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CAN CONCRETE “LISTEN TO”?

Imagine to be in the decades between the two World Wars…Imagine the devastation caused by enemy aircrafts which have bombarded coasts, towns, churches…Imagine you have to find a solution to prevent future aerial bombardments. Well, Dr. William Sansome Tucker was in charge of that! And what did he conceive for that purpose? A network of large and massive concrete acoustic mirrors to be built along both the south-eastern and the north-east coast of England. The radar was far to be invented, and the function of those buildings was to be an early-warning device by military air defence forces to detect incoming enemies’ aircrafts by listening to the sound of their engines.


The Mirrors are much more complicated than they appear: they usually consist of a paraboloid or ellipsoid body made of concrete with a microphone positioned at the focal point which records the sound reflected by the mirror surface. These specific reflectors made by Dr. Tucker were mainly spherical, because this shape allowed to move the sensor (in this case, the microphone) instead of the mirrors (radar reflectors, being parabolic, are mobile and they can be directed from a point to another rotating by 360° if needed, while the concrete mirrors, being static, couldn’t rotate in any way). As sound waves reached the concave sphere in the center of the concrete structure, the microphone picked up the engine noise and transmitted it to the headphones of an operator staked out or in a listening chamber below or in a trench nearby. The concrete sound mirrors made the tone almost four times louder.


The position needed for the reflectors was also very important: it should have been the flat top of a low ill, far from cities or village’s sounds that could interfere with the sound reception. Absence of trees was also an advantage since the rustling of the leaves could interfere with the earing. If mounted near the coast, the reflector should have been kept two or three hundred yards from the edge of the cliffs to eliminate the noise of the waves.


The most famous of these devices still stand on an island at a Denge, a former Royal Air Force site near the Dungeness peninsula, and in Kent. Other examples still exist in other parts of Britain and also in Malta. These structures could have been till to 9m of diameter, giving Britain no more than 15 minutes warning of an impending attack. The increasing speed of aircrafts in the 1930s meant that they would have been too close to engage by the time they have been detected: while during the First World War few aeroplanes could fly at 120mph top speed, by 1939 combat aircrafts had 1000hp engines and could easily fly at 350mph speed, rendering the concrete sound mirror useless.


These days, visitors to the UK could take special trips to see the hulking defense ruins, which are now pieces of art and also archeological sites. The book “Echoes from the Sky” by R.N. Scarth represents the definitive work on the post-WWI sound mirrors, including the Deng ones and the Maltese example.


Although the Acoustic Mirrors program (started in the early ‘30s) had been cancelled close to the WWII owing to the development of radar technology, its importance has been recently recognized as it gave Britain the methodology to use interconnected stations to pinpoint the position of an enemy in the sky, that is the main principle of radar itself.



Concrete acoustic mirror near Kilnsea Grange, East Yorkshire, UK. The pipe which held the "collector head" (microphone) can be seen in front of the structure (picture by Wikipedia)



Concrete acoustic mirrors at Denge, former Royal Air Force site (picture by Wikipedia)




REFERENCES

Tijana Radeska, Giant Concrete “Sound Mirrors” were Used to Detect Enemy Aircraft Before Radar, 2018/12/11, www.thevintagenews.com.

The concrete blocks that once protected Britain, 7 January 2019, www.bbc.co.uk.

Acoustic mirror, www.en.wikipedia.org.

NO CONCRETE … RADAR AND SIR ROBERT WATSON-WATT,

BRITAIN’S GIANT CONCRETE EARS, BUILT TO WARN OF AN ENEMY AIRCRAFT ATTACK, 30th October 2017, www.ronimix.co.uk.

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