The Evolution and Demise of Acoustic Air Defense: From Sound Locators to the Strip Mirror in Malta

By Ron Hewlett

When man attempted to emulate the flight of birds it was inevitable that, if successful, it would attract the attention of the military and at the same time create a requirement for anti-aircraft defence and early warning. Over the centuries the would be “birdmen” were encouraged by scientists and engineers alike, not least by the brilliant artist and engineer Leonardo Da Vinci who produced drawings of flying machines, including a helicopter. The unfortunate and inept fliers with their manufactured wings achieved little more than a few broken limbs or worst. Winged flight was much further in the future because the first milestone in aviation happened in the late 1700’s with the introduction of the man carrying “hot air” balloon.

During the Napoleonic wars 1803-14, the first hint of the military use of aviation occurred with reports that the balloon was used for early warning of enemy troop movements. By the mid-1800’s the “hot air” balloon had been superseded by the lighter than air “gas” balloon. However, both types had one common shortcoming: they could only fly in the direction of the wind. In 1900, Count Von Zeppelin introduced his radical new balloon design that was cigar shaped instead of spherical. Known as a “dirigible” his aircraft was fitted with a rudder and elevator and driven by engine-powered propellers. The German Navy immediately saw the potential of this self-propelled manoeuvrable craft, which they called an “airship”, and introduced an air formation into the German Navy.

In 1903 the Wright brothers made the maiden flight of their “aeroplane” and fixed-wing powered flight technology advanced rapidly from then on. Samuel Cody, an aviation enthusiast in England, designed a kite for the Royal Navy. Sam’s kite was a huge affair which could carry a man as high as 1,000 feet (305 meters) and used for observation over the horizon to give early warning of approaching enemy ships. Having completed the naval project, Sam then designed an aeroplane for the Royal Engineers who wanted to evaluate it for military use. His maiden flight was in a field next to an Army Balloon Unit in Hampshire near the town of Farnborough, which is today the home of the Royal Aircraft Establishment (RAE).

In 1909 the Frenchman Louis Bleriot flew the 31 miles (50 km) across the English Channel from Calais to Dover Castle and win the £1000 prize offered by an English newspaper. While the people both sides of the Cannel applauded Bleriot’s achievement it should have at the same time rang alarm bells in Whitehall and especially in the Admiralty. For centuries Britain’s main defence was the sea around the island and the Royal Navy to defend it. Bleriot had now proved that the British would have to look to the skies as well as the sea for an enemy attack.

It was soon realised in the early days of the 1914-1918 war that the aeroplane was yet another weapon to use. Along with “land” and “sea” warfare, the aeroplane had now introduced a third form “air warfare”. In 1915 the Germans took air warfare a stage further when they attacked mainland Britain using their long range Zeppelin airship; the first attack was made on the coastal town of Great Yarmouth in May 1915. By 1917 the Germans had developed their Gotha bomber aircraft and made their first raid in Folkstone in May 1917. Because it seemed the German raiders had a free hand to attack Britain when and where they liked, it became at the time easy for the MPs and the general public to criticise the air defence forces.

However, it should be appreciated that the Zeppelin raiders came in numbers as few as three and as many as fifteen on any one night and could attack targets as far apart as Newcastle in the north and London in the south. With airborne radio in its infancy there was little or no ground-to-air communication. Navigational aids were very basic and the blackout not only hindered the enemy but also the air defence fighters. In many cases it was pure luck if a fighter found an enemy aircraft while on patrol. However, the raiders did not always get away because the first Zeppelin to be shot down was LZ.21 by Lt. William Leefe-Robinson of the RFC’s N° 39 Squadron at 0230 hrs on 3 September 1916.

During the day aircraft could be detected by ground forces both by sight and sound but since most raids were made at night it was only the engine and propeller sound which warned the observer of an approaching aircraft. The range that sound can be heard unaided is dependent greatly on climatic conditions, but in normal situations an aeroplane can be detected by ear at about 4 miles (6,5 kilometers) range. The Royal Engineers Board (REB) set up an Acoustic Research Unit (ARU) at the Anti-Aircraft Experimental Establishment (AAEE) based at Orford Ness in Suffolk, to extend the detection range of aircraft sound. Various mechanical devices were tried using large horns rather like the early deaf aid “ear trumpet” with limited success.

