In this era of satellite imaging, drone strikes and stealth bombers, it’s hard to imagine an air war the way it was experienced a century ago. I remember myself sitting in front of a TV in the comfort of my living room in January 1991, watching explosion flashes in an eerie green hue – the night vision images of Baghdad getting obliterated live by Coalition bombers and Cruise missiles during Desert Storm. Even before that, I was already familiar with powerful images from the London Blitz, or cities like Dresden or Tokyo, razed by massive Thousand Bomber raids in World War 2.

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A Zeppelin over London – German propaganda poster

Back in 1915, however, the first use of aircraft for aerial bombing was a less effective affair, albeit one without precedent : For Britain in particular, a nation comfortable with a centuries’ old notion of its powerful navy protecting her from danger or invasion, the first German aerial bombing campaigns against its cities came as a complete shock, and prompted the beginning of a military and civic culture of air defense that would carry the day several decades later, during the Battle of Britain.  The first aerial raids over the coast of England and London in 1915 and 1916, were performed by gigantic Zeppelins – the impressive cigar-shaped, engine powered dirigibles of the German Empire. The attacks were launched during night time, aiming to achieve stealth, and to break the spirit of the civilian population or damage military facilities – in what marked the beginnings of the doctrine of strategic bombing.  The airships introduced Britain’s civilian population to death and destruction raining from the skies, a yet unknown, and certainly terrifying danger never experienced before. In the first terror raid over London in 1915, incendiary bombs from German airships started fires that claimed the lives of seven civilians, among them a 3 year old girl. This incident prompted the general public to give them the contemptuous nick name they carried throughout the war : Baby Killers.

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The end of the “Baby-Killer” – British propaganda poster

The slow airspeed, sensitivity to weather conditions, and overall  vulnerability of the highly flammable dirigibles to incendiary bullets yielded questionable results : apart from sporadic casualties and circumstantial damage, the Zeppelins achieved little in the way of a definitive military success over Britain. At the same time, they suffered considerable casualties by the ever increasing air defenses and interception capabilities of defending aircraft, forcing them to fly higher and higher, reducing their crew’s capabilities for lack of advanced oxygen equipment, and further diminishing their already reduced bombing accuracy – for lack of advanced bombing sights. Over the course of the Great War, 84 airships participated in 51 bombing raids, killing 557 and injuring 1358, while suffering more than 1/3rd casualties. In military terms, this is still a far cry from the devastating 1250 Allied bomber raid that obliterated Dresden in February 1945, causing 25.000 deaths in the space of 3 days – but in 1917, strategic bombing was a terrifying new reality the civilian population was only just beginning to come to terms with. And they tried to do so with an appropriate mix of resilience, curiosity and romantic stoicism, as evidenced by a contemporary letter (1915) from the writer D.H. Lawrence to Lady Ottoline Morrell :

“Then we saw the Zeppelin above us, just ahead, amid a gleaming of clouds: 

high up, like a bright golden finger, quite small (…) Then there was flashes near the ground — and the shaking noise. It was like Milton — then there was war in heaven. (…) I cannot get over it, that the moon is not Queen of the sky by night, and the stars the lesser lights. It seems the Zeppelin is in the zenith of the night, golden like a moon, having taken control of the sky; and the bursting shells are the lesser lights.”

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The fearsome Gotha G.IV in flight

Things got worse in spring 2017, when Germany introduced the fearsome Gotha G.IV, the first ever aircraft created specifically as a strategic bomber, and built in considerable numbers to bear. Small numbers of earlier models had already seen successful action in the Balkan theater since the previous summer, so the German High Command decided to use them in greater numbers over London, in operation Türkenkreuz (Turk’s Cross) : This is the beginning of daytime raids over Britain, made possible by the superior speed, altitude and maneuverability of the new flying machines versus the cumbersome airships, which increased their effectiveness and survival rate considerably. To make things worse, Gotha bombers could fly higher than the British interceptors of that period, who struggled to climb after them, resulting in minimal losses for the attackers. The Germans achieved complete surprise with this new strategy, with each new raid causing hundreds of casualties  : Lt. Charles Cabot, a Royal Flying Corps pilot commended on the German daytime strategic bombing successes of that period : “…Raids hadn’t become a very serious thing, and everybody crowded out into the street to watch. They didn’t take cover or dodge”.  It is indicative of the degree of dread caused to the British population by the new flying machines, when the Royal House promptly proclaimed a change of their family name from Saxe-Coburg-Gotha to Windsor in July 1917 – one month after the first Gotha raid.

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The gargantuan Siemens-Schuckert R.VIII

An aerial arms race ensued for the remainder of the war, with the British ramping up their air defenses and air interception capabilities, and the Germans deploying larger and larger Riesenflugzeuge, giant bomber planes of unprecedented size and capacity : The largest of those, the monstrous Siemens-Schuckert R.VIII  had 6 engines, was almost 5 times bigger than the twin-engined Gotha G.IV , and at the time of its construction, it was the largest aircraft in the world – and about the same size as the humongous B-29 Superfortress , one of the largest aircraft flown in World War 2 over two decades later. By the time the hostilities ended, the world had entered what Air Commodore Lionel Charlton described in 1938 as “..the beginning of a new epoch in the history of warfare”.

