Geek's Guide to Britain Mention the development of air raid early warning systems in the UK and thoughts will most likely jump to the Chain Home radar network of the 1930s.
But a system was put in place twenty years earlier to give advance warning of raids by German Zeppelins. These were giant, cigar-shaped, hydrogen-filled airships that unleashed the first aerial bombing campaign to hit Britain and the civilian population.
What we are talking about here are acoustic mirrors – made of concrete.
A network of 13 such mirrors were built across the UK, concentrated in a tight clusters. Those in the South East – around Dungeness and Hythe in Kent – have been documented most.
Arguably the least documented part of this air-defence system thrown is to be found along the northeast coast of England in Sunderland, Durham and North Yorkshire.
One year in, the Great War had ground to a stalemate in the fields of Belgium and France. But while we popularly imagine the conflict as confined to the trenches of Flanders, the war also forayed into aerial combat. The Zeppelin raids were the first ever aerial strategic bombing unleashed on a civilian population.
Imperial Germany in 1915 began bombing the cities of Blighty using Zeppelins and other aircraft.
Precisely when the sound mirrors were built is not wholly certain. As for the cost, supporting infrastructure, manning requirements, success and lifetime of the installations, we are again in the realm of surmise and conjecture. We can't even say for sure if the purpose of the mirrors was to give advance warning to local industrial sites in order to evacuate or to summon up local air defences. Or both.
Trawl the nation's archives for information on sound mirrors and most of what you will find will be related to the better known, and later, sites on the southeast coast. The northern acoustic mirrors are even missing from most Ordnance Survey maps.
What we can do is marvel at these creations, the technological equivalent of that pill box you occasionally see poking out from some woodland or stranded in a farmer's field. Sound mirrors were a British can-do response response to Blighty's first Blitz based on a simple idea: to listen for the enemy.
It was radar without the electromagnetic stuff.
Two figures step forth from this era: Professor F Mather, an engineer seemingly taken with the concept of acoustic detection and involved in experiments using a 4.8m (16ft) sound mirror – sadly no longer extant – carved into a chalk cliff at Binbury Manor in Kent.
The other: a lecturer in physics at Imperial College London named William S Tucker, a major in the British Army who – in the heat of battle near the horror of Ypres – discovered a way to pinpoint the source of incoming artillery. It was Tucker who kept the cause of sound mirrors going almost up until the eve of the Second World War.
Three of the northeastern concrete cubes are on the coast at Fulwell, Redcar and Boulby. A fourth – somewhat removed both in style, location and probably period, but still worth a visit – can be found near Kilnsea on the north bank of the Humber estuary.
There is also good evidence that at least two more existed at Dalton-le-Dale near Seaham and High Springwell near Hartlepool.
High Springwell's was demolished some time in the late 1960s to make way for a housing development. Seaham's was apparently demolished to make way for the B1287 during the late 1970s.
The Fulwell mirror is the best place to start. Thanks to a £68,000 restoration project completed in 2015, it is by far the best preserved and presented.
The trees and scrub that once crowded the structure have been cut away, the earth that had built up in front cleared, and a hardcore path laid up to its base so you can walk right up to it without getting covered in mud. There's a handy information board too.
Access is easy. On the A1018 Newcastle Road in Fulwell, take a right onto the unnamed lane and then turn immediately right again to avoid ending up in the dealership car park and follow the narrow lane for a quarter of a mile.
As large lumps of concrete go, Fulwell is impressive. The rear wall of the structure is some 5.8m (19.02ft) wide and 4m tall with slightly splayed flanking walls extending forward from the face. The face of the mirror is a shallow 4.5m (14.76ft) diameter dish (following the concavity) inclined at approximately 11° from the vertical.
The mirror faces east, positioned to cover the approaches to the Tyne and Wear estuaries. It has been suggested that other mirrors were built nearby to enhance coverage of this vital industrial area but there is no evidence for this.
In front of the mirror is the concrete stand for the receiving instrument that would have allowed the operator to gauge the range and direction of incoming raiders. Whether the operator sat in a trench or stood on a wooden platform in front of the mirrors is unclear.
