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  • Ashley Cochrane

Hythe Acoustic Research Station

I raced into Hythe, riding my bike along the sea wall from Folkestone with a fair, but strong, tailwind. It felt like I had hardly blinked and I was emerging at speed on the other side of town, fast approaching 'The Roughs'. I had intended to continue along the beach at Dymchurch, but I decided instead to veer into Palmarsh and head up the hills outside of Hythe to check out an interesting artefact of the 20th century. At the top of 'The Roughs', to the north of Palmarsh, is a lonely acoustic sound mirror, stranded helpless at the hills crest. Acoustic Mirrors were put into development between the two world wars as a means of detecting enemy aircraft via sound waves. The bombing raids of the First World War had wreaked havoc on English towns and cities, which encouraged the need for a way to detect bombing raids, and scramble our own planes to disperse them before they could cross the Channel.

Figure 1.0. The Hythe Sound Mirror (right) and an accompanying bunker (left). It is believed the bunker was built during World War II and not directly associated with the structure above.

To get there, you must cross the canal bridge on Burmarsh Road, opposite Peregrine Close, turn left once across the canal and join the footpath. After around 200 metres, you will be met with a gate on your right. Jump over the sty and stick to the steep path as it snakes it's way towards the site. This land is owned by the MoD but is accessible to the public unless signed otherwise. I reached the mirror in steady time, I only sat down three times to recover my breath, and even had a little lie down in the shrubbery after a particularly steep scramble. I was not alone on my arrival, as I was met with the disgruntled faces of three pissed off sheep, who looked to have not seen a human in some time. Accompanying them was the jolly bounce of their new born lambs, so I sat for a while and enjoyed the views, and to regain what I could of my breath.

Figure 2.0. Map of the site via Google Earth.

  • Point A: Remains of 30 foot 'Bowl Mirror' and WWII Bunker.

  • Point B: Remains of 20 foot 'Slab Mirror' and adjoining buildings.

  • Point C: Aerial photography shows more buildings associated with the complex in this area. Saturated grass in this image may be indicative of concrete remains below the surface.

  • Point D: The gate on the northern side of the canal used to gain access to the site.

The Hythe Sound Mirror is a strange case, dwarfed in popularity by the Denge Sound Mirrors on the Dungeness peninsula, and the Abbot's Cliff mirror between Folkestone and Dover, both sites are much more accessible and therefore had the fortune of being maintained by their owners. The Hythe complex however, is the runt of the litter, lying to rot in fields and left to the mercy of the nature around it. It's easy to overlook the fact that the Hythe Sound Mirrors were at the forefront of the development of acoustic detection technology, and the epicentre of command and jurisdiction of the three sites.

Our story does not start with the decaying mirror that can be seen today, but along the hilltop to the north east, where a more fascinating story is concealed in earth at the foot of the escarpment. I tiptoed anxiously past the resting sheep, a stealth mission that was compromised when I accidently dropped my bike in a fresh sheep turd. I decided to leave it there, I was fairly confident no one would take it, not because it was covered in sheep dung, but because to cycle it away from here would be near certain death, and, I'd be able to hear any opportunistic criminals using the aforementioned facilities.

Figure 3.0. The escarpment that separates the two mirrors. To the left, the path winds back to the 30 foot mirror, and to the right the 20 foot mirror and surrounding buildings. On top of this bank would have been a larger collection of buildings associated with development and research at HARS as seen on aerial photography in 1946. By 1966, the site had been dismantled and only the foundations and the sound mirrors remained.

Just along the path, face down in a field strewn with concrete obelisks, lies the first mirror erected at this site. In 1922, a 20 foot 'Slab Mirror' was installed here, with the means of detecting low-lying enemy aircraft. The mirror was carved out of a solid slab of concrete and supported with buttresses to keep it upright. A landslip in the 1980's caused the mirror to fall on it's face, where it has remained ever since. Subsequent slips have half buried the mirror and vegetation endeavours to consume what's left at surface level. The date of the landslip is not known, but there are testaments in forums online that say it was still upright in 1984.

Fig 4.0. The fallen remains of the 20 foot 'Slab Mirror'

It is at this mirror that you can gauge a scope of how different this landscape would have been one hundred years ago. What now is a seemingly random scattering of concrete, was once a sizeable complex, home to the Acoustic Research Station, which was at the pinnacle of early defence strategies preceding radar. Acoustic mirrors had been put into testing following World War I as a means to listen out for enemy aircraft. The early prototypes had deduced that bombing sorties could be detected in half the time, which often amounted to seconds and minutes, but they were precious seconds and minutes, in order to alert the nearby towns of raids and scramble the planes at nearby Lympne Airfield. Amazingly, the mirrors were never required, by 1939 they had become obsolete due to the invention of radar, which could boast an 8 minute warning for enemy formations.

