Concrete Acoustic Mirrors Listened for Aircraft before Radar｜雷達之前，人用巨大的混凝土鏡子聽遠方飛機

Historic England records a pair of First World War and interwar acoustic mirrors at Fan Bay, built around 1916 and 1920–23 as part of early-warning experiments intended to detect approaching aircraft by concentrating their engine sound.

它們是雷達前的特定歷史技術，不代表所有聲學鏡都具相同尺寸、效能或軍事角色；也不應把後來雷達原理倒套回去。