The site of the next mirror is 13.67km (8.5 miles) down the road at Dalton-le-Dale.
Though the mirror has long since vanished, walking the site I fancied I could see the remnants of the leveled platform where the mirror could have stood covering the northeastern approaches to Seaham harbour.
A further 17.79km (11 miles) down the coast is the probable site of the next mirror, near Hartlepool. The area has been entirely redeveloped and all trace of the mirror and the landscape where it sat has been erased.
According to resident J W Perrin in the Hartlepool Mail in 2007, this mirror was built in 1916 and played a part in shooting down a Zeppelin over Teesmouth on 27 November that year. This was L 34 commanded by Kapitanleutnant Max Konrad Dietrich (uncle of Marlene Dietrich), shot down by 2nd Lieutenant Ian Pyott flying a Royal Aircraft Factory BE2c.
The Museum of Hartlepool houses a rather fine model recreating Pyott's attack but sadly there is no corroborative evidence that the Hartlepool sound mirror played any part in the action. All available sources put the demise of L 34 down being caught by the Castle Eden searchlight.
The Redcar mirror is 19.31km (12 miles) further to the south as the crow flies but the drive covers twice the distance thanks to the need to cross the Tees at Middlesborough.
The mirror sits amid a modern housing estate, on the corner of Greenstone Road and Holyhead Drive. A plaque at the site states that it was built in 1916 by the Royal Engineers. In size and shape the Redcar mirror is very similar to that at Fulwell and here again the stand for the acoustic receiver has survived.
The surrounding housing estate has robbed the site of its geographical context, though. Originally it commanded an uninterrupted view of the North Sea.
A further 19.31km (12 miles) east sits the Boulby Mirror from which – on a fine day – there are spectacular views over the North Sea and the Cleveland Way footpath. On the day I visited a fog bank rolled in that cut visibility down to 20m (65.61ft).
The mirror is situated on privately owned farm land, the farmhouse being immediately next to the mirror heading west.
Talking to the farm owner, I discovered they have a pretty steady stream of visitors asking for permission to hop over the gate and take a shufti. They could not have been more accommodating so please ask before you enter the field.
J R V Carter reported in the Yorkshire Archaeological Journal Vol. 61 for 1989 that the Boulby mirror was also built in 1916. The mirror is set some 500m (1,640.42ft) back from the cliff edge to reduce the interference from wave noise and faces NNE to cover the approaches to the Tees estuary.
Like Fulwell and Redcar, the Boulby mirror is a listed Grade II structure, but – unlike Fulwell and Redcar – there is no plaque and the mirror has undergone no restoration. The concrete rendering is flaking off, giving a view of the underlying concrete cast and corrugated-iron shuttering.
The Historic England description of the structure speculates that two small protrusions above the mirror may been used to mount microphones at some point.
Unique to Boulby is a surviving earthwork in front that measures 4m (13.12ft) wide and 0.5m (1.64ft) high. Its purpose is unclear. The ground immediately in front is slightly hollowed and this is thought to be the remains of a trench or dugout in which the listener would have sat.
The Boulby mirror leans back at a greater angle than Fulwell or Redcar but this seems more down to subsidence than design.
The final mirror is at Kilnsea on the road to Spurn Point. Getting to this involves a 144.84km (90 mile) drive from the Boulby mirror to the Kilnsea Wetlands Nature Reserve. Mark the word "wetlands". Unless you visit the site in the height of summer, take wellies.
Even a cursory glance at the Kilnsea mirror shows a distinct difference in design. It has more in common with the post-war, 6m mirror at Denge.
Architecturally, Kilnsea is more advanced than the structures to the north: the mirror bowl is deeper, profile thicker, side walls absent, and the quality of the concrete seems higher. The hollow iron post in front of the mirror suggests some use of microphone-based electrical directional finding.
That the Kilnsea mirror was not operationally related to the mirrors further north is perhaps explained by the large pieces of concrete that litter the nearby beach. They are the remains of the Godwin Artillery Battery. Built in 1915, the fort housed two 9.2-inch guns and was designed to protect the ports on the Humber from seaborne attack.
