Testing The Sound Mirrors That Protected Britain

Testing The Sound Mirrors That Protected Britain


These are the Sound Mirrors on Romney Marsh
on the south-east coast of England. And in the 1930s, these were Britain’s
experimental early warning system against aerial attack. And with the help of a few folks,
today we’re going to test them. Now the small dishes over there,
they were the first mirrors at this site. But sound waves from the bass
of a plane engine have a wavelength that is much longer than the whole surface of
those dishes. So in 1929, this 200-foot wall of
concrete was constructed and to test it, we’ve got a drone. Now back in the ’30s, there would have been
microphones set up all along the edge of
this concrete forecourt. And those microphones were
specially designed and tuned to only pick up certain frequencies. The project lead, Dr. William Tucker, wrote that they wouldn’t react
to a loud air horn nearby, but when conditions were
right, they would pick up the low hum of an aircraft
engine 27 miles out to sea. That signal would become the
twitch of a needle on a gauge in a control room behind that wall. That hole just there,
that was the operator’s window. Now we don’t quite have
that much bass today, but… Take her up! Now our engineer Ben has
put a microphone here at the mirror’s central
focus. Thank you, Ben. And also one outside the mirror,
so you can compare [drone buzzing] …that sound. [drone buzzing] [wind noise] And, sure, 27 miles
is asking a bit much, the drone is a lot quieter,
conditions are nowhere near perfect, there’s a lot of other noise from birds and from everything that’s
been built in the last century and that mirror has 90 years
of damage on it, but listen. [drone buzz] [wind noise] This does still work.
Okay, take it sideways! Now the mirror’s focal point is different
depending on the direction of the incoming plane, so the operators could work
out the direction of approach based on which microphone
was picking up the sound. So as the drone goes sideways, listen, [insects chirp] the focus moves and that mic
doesn’t pick up anything. Alright, bring her in! Now at one point, there were plans
for a long string of sound mirrors across the whole of the
southern English coast, but there were a lot of
problems with that idea. Not only does it require
a huge amount of resources to construct and maintain
anything like that, it’s easily swamped by other noise, cars and trains and new-build houses
drowned out the signal. Any other engines in the same
direction get picked up too, which meant that you could
either have friendly aircraft patrolling or you could have people
listening at the sound mirrors, but never both at the same time. [drone buzzes] And then humanity invented radar and building 200-foot walls of concrete
became obsolete very, very quickly. So this was never used in wartime,
but it does still work. There’s not much technology that, er, [drone buzzes] that can survive 90 years of decay, but when your technology is
a massive lump of concrete, I guess that’s a little bit easier. I’m talking about the history of radar
over on the RAF Starrship channel, everything that replaced this. Thank you very much to
all the team from that, who helped make this happen. Humanity invented radar and building 200-foot walls of concrete [stuttering drone] became a little bit obsolete.

100 Replies to “Testing The Sound Mirrors That Protected Britain”

  1. I've wanted to make this video for literally four years, and – thanks to the RAF team – all the pieces finally came together. Do go check out my history of radar video over on their channel!

  2. I've been here! Not onto the island itself, but I looked at them from across the water. I'm jealous you got to go that close to them.

  3. 2,000 years in the futures, humans will think we built those concrete structures to worship a cat-headed deity.

  4. There is a bar in Vegas, I think it's the Rio, that is perfectly round with a really high domed ceiling. You can sit on one side and talk in a soft voice to a person sitting on the other side 30 feet away and they can hear you just like you're sitting next to them leaning over to whisper in their ear. It is the coolest thing ever. When it's really busy and full of people the effect is cancelled and it just sounds like a normal bar. But in the morning when it's empty we would sit on opposite sides and have a conversation. So cool

  5. They used the wrong kind of microphone, and if they'd have put a microphone at the throat of a horn they'd have had more signal and less outside noise; it's the same principle as a satellite dish's feedhorn. In fact, replacing the feedhorn in a satellite dish with a microphone in a horn makes an extremely sensitive and directional listening device, you can even use two horn microphones with one oval dish as those have two focal points. It looks like they used an omnidirectional mike which is the worst for that application because it picks up sound from all around it.

