Sound Mirrors

Saw these beasts today:

Built around 1930 as part of an experiment to develop acoustic defenses in the UK, but sadly superseded by radar shortly before the 2nd world war. The sound mirrors made it possible to hear enemy aircraft and pinpoint not only their direction of travel, but also the makes and models of the aircraft. Now they just sit as eerie monuments to an obsolete technology… (Hidden behind a caravan park and thousands of blackberry brambles).