Concrete blocks that once protected Britain | Hacker News Reader

Concrete blocks that once protected Britain

bbc.co.uk

114 pointsteh_klev13 comments

A video on the sound mirrors https://youtu.be/04F5osXK4vw

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

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...

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

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... and 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

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?

Nope. No computers with signal processing capability yet. There was usually just a wave guide (or voice tube) leading to a man listening with his ears, or later models had a microphone.

The waveguide was rotatable so that sound direction could be roughly determined. This was done by slowly turning the guide until the sound was at its loudest and then reading off the angle.

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.

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.

The intent was there would be several microphones that could be triangulated. With a hut full of human computers and sets of trig tables presumably.

The chap mentioned here, Tucke, was active in developing sound artillery ranging, which survived well after WW2. Artillery didn't have the speed issues that killed off sound mirrors before they ever really got going.

Watson Watt and his team fortunately gave us workable radar with height and direction finding by war's outbreak. Just.

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!

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?

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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