An acoustic mirror is a passive device used to reflect and focus on sound waves.
Before radars were invented these large acoustic mirrors would have been used during WWII to detect incoming enemy aircrafts. Along the coast of England the construction of giant acoustic mirrors was in the process before being canceled due to delvelopment of the Chain Home radar system. Today most of these acoustic mirrors still stand.
Links: www.artificialowl.net, en.wikipedia.org