Application of aircrafts in military operations in early twentieth century notably began new technology race. Early aircraft were low-flying, noisy, and slow. Yet, with lack of air defence systems invented, this would not stop them being extremely advantageous in scouting and warfare.

In response to the threat of aircraft attacks during the First World War Britain commissioned  a number of early-warning systems experiments, including sound wells, sound disks, sound trumpets and sound mirrors. All of those devices were based on geometry of distribution of sound waves and were made obsolete by radio radars within decade of their erection.

While most of the other types of early early-warning systems are hard to come by, sound mirrors became a permanent feature in some areas of British (and Maltese) landscape.

https://www.fastcompany.com/90135167/the-concrete-sound-mirrors-that-influenced-wwii-science-and-design: What are Sound Mirrors and where to find them?