  6. I can't believe that phantom 4 hit a tree, did a full fov quad flip then saved itself. Astonishing technology now that shows how far we have come from giant concrete walls with microphones

  7. Hey Tom, I live on the South Coast town called Selsey and we still have our listening post nothing on the size of that but I was still led to belive that the observer corps used it during Ww2.

  8. Neat little vid as always. Not too far away from where I live. Never had the opportunity to visit them; does anyone know if they are accessible to the public?

  9. Fascinating video, Tom. Some years ago I watched a documentary about this, but, unlike you, the maker did not sound test them, which left me, and many others, wanting just what you have done here. Many thanks.

  10. It's gotta be somewhat disappointing to be the scientists and engineers that come up with such a remarkably ingenious what to detect enemy aircraft, only to have it almost immediately superceded by a more advanced technology.

  11. Not too far away from my home town of Rye. My grandfather worked at Dungeness for around 20 years. He told me all about the sound mirrors when I was a kid. Great stuff.

  12. Isn't it amazing how many different obscure technologies were designed and even built or used but were very quickly (in just a few years or so) surpassed by superior technologies which are much more commonly known? Thank you Tom Scott for letting the internet know about these fascinating reminders of niche obsolete technologies. I've found them quite interesting.

  13. Why do you need authorization to fly a drone there?
    If the "nature reserve" isn't scared by the airport traffic your little styrofoam drone isn't gonna do anything either and the closest airport is 80Km (50mi) away.

  14. 2:32 why can't we have people listening to the sound mirrors and friendly air patrolling at the same time? Just be aware that the sound mirror might be picking up the friendly aircrafts.

  15. I believe the Soviets were still using audio mirrors as part of their air defence network, right up until the collapse of the USSR… I know Moscow is still the most heavily defended city in the world (against air attack, that is) so I wonder if they kept any of the old sites running as part of that air derence system… Anyone know one way or the other?

  16. Tom, between you and Neil deGrasse Tyson I don't know who I'd rather listen to while being educated. Your videos are awesome! Please, never stop producing them!!

  17. this is actually the second video i’ve seen of this place. the other was og some bmx-riders who snuck in to do tricks in those dishes

  18. A similar idea was the "war tubes" used after WWI by many of the countries that were developing aerial warfare, basically giant horn shaped tubes to amplify sound

  19. "And building 200 foot walls of concrete… became a little bit obsolete" —
    I couldn't help but think of the border walls between nations, even new ones that some are trying to build, also potentially falling under this category of "a little bit obsolete".

  20. If you didn't include the outtake at the end I'd have totally believed you got all the shots in one take. (Not because I'm unaware of camera trickery, but because you so often do!)

  21. Why do you need permission? it's just a bunch of concrete, why flying a quad would cause any problem ?
    I though you would fly a real plane in low altitude, that's why the permission.

  22. That is absolutely brilliant. Really, a clever way of detecting aircraft before it's coming. Sure, radar was invented, but this was ahead of it's time. If something had happened that required this system, they would be the only ones in the world that had it, and it's super well thought out, too. Amazing.

  23. Why has it not been demolished?. It's such a strange thing with humans, we leave our old garbage all over the place.

  24. There is an almost identical acoustic mirror built near Baħar iċ-Ċagħaq in Malta. It is known locally as il-Widna.

  25. "Humanity" didn't invent radar, Sir Robert Watson-Watt did. "Humanity" without the insights of a tiny number of inventive men (and virtually no women) would still be living in caves.

  26. Old satilite dishes can be lined( coated) .and used to comunicate .smply point two at each other and stand listen speak from focal point..

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