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Emperor Hirohito of Japan inspecting his War Tubas

But lessons learned from the strategic air raids of the Great War also prompted an international race for relevant technology during the 20s and the 30s. The increasing speed and efficiency of early bombers created a need for more effective response, but also importantly, for early detection technology – for all major nations. With the Zeppelins phased out of air operations, replaced by multi-engined behemoths, and the lack of RADAR technology (that won’t become fully practical until the mid-30s), the quest for an efficient early warning technology focused on the science of acoustics, and of applicable ways of listening to the skies for the roar of oncoming bombers. This resulted in the establishment of civil defense organizations tasked with identifying, tracking and reporting airborne danger (such as the British Royal Observer Corps in 1925), and the rapid development of military grade listening devices during the Interwar period. Giant listening horns, war tuba arrays, stethoscopes attached to gramophone horns, and other weird acoustic devices were deployed by the military forces of the time – some of which look quite amusing by today’s standards.

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U.S. Army Interwar acoustic device

One of the applications of the acoustics technology in Britain came in the form of parabolic mirrors, such as the ones that can still be seen at Denge Marsh near Dungeness, built circa 1928 for experimental purposes and participating in air defense exercises to establish the acoustic quality of the various shapes and sizes tested. These concrete mirrors range from 20ft (6m) to 200ft (60m) wide, and were meant to concentrate the sound of oncoming aircraft from a range of 24 miles (38km) away, collecting it with the use of microphones placed in focal points in front of the mirrors. Triangulation and Time Difference On Arrival techniques were then used to roughly estimate the angle of attack, elevation and speed of the attackers. Parabolic mirror arrays such as the one in Dungeness are known to have been built in about 14 locations along the British coast, and many of those are still standing. Some of the earlier versions of those acoustic mirrors existed since late 1917, and they would be able to pick up the engine sound of a formation of incoming Gothas 20 minutes before they reached the English coast – and that’s 10 times faster than the average observer would have spotted the planes over the sea on a clear day !! This is exactly what happened at the Fan Bay mirror near Dover, when during a raid in October 1917, it is documented to have been  able to detect the sound of oncoming aircraft over the Channel at 12 to 15 miles (or 20 to 24km) away.

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The abandoned Acoustic Mirrors at Dungeness

That’s a lot of early warning by Great War standards, more than what Londoners would have hoped for in those early days of terror bombing. By 1935, however, Hitler’s new generation of super fast Schnellbombers – such as the Dornier 17z or the Heinkel 111, who would indeed end up blitzing Britain in 1940  –  would already be flying at much higher air speeds, cutting the early warning time down to a mere 5-6 minutes. And what’s more, rumors of a Death Ray being developed by Nazi Germany are abound, and parabolic mirrors would certainly be useless in defending against such an advanced technological threat. With the clouds of war gathering over Europe once more, Britain swiftly abandons acoustics and switches their early warning research effort towards Radio Detection And Ranging, known as RADAR. The development of the Chain Home radar system along the island’s coast begins in 1937, a decision that will prove crucial in deciding the fate of the nation during the Battle of Britain in 1940 : enemy aircraft formations can now be detected as they form up over their airfields in France, and their size, speed and direction can be calculated with unprecedented accuracy.

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The 200ft (60m) parabolic mirror near Dungeness

The advent of radar combined with the ever increasing bomber aircraft speeds quickly made acoustic mirrors obsolete by the late 30s, however more advanced, and often mobile military acoustic devices are known to have still been employed by combatants well into the World War 2, before their effectiveness was entirely eclipsed by the advent of the sonic speed jet bombers of the 50s. Today, the abandoned remains of this forgotten technology have a mysterious appeal, and can still be admired for their ingenuity – as monuments to an obsolete military technology.

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The 20ft (6m) parabolic mirror near Dungeness

Recently, I discovered that acoustic mirrors can also be enjoyed as a game while on a day out with the family !! Here’s one of a set of parabolic play mirrors I found in a playground – my boys had no idea how to play with those strange looking objects, and largely ignored them in favour of more familiar features, such as swings and play castles (another one of those defunct military applications presently confined to play areas).  But I had a really fun time whispering and growling into the mirrors, looking at the boys bursting with laughter each time the sound of my antics was carried an unbelievable distance away with clarity – while I found myself subconsciously checking the skies above our heads for Gothas… These miracles of physics will certainly remain with us for the next generations, hopefully to be used exclusively for peacetime applications of the future.

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It is but a toy

Sources :

  • www.wikipedia.com
  • http://www.andrewgrantham.co.uk/soundmirrors/locations/denge/
  • https://theromneymarsh.net/soundmirrors
  • https://historicengland.org.uk/listing/the-list/list-entry/1442235

All Acoustic Mirror photos are © explorabilia , all other photo material is public domain

 

 

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