A number of sites give some detailed historic information about the development of the early sound detection systems and are well worth visiting and reading.
IF IN DOUBT ASK !!!!
There have been a great variety of devices that used acoustic (sound) detection for ranging -
( distance and direction finding )
Checking some of these sites and images should be informative as well as fun.
Sound Mirrors on the South Coast of the UK
Early warning sound mirrors around Britain
Down amateur radio society
Acoustic Deficiencies > Birth of Radar
German Communication equipment
Helge Fykse’s site