The Godwin Battery was in use throughout both world wars and, given the remoteness of the location, popping a sound mirror next door to give advance warning of attack by air or sea would have made sense. The consensus is that the Kilnsea mirror dates to the early 1920s.
The mirror can be accessed via a footpath from a new Yorkshire Wildlife Trust car park just north of Kilnsea on the Easintgon Road or you can start at the main car park by Sandy Beaches caravan park and walk up the beach past the massive remnants of the artillery platforms of the Godwin Battery then cut inland.
Sticking to the footpaths you can only get to within a dozen metres of the mirror itself, which stands on farmland.
In our world of electronic warfare and by the standards of radar two decades after the Great War, using concrete to build what amounts to a giant earhorn to detect the enemy seems quaint – almost steampunk. But the thinking behind sound mirrors was a briefly serious pursuit.
Enter Professor F Mather. In 1915 he was a Fellow of the Royal Society at City and Guilds (Engineering) College. This we know from a letter dated 29 June 1915 from the Superintendent, Royal Aircraft Factory, South Farnborough to the War Office in regards to "the subject of listening for aeroplanes by means of acoustical arrangements".
According to the letter, Mather was "very keen" on the subject of acoustic detection and rocked up to a meeting with a 1.2m (4ft) mirror to demonstrate their potential.
That year Mather wrote a report about its experimental work with a 4.8m (16ft) "sound mirror" – sadly no longer extant – carved into a chalk cliff at Binbury Manor in Kent.
A later record states that the earlier of the two sound mirrors built at Fan Bay near Dover in 1917 (also carved into the cliff) was the result of work by a Major Mather RE and Lieutenant Roger RE. Were Professor and Major Mather the same man? We simply do not know.
According to Mather's 1915 report, a 4.9m (16ft) diameter dish would be able to detect the engines of a Zeppelin airship at 32km (20 miles) and of a surface running submarine at 19km (12 miles). Mather's report states that a mirror with a 7.6m (25ft) reflector mounted on gimbals would be ideal but that fixed mirrors made on the spot "need not be very expensive".
Mather said the Binbury Manor mirror could be replicated for the princely sum of £10 – or £1,100 in modern terms.
The noisy, low-frequency engines that propelled the slow German airships made them perfect targets. With the best Zeppelins of 1915 capable of speeds of between 91.73 and 136.79kmph (57-85mph) in good conditions, these early sound mirrors could theoretically give defenders 15 minutes more advanced warning than from unaided ground-based listening.
Archive photographs of the experimental mirror at Joss Gap show two types of sound collector mounted on a pivot and connected to a stethoscope-type contraption. The first is simple in the extreme.
The operator simply moved the collector around until the sound was the loudest and then recorded the elevation and azimuth. A second shows a device to move the sound horn closer to the mirror surface to further improve the "focus".
A third picture shows an electrical sound connector with the operator using earphones. The exact dates of all three photographs are unclear but the final image seems to date from the post-war period.
Here, Major William S Tucker's pivotal involvement deserves a brief mention even though they are not directly related to the northeastern mirrors.
A lecturer in physics at Imperial College London before the war, Tucker found himself posted to the Experimental Sound Ranging Station at Kemmel Hill near the muddy hell that was Ypres in Belgium, which was under the command of Lawrence Bragg. Bragg, an Australian-born physicist, was pioneered the field of X-ray crystallography and was directed by the War Office to refine early work using sound waves to detect the position of the enemy's artillery. The idea, successfully developed, was to detect the position of the Germans' guns using the boom of their firing.
Tucker invented a "hot wire" microphone capable of identifying the shell sound wave and the following report of the gun that fired it, a key technical development in being able to accurately detect the origin of incoming artillery fire.
The theoretical breakthrough is said to have come from Bragg, who found that the toilet in the farmhouse where he was billeted allowed him – once seated inside – to detect the sound and pressure differences of shell and gun sound waves from a 6-inch field gun.
