Acoustic location was used during mid-WW1 for the passive detection of aircraft by picking up the noise of the engines. It was rendered obsolete by the introduction of radar.
![frloc1a[1].jpg](https://i2.wp.com/www.we-make-money-not-art.com/yyy/frloc1a%5B1%5D.jpg?w=940)
This one is French and each of the four hexagons carries 36 small hexagonal horns.
Via del.icio.us.