This strange object was built on the coast of Great Britain. What is it?

sun clock
storm detector
acoustic mirror
emergency radio receiver
Prior to World War II and the in­ven­tion of radar, acoustic mirrors were built as early warning devices around the coasts of Great Britain, with the aim of de­tect­ing airborne in­va­sion­s. The de­vel­op­ment of radar put an end to further ex­per­i­men­ta­tion with the tech­nique.

Answers distribution

sun clock
storm detector
acoustic mirror
emergency radio receiver
answers
flagFinland
0%
0%
100%
0%
57
flagItaly
0%
3%
95%
1%
66
flagBulgaria
7%
0%
92%
0%
13
flagthe United Arab Emirates
6%
2%
88%
2%
44
flagGreece
5%
0%
84%
10%
19
flagCzech Republic
5%
11%
83%
0%
18
flagSpain
4%
8%
80%
8%
25
flagGermany
10%
5%
74%
9%
219
flagthe Netherlands
9%
11%
74%
4%
62
flagPoland
10%
6%
72%
10%
15013
flagRepublic of Ireland
14%
7%
71%
7%
42
flagthe United Kingdom
14%
6%
69%
9%
584
flagNorway
15%
6%
68%
9%
32
flagthe United States
12%
5%
68%
12%
259
flagLithuania
0%
11%
66%
22%
18
flagUnknown
11%
8%
64%
15%
919
flagFrance
20%
0%
60%
20%
20
flagCanada
20%
0%
53%
26%
15
flagRwanda
42%
14%
28%
14%
14
flagIndia
27%
18%
27%
23%
65

Gender

men73%
women77%
sun clock
8%
9%
storm detector
6%
5%
acoustic mirror
73%
77%
emergency radio receiver
11%
8%

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This strange object was built on the coast of Great Britain. What is it?
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