Tucker turned this eureka moment into a practical use when he discovered how to cool platinum wire using the air currents caused by the sound waves. This separated the low frequency sound made by the firing of the gun from the sonic crack of the shell passing overhead.
Using a microphone consisting of a thin, electrically heated wire, stretched over a small hole in a container the decrease in the electrical resistance of the wire as the sound wave of the gun firing was recorded by a galvanometer. By 1917 British Army sound ranging units armed with Tucker microphones were pinpointing German artillery with uncanny accuracy.
Known for this work in France, Tucker ended the war at Joss Gap in Kent. Tucker went on to become director of acoustical research at the Air Defence Experimental Establishment, Biggin Hill, which researched gun sound ranging and acoustics, and continued to champion acoustic aircraft detection throughout the 1920s and 30s until it became clear that faster aircraft and radio detection had rendered the technology wholly obsolete.
The project effectively was moribund by 1937 as Biggin Hill also began to investigate radar – a technology pursued by the Air Ministry.
The South East gets the majority of the attention in the sound mirrors' tale and that is understandable, given its proximity to France and the capital.
So why was the northwest, hundreds of miles away, chosen as a site for the sound mirror network? There is no documentary evidence, but I would suggest that quite simply because that was were many of the early Zeppelin raids were directed.
To take the Boulby detector as an example, it was clearly situated to give early warning of any attack directed at the nearby Skinningrove iron works – now Tata Steelworks – and explosives factory.
The Kaiser, Germany's imperial ruler, had approved the first Zeppelin raids on Britain in January 1915 under the command of naval chief Peter Strasser. That same month, the first ever bombs to land on British soil from the air hit Great Yarmouth and then Kings Lynn, killing four, injuring scores more and resulting in thousands of pounds of damage.
Documented Zeppelin raids on Skinningrove took place on the nights of 8 September 1915, 1 and 5 April, 2 May and 8 August 1916. London was not bombed until May 1915.
The impunity with which Zeppelins entered the skies above Britain doubtless hastened the development of an early warning system. In 1915, 81 sorties involving 37 airships resulted in only three confirmed enemy sightings and no engagements. Zeppelins could reach altitudes of 10,500 feet, safely above the range of anti-aircraft fire and beyond the capabilities of most contemporary aircraft and pilots.
Zeppelins could casually mete out death and destruction to those on the ground whose only course of action was to hide at short notice.
The sound mirrors seem to have been the response. Improved anti-aircraft guns and searchlights, plus the introduction of new tactics and incendiary bullets by the embryonic Royal Flying Corps and Royal Naval Air Service in mid-1916, led to an increase in Zeppelin kills and reduction in raids.
Only a handful of Zeppelin raids went north of the Humber after 1916, the last two being on the nights of 12-13 March (when the only casualty was a cow in a field in Cottingham) and 12-13 April 1918.
The Germans escalated the conflict: introducing the Gotha and Giant bombers, capable of 140kmph (87mph) and higher altitudes. They raided targets in South England and the Midlands from occupied Belgium.
This is presumably why all the post-war sound mirror sites are in the southeast, Gothas and Giants, while capable of flying higher and faster than Zeppelins, lacked range to effectively threaten the northeast. As the focus of German raids moved south so did the British defences, both active and experimental. The threat the northeastern mirrors were built to counter had ended almost before they entered service.
The First World War resulted in a number of technological breakthroughs that would be developed and refined in later conflicts. Born of necessity and driven by inventiveness, sound mirrors were a chapter of that advancing era, not an end point.
An historical cul-de-sac they may be, but visiting these sites is an atmospheric and poignant way to learn about a defence system that was briefly on the front line.
Fulwell 54.929724, -1.393937
Seaham Dalton-le-Dale 54.830041, -1.363016
Hartlepool 54.710461, -1.246876
Redcar 54.598064, -1.049855
Boulby 54.561587, -0.836114
Kilnsea 53.627804, 0.131642
Car: 145-mile trip from Fulwell to Kilnsea
Echoes from the Sky by Richard N Scarth
The Baby Killers: German Air Raids on Britain in the First World War by Thomas Felgan