Mirrors can actually reflect sound as well as light. Mirrors that reflect sound waves are known as “acoustic mirrors,” and were used in Britain during World War I to detect certain sound waves coming from enemy aircraft from 8 to 15 miles away. This was before the development of radar.
Several were built around the coast of Britain, and are still standing today. They are located on both the north and south shores of England. They are also called listening stones.
Concrete acoustic mirrors were built on the south and northeast coasts of England between about 1916 and the 1930s. The ‘listening ears’ were intended to provide early warning of incoming enemy aircraft.
They did work, but the development of faster aircraft made them less useful, as an incoming aircraft would be within sight by the time it had been located. Increasing ambient noise made the mirrors harder to use successfully, and then radar rendered acoustic detection redundant.
There is also an example of one that is a parabolic sound mirror carved into boulders to dramatically magnify the sound of a nearby stream for listeners. It is inspired by satellite dishes, the seating in choir lofts where curved walls reflect sound and the antique hand-held sound magnifiers used in the days before hearing aids.