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%
47
flagItaly
0%
1%
98%
0%
51
flagVietnam
0%
0%
92%
7%
13
flagBulgaria
9%
0%
90%
0%
11
flagGreece
6%
0%
87%
6%
16
flagSpain
0%
9%
85%
4%
21
flagCzech Republic
8%
8%
83%
0%
12
flagthe United Arab Emirates
13%
0%
82%
4%
23
flagGermany
9%
3%
76%
9%
169
flagRepublic of Ireland
12%
4%
76%
8%
25
flagPoland
10%
6%
72%
9%
9891
flagthe Netherlands
12%
9%
72%
6%
33
flagNorway
18%
4%
72%
4%
22
flagthe United Kingdom
17%
6%
65%
9%
411
flagthe United States
13%
5%
65%
14%
207
flagUnknown
13%
2%
65%
15%
44
flagCanada
7%
0%
61%
30%
13
flagLithuania
0%
14%
57%
28%
14
flagFrance
22%
0%
55%
22%
18
flagIndia
27%
19%
27%
24%
62

Gender

men78%
women75%
sun clock
6%
9%
storm detector
5%
5%
acoustic mirror
78%
75%
emergency radio receiver
8%
9%

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