
Concrete blocks that once protected Britain - teh_klev
https://www.bbc.co.uk/news/in-pictures-46348917
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protomyth
A video on the sound mirrors
[https://youtu.be/04F5osXK4vw](https://youtu.be/04F5osXK4vw)

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Paraesthetic
Legend, I was about to post a link to the Tom Scott video as well. Its a
fantastic video and explains it very well.

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m-i-l
From the headline I thought it was going to be about the variously shaped
concrete blocks, e.g. Dragon's Teeth [0], used as anti-tank defences, which
can still be seen all over the British coastline[1].

[0]
[https://en.wikipedia.org/wiki/Dragon%27s_teeth_(fortificatio...](https://en.wikipedia.org/wiki/Dragon%27s_teeth_\(fortification\))

[1] [https://citizan.org.uk/blog/2017/May/31/humble-anti-tank-
blo...](https://citizan.org.uk/blog/2017/May/31/humble-anti-tank-block/) &
[http://www.pillbox-study-group.org.uk/other-wwii-
defensive-s...](http://www.pillbox-study-group.org.uk/other-wwii-defensive-
structures/anti-tank-obstacles/)

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Steve44
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-19...](https://rarehistoricalphotos.com/aircraft-detection-
radar-1917-1940/) 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)

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CaliforniaKarl
Cool! Lots of little radio telescopes, except tuned for audio. I wonder how
the triangulation was done: Were the relative sound intensities fed into a
computer, to work out the approximate position of the incoming zeppelin?

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mattlondon
Not many computers about from 1910s to 1930s! :) (apart from mechanical ones
used for e.g. artillery)

My guess is basic lines being drawn on a physical map using observations
spaced over time, and then humans trying to spot the pattern. I think these
sort of sound mirrors are fairly directional, so with a bunch pointing in
various slightly different directions you might be able to find the "sweet
spot" and at least get an _idea_ of where the enemy is, even if it is not
precise.

From my own personal half-forgotten memories of seeing these as a kid in the
90s, they always seemed to be aligned with the coast, e.g. facing directly out
to sea, not at an angle. This I guess would get you a lot of subtlety
different directions to take a sounding from.

Very interesting.

~~~
londons_explore
The mirrors focus sound to a different point depending where it originated
from. With a movable microphone, an operator can pinpoint to within a fraction
of a degree the direction the sound came from, and if it can be heard on two
sound mirrors, that gives an exact location.

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buserror
I was left very disappointed with the quality of his images. He used a large
format camera, so there ought to be a lot of detail and grit in these images,
but the framing and the processing leads to pretty dull images... Too much,
and too dull skies, boring central compositions etc...

Traditionally, large format photographers would produce much more interesting
work!

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benj111
I'm somewhat surprised these weren't constructed as decoys even after their
usefulness had passed.

The second to final one looks very much like an early radar station (or was
that the point?).

Would it not have been preferable for enemies to believe all our radar
stations were acoustic mirrors?

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londons_explore
Accoustic mirrors could actually be rather effective - a large mirror could
perhaps hear a plane in good conditions up to 75 miles away, well over the
horizon.

Early radar only had about the same range, but the advantage was it could work
in nearly all conditions, rather than just windless sunny days.

They also weren't used by the Germans, so perhaps the accoustic mirrors were
as much of a secret as radar.

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quietriot
see also: [http://www.douglas-
self.com/MUSEUM/COMMS/ear/ear.htm](http://www.douglas-
self.com/MUSEUM/COMMS/ear/ear.htm)

