hear" approaching aircraft from a very impressive 100km away! (England)
Acoustic "radar" 1920s U.S.A.
Japanese air locator 1930
US Army 1940
France 1930s
