Sound mirrors – by Peter G0FUU

On Sunday 10th September 2007 I went on a short trip to the bird sanctuary at Denge Marsh on Dungeness. However, I was not there to look at the birds because although some may consider me to be a twit I am not a twitcher (someone who watches birds). I met up with Phil Parkman and the reason for our trip was to visit the series of sound mirrors on the marsh.

History

These devices were constructed from early 1920 to the middle of 1930 as an early warning device for detecting approaching aircraft, by listening for the sound of their engines. A number of these devices exist in the South East, and elsewhere around the eastern and southern coasts of England and they are either 20’ diameter, 30’ diameter or a 200’ wall, all made of concrete.

The Royal Flying Corps, later to become the RAF, came into being during the First World War. Initially our air force was only really used for spotting for artillery and bombing; it was the Germans who first used small aircraft in an offensive role. After the war it was realised how useful attacking aircraft could be and at the same time how vulnerable we would be from a mass air attack. Hence the need for some form of defence and knowing as soon as possible that an aircraft is approaching is a good first step in defence.

Principle of operation

The idea is simple. In the same way that a dish will collect light or radio waves and direct them towards a focal point, this also applies to sound. If you make the ‘mirror’ large enough, with the correct focal length or curvature of the mirror and point it in the right direction sounds will be collected and concentrated at the focal point. Listen at that focal point and when you hear an aircraft you know they are on their way.

At Denge Marsh there are three sizes of mirror. 20’ 30’ and 200’.
20’ mirror             30’mirror            200’mirror

Twenty-foot mirrors

These have a fairly flat curve and were quite upright and an operator stood on a small wooden platform in front of the mirror. The operator, known as a listener, wore a stethoscope type contraption (the only word I can think for it) complete with rubber tubes which were connected to a horn type device, similar to a microwave con a modern microwave dish. The collecting horn was mounted on the end of an arm, which was in turn mounted on a pivot mounted in front of the mirror. The listener would move the arm and therefore the collecting horn over the dish listening for the sound of aircraft. The length of the arm was a little shorter than the centre of the arc of the dish so that the listening cone would always be near to the dish surface.

When the listener heard an aircraft he could move the collector around to find the point of maximum sound. He could then read off from two graduated scales, one horizontal one vertical, which would indicate both the horizontal and vertical angles of maximum sound. At the time, in the early 1920s it was believed that aircraft would be flying low over the English Channel and so the height graduation would give some crude reckoning of distance. Low sound meant further away. Horizontal angles were the bearing. Two bearings from different locations would give you the range and the elevation would give some measure of height.

The main flaws with these mirrors were the shallow dish curvature and the low angle of detection plus the listener was out in the open air in all weathers.
There are mirrors at Denge and another on top of the cliff between Folkestone and Dover. Another at Hythe fell down flat on its face many years ago.

Thirty foot mirrors

Not only were these larger but they had a much deeper curvature. Much more like a deep breakfast serial bowl rather than a shallow dish as with the 20’. Also the listener sat in a small room beneath the dish. Much more hospitable.

30’ mirror with remains of the staircase on the right. (see enlarged image) The original ground level was at the top of the stairs.

Because the listener was now remote the sound collecting mechanism were operated with a series of wheels and leavers from below. The principle of operation was the same. The listener wore stethoscope ‘head phones’ connected to the sound-collecting cone, which was moved about within the bowl. This time when sounds were heard the horizontal and vertical angles were read from graduated scales fixed around the operating wheels or levers. These dishes were more inclined backwards than the 20’ ones and the much deeper bowl collected more sound.

200 foot mirror
200’- mirror with group. Note the reinforcing lines that can be seen as the concrete weathers. These make the vertical and horizontal curves clearer. Somewhere in the crowd is Phil.

These are the daddy of all sound collecting mirrors. Not only are they 200’ feet long and 20’ high but they have a curvature calculated to be a segment of a giant sphere, both horizontally and vertically. Built in concrete in the early 1930s they  are a great feat of engineering.

This time electronics came on board. In front of the mirror was a concrete platform with a trench along its edge furthest from the sound wall. The mirror was constructed so that it would focus its collected sound down toward the trench. In the trench was placed a series of special microphones. The microphones were not used for the direct collecting of audible sounds but were tuned to react to certain frequencies made by aircraft engines. This in turn would change the voltage or current, which in turn would move a needle on a scale inside a control hut where the operators were sitting. The microphones could be selected in and out of circuit to ascertain horizontal direction and needle movements would help decide the size of an attacking force and its height. Does this have a familiar sound (no pun) to it?

