WEBVTT
Kind: captions
Language: en

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These are the Sound Mirrors on Romney Marsh
on the south-east coast of England.

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And in the 1930s, these were Britain's
experimental early warning system

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against aerial attack.

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And with the help of a few folks,
today we're going to test them.

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Now the small dishes over there,
they were the first mirrors at this site.

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But sound waves from the bass
of a plane engine

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have a wavelength that is much longer

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than the whole surface of
those dishes.

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So in 1929, this 200-foot wall of
concrete was constructed

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and to test it, we've got a drone.

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Now back in the '30s, there would have been
microphones set up

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all along the edge of
this concrete forecourt.

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And those microphones were
specially designed and tuned

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to only pick up certain frequencies.

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The project lead, Dr. William Tucker,

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wrote that they wouldn't react
to a loud air horn nearby,

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but when conditions were
right, they would pick up

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the low hum of an aircraft
engine 27 miles out to sea.

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That signal would become the
twitch of a needle on a gauge

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in a control room behind that wall.

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That hole just there,
that was the operator's window.

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Now we don't quite have
that much bass today, but...

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Take her up!

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Now our engineer Ben has
put a microphone here

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at the mirror's central
focus. Thank you, Ben.

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And also one outside the mirror,
so you can compare

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[drone buzzing]

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...that sound.

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[wind noise]

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And, sure, 27 miles
is asking a bit much,

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the drone is a lot quieter,
conditions are nowhere near perfect,

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there's a lot of other noise from birds

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and from everything that's
been built in the last century

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and that mirror has 90 years
of damage on it,

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but listen.

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[drone buzz]

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[wind noise]

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This does still work.
Okay, take it sideways!

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Now the mirror's focal point is different
depending on

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the direction of the incoming plane,

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so the operators could work
out the direction of approach

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based on which microphone
was picking up the sound.

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So as the drone goes sideways, listen,

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[insects chirp]

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the focus moves and that mic
doesn't pick up anything.

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Alright, bring her in!

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Now at one point, there were plans
for a long string of sound mirrors

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across the whole of the
southern English coast,

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but there were a lot of
problems with that idea.

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Not only does it require
a huge amount of resources

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to construct and maintain
anything like that,

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it's easily swamped by other noise,

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cars and trains and new-build houses
drowned out the signal.

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Any other engines in the same
direction get picked up too,

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which meant that you could
either have friendly aircraft patrolling

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or you could have people
listening at the sound mirrors,

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but never both at the same time.

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[drone buzzes]

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And then humanity invented radar

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and building 200-foot walls of concrete

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and building 200-foot walls of concrete
became obsolete very, very quickly.

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So this was never used in wartime,
but it does still work.

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There's not much technology that, er,

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[drone buzzes]

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that can survive 90 years of decay,

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but when your technology is
a massive lump of concrete,

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I guess that's a little bit easier.

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I'm talking about the history of radar
over on the RAF Starrship channel,

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everything that replaced this.

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Thank you very much to
all the team from that,

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who helped make this happen.

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Humanity invented radar

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[stuttering drone]

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became a little bit obsolete.