sound mirrors on the south coast
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Sound Mirrors on the South Coast by Phil Hide
Origins in the First World WarPrimitive sound locators were used on the Western Front to locate artillery and enemy aircraft as early as 1914, however it was events away from the Western Front that provided the real impetus for developing means of detecting and tracking aircraft by sound. In May 1915 Zeppelin and Shutte-Lanz airships of the German Army and Navy started bombing targets around the Humber and Thames estuaries. London was attacked for the first time on the 31st of that month and by 1917 the airships were being replaced by twin engined Gotha and Giant aeroplanes. In total 300 tons of bombs were dropped on Britain during the First World War causing some 5,000 casualties, a third of which were fatalities. Some form of early warning system was badly needed, especially to counter the night raids. Following encouraging experiments with a four foot diameter prototype built by a Professor Mather a 16' mirror was cut into a chalk cliff face at Binbury Manor between Sittingbourne and Maidstone in July 1915. The mirror was shaped to form part of a sphere and a sound collector was mounted on a pivot at the focal point. The collector was usually a trumpet shaped cone connected to the ears of the listener with rubber tubes but experiments with microphones were under way before the end of the war. The listener would move the sound collector across the face of the mirror until he found the point where the sound was loudest. Bearings to the target could then be read from vertical and horizontal scales on the collector.
Professor Mather and his colleagues carried out a series of experiments with this mirror and produced a report which claimed that it could detect a Zeppelin at a range of twenty miles. The Army conducted it's own experiments at Upavon which were so disappointing, something Professor Mather blamed on "the ineptness of Army personnel", that it wanted to cancel all further work. Despite this it seems that several mirrors of 15' diameter were constructed around the South East Coast, Thames Estuary and on the North East Coast. No information regarding precisely when these mirrors were built survives but it seems that the ones around Kent were probably built first. They are similar in construction to the first mirror at Binbury Manor, being cut out of a chalk cliff, but were lined with concrete which made a better sound reflective surface. Later mirrors, such as the ones on the Yorkshire coast, were free standing and made entirely of reinforced concrete. Certainly the mirrors at Fan Bay, Dover (also identified as Langdon) and Joss Gap near North Foreland saw action in 1917 and 1918, the Fan Bay mirror detected an enemy raid at a range of 12-15 miles in October of 1917 and in 1918 both mirrors were able to detect aircraft heading for London several minutes before they were audible to the unaided ear. It's worth noting that late in the war the mirrors reported to a central command centre which plotted the positions of raiders on a map and organised defensive measures. Post-War Experiments Despite the early scepticism shown by the Army the sound mirrors must have performed well enough to warrant further development. An experimental station to develop sound mirrors and other sound detection devices was established at Joss Gap before the end of the war. Work continued there up until 1922 when the research centre was moved to an area called The Roughs near Hythe. This land was already owned by the Army and was situated near to the flight path for commercial aircraft flying between London and Paris. One problem the researchers had at Joss Gap was persuading the newly formed RAF to provide target aircraft.
It was hoped that by siting the new centre at Hythe commercial aircraft could be tracked as they made their regular flights overhead. A new 20' mirror and several wooden huts for personnel and
equipment had been constructed at Hythe by the end of 1922 and the mirror was operational by early 1923. The mirror was built as a solid slab of concrete set against the face of a cliff with a steel mast set in a concrete pedestal supporting the sound collector. In September of 1923 the mirror detected an aircraft at a range of 12 miles. Further research was carried out with microphones at Hythe but stethoscope tubing remained the method of choice right up until the 1930s. In 1925 Doctor W S Tucker was appointed Director of Acoustical Research and he was to play a major role in the development of sound mirrors and locators. In 1927 he put forward a proposal to build a chain of 20' sound mirrors along the south coast. Only two were constructed, at Lydden Spout (Abbots Cliff) Dover, and at Denge on the Dungeness peninsula. Like the Hythe mirror they were cast as one solid slab of concrete but were freestanding and not cut into the face of a cliff. Both were completed in July 1928 at an estimated cost of 650 each which included one Nissen hut and a fence around the entire site. Even before these mirrors had been finished Dr Tucker and his team had finalised the design of a 30' diameter mirror. Incorporating lessons learned from experiments with earlier mirrors they were angled upwards and of a more sophisticated construction than the previous slab designs.
