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%
98%
1%
62
flagItaly
0%
4%
94%
1%
74
flagSwitzerland
5%
1%
92%
0%
52
flagthe United Arab Emirates
5%
1%
90%
1%
53
flagGreece
8%
0%
83%
8%
24
flagthe Netherlands
8%
6%
80%
3%
104
flagSweden
13%
2%
80%
2%
36
flagCzech Republic
10%
10%
78%
0%
19
flagSpain
7%
7%
77%
7%
27
flagGermany
9%
5%
75%
9%
269
flagAustria
7%
11%
74%
7%
27
flagPoland
9%
5%
73%
10%
18435
flagthe United Kingdom
13%
6%
69%
9%
668
flagNorway
15%
6%
68%
9%
32
flagthe United States
13%
6%
67%
11%
325
flagLithuania
0%
11%
66%
22%
18
flagUnknown
11%
8%
64%
15%
1094
flagRepublic of Ireland
12%
10%
63%
12%
55
flagFrance
20%
0%
60%
20%
20
flagIndia
28%
18%
32%
18%
87

Gender

men73%
women79%
sun clock
8%
8%
storm detector
6%
4%
acoustic mirror
74%
79%
emergency radio receiver
11%
7%

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