Listening Ears

These concrete 'Listening Ears' in Denge, England, work the same as the Sonic Marshmallows we have seen before. They serve a different purpose though, as they were used to detect slow moving enemy planes in the years before radar was invented. 

The sound waves reflect on the surface and concentrate in the middle, where most probably a microphone was attached. This technique does work - try holding the palm of your hand behind your ear, you will hear things better! - but as planes became faster and radar was invented around 1935, these Listening Ears became obsolete.