25cm x 18cm x 12cm
Sound mirrors were constructed at key points along the East British coast in response to the perceived military threat of early aircraft during the latter stages of the First World War. These concave shapes (originally carved into chalk and later cast in concrete) provided a concentration of sound for an operator to record and listen to any slight changes or disturbances. In the 1970s as part of a coastal beautifying program many sound mirrors were buried.
The Bombonera football stadium in Buenos Aires, Argentina is famous for its excellent sonic atmosphere owing to three of its stands curving around the pitch, with the adjacent flat fourth stand along one side.