What is "acoustic mirror"

n. A passive device for reflect and often focusing sound waves, chiefly used to provide early warning of incoming aircraft (by detecting the sound of their engines).

An acoustic mirror is a passive device used to reflect and perhaps to focus (concentrate) sound waves. Parabolic acoustic mirrors are widely used in parabolic microphones to pick up sound from great distances, employed in surveillance and reporting of outdoor sporting events. Pairs of large parabolic acoustic mirrors which function as "whisper galleries" are displayed in science museums to demonstrate sound focusing.

Between the World Wars, before the invention of radar, parabolic sound mirrors were used experimentally as early-warning devices by military air defence forces to detect incoming enemy aircraft by listening for the sound of their engines. During World War 2 on the coast of southern England, a network of large concrete acoustic mirrors was in the process of being built when the project was cancelled due to the development of the Chain Home radar system. Many of these mirrors are still standing.