The listener, previously required to stand outside in all weathers, was now seated in a booth beneath the mirror. He rotated the sound collector horizontally with a hand wheel and vertically by way of foot pedals. The booth was largely underground and the upper section glazed with bullet proof glass. Two were built, one 200 yards west of the Acoustical Research Station at Hythe and another adjacent to the 20' mirror at Denge. Both were completed by the spring of 1930. Although these mirrors did not have a greater range than the 20' versions they did provide greater accuracy, especially in the vertical plane. At the same time as the 30' mirrors were being constructed work started on something even bigger. In Dr Tucker's words "Whereas the 30' mirrors are very efficient for (sound) waves up to 3' or so.....the sounds we wish to deal with have waves of 15' to 18'... This involves extension of the mirror surface to about ten times that hitherto employed." As the mirror was intended to operate only at long ranges elevation angles would be small so the height of the mirror could be reduced. Even so the eventual design was 200' in length and 26' high with a curvature of 150'. It's size precluded the use of a moveable sound collector so it was proposed to use a line of static listeners and microphones. The mirror was built at Denge, close to the existing 20' mirror and was completed by the middle of 1930. At first the microphones, 20 in all, were connected to apparatus housed in a hut but in 1933 a two storey listening chamber was built onto the rear of the mirror with a small window cut through so that the listeners on the forecourt could be observed. Sound Mirrors in Action From 1930 until 1935 the mirrors participated in the annual Air Defence of Great Britain exercises with the RAF. The 200' mirror was the long range lookout, telling the operators of the
30' and 20' mirrors where to listen. They in turn tracked the incoming aircraft and reported their readings back to a central control centre which calculated and plotted the raiders' position. In 1932 the 200' mirror detected aircraft at a range of 20 miles when the unaided ear could only hear them at 6.5 miles and on another occasion at 30 miles when an unaided listener could only hear them at 5.5 miles. Problems were experienced with high winds blowing across the face of the 200' mirror and unwanted noise such as ships in the English Channel, however the story about the mirrors being jammed by a passing milk float appears to be untrue. Canvas screens were erected either side of the 200' mirror to reduce wind noise and to provide some shelter to the listeners on the mirror forecourt. The main benefit of the exercises was the development of a centralised reporting and command structure for air defence which formed the basis of the system used in World War 2. In the 1935 exercise information was passed to a control centre set up in central London, Dr Tucker also identified the need for bearings to be transmitted from the mirrors automatically as transmitting them by 'phone was too slow. It was also discovered that the job of listener was very tiring, duty periods of no more than an hour were recommended, and that great care had to be taken in selecting the right men. The Thames Estuary Scheme and Finale During the 1930s it was proposed to build a chain of 200' and 30' mirrors along the south coast from The Wash to Swanage. The first stage of this scheme was to be a system to protect the Thames Estuary consisting of two 200' and eight 30' mirrors.
In mid-1935 approval was given to commence work on the construction of this system, sites were selected and a budget of 10,000 was approved. One of Dr Tucker's team, Percy Rothwell, had designed a 200' mirror constructed from pre-fabricated blocks which would have been cheaper and quicker to build than the one at Denge but the end was already in sight. In February of that year Robert Watson-Watt had performed his first experiment with Radar at Daventry, with uncharacteristic speed a Radar research station had been established at Orford Ness by March and by July aircraft were being detected at ranges of 40 miles. The Thames Estuary scheme was at first postponed, then cancelled altogether. In May 1936 the 15' mirrors at Fan Bay, Joss Gap and Warden Point were abandoned, the 200' mirror followed in 1937 and the remaining 30' and 20' mirrors in 1939 when the Acoustical Research Station was closed down. The mirrors at Denge were used for a series of experiments involving the detonation of small explosive charges at the focus of the mirrors but fortunately the War Department's order that