Testing

Throughout the dozen years of so years of testing they simply used to fly aircraft towards and around the sound mirrors to what could be heard, how, when, how soon and how accurate. The results were marginal. Although my trip and this short piece is about the mirrors at Denge there were also 20’ & 30’ mirrors on top of the hills behind Hythe but no 200’ mirror.

Hythe was also where all of the control and testing research buildings were based. Hythe was chosen partly because it was under the flight path of the early civil aircraft and so the ministry got ‘free’ test flights to listen to; perhaps several a week. It is still easy to see the 30’ mirror from the A259 coast road on the western approaches to Hythe. (Our side).

The plan

The plan, if there was one, was to build a series of these mirrors up the East and Southern coastlines of Great Britain. Signals from the various, and if the plan had been fully put into action numerous, listening points would be fed by post office phone lines back inland towards sector control rooms and then overall control rooms.

Back of 30’ mirror showing phone line insulators on the right. (see enlargement) Note figure walking away on the right which gives some idea of the scale.

The control room would have a big layout map where enemy aircraft and our responses to them would be laid out and pushed around with long cues. Now we all know where we have seen that before.

It is believed that the radar fed control rooms in the Second World War is the direct descendent of this plan of detection and signal being fed back to a control room.

The demise of the system

It is easy now with hind sight to see the flaws in the system and by the early 1930’s the system was falling out of usefulness. Aircraft got much faster and between hearing them and directly seeing them with visual spotters was down to 10 or less minutes. The structures themselves were a huge resource and obvious target in them selves, again what we found to our cost at the start of WW2 with huge radar masts.

And finally the noise

The Romney Hythe and Dymchurch Railway was used to help carry materials to build the mirrors and together with the Greatstone and Littlestone coast roads was extended right across in front of the mirrors at Denge. Although the Hythe mirrors were much higher up on the hills, and there was no 200’, sounds would still be heard from the growing town below. Imagine a quiet coast road, the man in his chamber intensely listening for the sounds of aircraft and then an old 1930’s lorry with a lumpy old side valve engine went along the road! It was not going to work but was a good fist effort. There is even a report about the 200’ wall operators hearing strange noises on one occasion. It was a long walk around to the front of the wall where they found a family having a picnic in front of this huge 200’ long by 20’ suntrap.

Maybe the system was either kept going long after its real use was over as a means of testing the communications network that we know really did work well, or maybe it was a cover for the advancing new technology of radar. A study of the dates of one technology’s demised while another was starting up would prove interesting.

Today

Today the mirror at The Roughs behind Hythe has a fence around it but it is still easy to see from the A259 road and it is possible to climb the hill and take a close look.

At Denge the three mirrors are surrounded by water and can only be reached by crossing a retractable bridge. The bridge and the whole area is under the control of a Nature Reserve warden who arranges free walks several times each year.

A personal view

Broken pottery or bits of rusty old metal do not do anything for me but I find industrial archaeology fascinating. These devices which were at the cutting edge of technology in their day were built before my time but my father was

certainly alive by then. These are both very near to us both in time and geographically and as a direct ancestor of radar we owe a great deal to the brains and forethought of those involved.

During my visit I was able to walk around all three mirrors, touch them and even go inside the 30’ mirror underground chamber. We could of course walk on the front concrete apron on the 200’ mirror. This is real archaeology in my humble opinion and well worth a visit. I will definitely be going back some time again in the future.

Note the deep bowl of this 30’ mirror. Peter is standing with his elbow on what would have been the windowsill of the listeners’ room. The original ground level was just a few inches below that sill.

Further reading

I hope my limited scribblings and the photographs have whetted your appetite. If so, before you make a visit, or in case you cannot actually get there, there is a great little book called Mirrors by the Sea written by Richard N Scarth. I have a copy and will let you have a look at it but not take it away. Richard came along on the trip I went on although he does not go that often these years. To top it all I got him to sign my book so that is why I will not let it out of my sight. Don’t want to have to buy it back on eBay do I?

Please enjoy.

Peter Firmin from Vital Spark published November 2008.

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