Figure 5.0. The remains of a building south of the fallen mirror.

Aside from the collapsed mirror, there are plenty of remains to be seen here. The foundations of possible laboratories and workshops, a strange circular mound, which is almost henge-like in all but size. Supposedly, there was a horizontal mirror being developed here, which this may have been the footings for, but I couldn't find anything in my research to elude to that aside from a passing comment on Kent's Heritage Map database. You can clamber up the escarpment, where the road that was used to link this site to nearby Pedlinge once ran. From here, you get a wonderful birds eye view of the site, views to the south over the Marsh, and to the north, the rolling fields that spread from Pedlinge to Saltwood, but be careful, the path is incredibly narrow and runs perilously close to the edge. There were times I wondered if I was destined to meet the same fate as the sound mirror, face down, in a field, covered in two metres of shite. In the fields above the escarpment was an extension to the complex, aerial photographs in 1946 show a substantial collection of buildings up here, likely all used in the development and testing of the acoustic mirrors.

Figure 6.0. Atop the escarpment you get a better view of the complex, including the 'Slab Mirror' (bottom left), the circular mound feature (centre), the remains of the southern building (bottom) and a concrete foundation for perhaps another building (far left).

Seven years after the completion of the 20 foot mirror, came the 30 foot 'Bowl Mirror' which can be seen today, and to where I head now, stopping briefly to wake my bike from its sheep shit slumber. The sheep had fled beyond the Bowl Mirror onto the hill crest, to get a panoramic view of me cursing as I wipe sheep muck from the handlebars. As soon as the 'Bowl Mirror' was built in 1929, it rendered the earlier creation obsolete. This mirror had a better range of detection, a more concentrated sound, thus a more accurate calculation of distance, as well as muffling external sounds more efficiently. Picking up other sounds was a major flaw of the Acoustic Mirror. As efficient as it was for finding the low hum of approaching aircraft, it was also known for amplifying the sound of waves, traffic and presumably, sheep forcing out droppings. The sound was picked up through a microphone, elevated in the centre of the mirror, and an operator would be stationed at the microphone to listen for the fateful drone of the enemy, if he could decipher it out of the relentless groaning of constipated sheep.

Figure 7.0. Front view of the 30 foot 'Bowl Mirror', built in 1929.

Another unique function of the new mirror was a control room located underneath the bowl. This meant the operator was protected from external noises, boosting the efficiency of the new design. The operator could move the microphone using a steering wheel and pedals located in the control room. The standing mirror is fenced off, and rightly so, the outer shell of the mirror looks like it has the structural integrity of a Dorito. If had not been fenced off, I dare say there may be nothing left of it. The new design incorporated a steel mesh into the concrete to give it its deep bowl shaped profile. In the century since its erection, it has done well to withstand the turbulent weather of the Channel. But it will not stand forever, the rain has eaten at the frame mercilessly for decades, and with every visit, it pains me to see ever more of this relic has fallen to the ground. I wandered slowly around the perimeter fence, trying to get sneak peaks inside, but the best views inside seem to be past us. Since I was last here, the foliage around its base has seemingly grown exponentially. Pictures from Wessex Archaeology's survey of the site in 2005 show a more detailed look at the site compared to its current state. The garden aesthetic it has gained cares little for it's size and stature as the branches weave undaunted through the cracks and holes at the structures base. The control room, once clearly visible, is now almost entirely shrouded by shrub.

Figure 8.0. From the western side of the dish was the entrance to the control room underneath the mirror, the doorway is between the supporting leg and the bush on the right, under the bowl. The bush also conceals the platform that would have once held the listening device used to record sound waves, which was controlled by the operator underneath.

The Hythe Sound Mirror does not get the attention of it more famous successors. The mirrors at Denge and Abbot's Cliff were birthed through the initial developments that took place here. The Denge mirrors in particular seem to stand proud in the minimalist abandon that is the Romney Marsh. In comparison to the decay and despair at Hythe, which was once the hub that allowed such monuments to come into existence. The mirrors may never have been used in wartime, but their importance in the story of radar and the fortification of the south east coast during the pre-war years can never go understated. The mirrors form part of a complex of innovation and development, a network that nestles into every community in this part of England. The Hythe Mirror stands valiantly over it's fallen predecessor, soon to meet up with it in radar heaven. In another hundred years, it'll be unlikely that any of these relics remain, above ground at least. I hope that good fortune may rescue this site soon, as it's story and importance owes it at least that.


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