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.
[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
[stuttering drone]
became a little bit obsolete.