In addition to the concrete sound mirrors there was a lot of development work done pre radar. I'm sure I've seen the Observer Corps using them south of London

In addition to the concrete sound mirrors there was a lot of development work done pre radar. I'm sure I've seen the Observer Corps using them south of London in WW2 to help track incoming aircraft but can't find anything specific. I did find https://rarehistoricalphotos.com/aircraft-detection-radar-1917-1940/ https://rarehistoricalphotos.com/aircraft-detection-radar-19... and http://www.douglas-self.com/MUSEUM/COMMS/ear/ear.htm http://www.douglas-self.com/MUSEUM/COMMS/ear/ear.htm which have quite a lot of photographs of various devices. On a related note last time I visited the Jodrell Bank radio telescope they had whispering dishes you could play about with. https://www.geograph.org.uk/photo/4117915 https://www.geograph.org.uk/photo/4117915