A German observation post in 1917 with two strangely equipped sentries,

The following pictures depict a very odd type of hearing aid that first appeared on the battlefields of World War 1.

This was a war when fighter planes and bombers made their debut.  It was also a time before the invention of radar so the best way to detect incoming enemy aircraft was to listen for the sound of their engines.

Of course, the further away you could hear them the better you could prepare so these early warning “acoustic listening devices” were developed.  Here are some weird and wonderful examples:

 

 

The “acoustic detection” era ended with the development of radar in WW2.