Based on a multi-horned device, the manifacturing order for the Sound Locator Mk.1 was placed in October 1917 and went into service early in 1918. Meanwhile, Dr. William Tucker, a Captain in the RE, had in 1917 developed a microphone for the detection and ranging of enemy guns. Having completed this project by late 1917, the REB asked him to design a long range aircraft sound detector; the Acoustic unit at Orford Ness was considered as short range (up to 10 miles, 16 kilometers) research. The Gotha bomber raids that had started in May now made long range detection an urgent requirement. Within a few weeks Capt. Tucker produced a design in which he had adapted his gun-ranging microphone and the REB eagerly provided him with another RE office, Lt Tabor Paris, four sappers and accommodation at Thames Ditton to develop the sound detector.

Lt Paris, a pre-war scientist, had worked with Dr. Tucker on the gun ranging microphone project. Unfortunately, tests early in 1918 on the microphone detector were not a success and Dr. Tucker’s thoughts turned to using other forms of sound locating devices. For the first half of 1918, Dr. Tucker’s team studied the physics of acoustics, the climatic effects on sound and the wavelenghts of aircraft noise. They had also developed a resonant disk system, a large concrete or wooden disk pivoted at the centre which acted like a tuning fork when excited by the sound of an aircraft as it flew overhead, the resonant vibration being detected by a microphone.

At the Orford Ness ARU the team had devised an extremely clever although ambitious sound detecting system. It was discovered that an observer in a deep well could hear an aircraft at almost the same time as he could see it. A well six feet (1,9 meter) wide and thirty feet (9 meters) deep provided a sound aperture of 40°, the sound cut off being very sharp at the edge of the cone. The plan was to have a row of wells close to the coast at half mile (0,8 km) intervals with a second row four miles (6,5 km) inland and parallel to the first. A sentry in each well would be in contact with an operation centre and alert the operations staff when an aircraft was first heard and when it had past out of audible range.

A sound mirror in the UK (Credit: Major Tony Abela)

Using a stop watch, the operations staff would time the aircraft across the 40° cone and also when it was picked up by a sentry in the second row of wells. Knowing the distance the aircraft had travelled and the time it took to travel across the 40°cone, the height could be calculated. The aircraft’s course would depend on which pair of wells it passed over. Assuming the attacker maintained his course, height and speed, its position inland could be plotted by extrapolation.

This ingenious scheme was however flawed because of the enormous amount of manpower it required. It was estimated that a minimum of eight miles (13 km) of wells, 32 in all, would be required to protect an inlad medium sized town. Taking into account the listening sentries, operation centre staff and all the support personnel for a 24 hour three watch system, it was calculated that the manning level would be 350 personnel, a huge number for such a short stretch of coastline. As a compromise, shallow 2 feet (0,6 m) deep wells were used and the sentries replaced with Dr. Tuckers resonant disks. However, limited trials of the system started at the end of October 1918 and within two weeks the war was over and no further trials were conducted.

By early new year 1919, demobilisation was in full swing. The War Office had decided that air defence was a priority but their experts were leaving the army and returning to their civilian jobs. To keep up the momentum in the air defence Research and Development (RD) programme, the War Office formed the Air Defence Experimental Establishment (ADEE), centralising all air defence research and managed by the army but staffed by civilian engineers and scientists. Dr Tucker was appointed Head of Acoustic RD with Mr Tabor Paris as his deputy and the section was accommodated at the new RAF station at Biggin Hill. Because at the time the sound of an aircraft engine was still the only parameter they had to work with, the scientists centred their research on acoustic detection.

When planning the Acoustic Sections areas of RD, Dr. Tucker and Paris decided that the “in service” short range Sound Locator Mk.1 could be improved and further work on the short range horn type locators was required. Also the listening well system using resonant disks was considered useful as a “gap filler” between manned locator systems. However, the main thrust of the research was to developing a long range sound detector that provided sufficient time to alert defence fighter aircraft.

In the summer of 1915, Professor T. Mather had experimented with parabolic shapes cut into the high cliffs along the Kent coast with the operator standing on a platform at the focal point to listen for aircraft engine sounds. The advantage of this shape is that it magnifies the sound by concentrating the reflected sound at a focal point in front of the dish.

Prof Mather’s 16-foot (4,9m) diameter dish located a Zeppelin at twelve miles. Although it established a scientific principle and an interesting experiment, Prof Mather’s dish was of limited value in air defence at the time. However, Dr. Tucker had now decided to revive the idea of a dish type locator. The scientists experimented with many and various types of large concrete sound collecting mirrors and as radio and electronic development progressed in the 1920s, so the use of microphones and thermionic valve amplifiers were used in this research with advantage.

One factor was clear: the locator dish had to be substantial, made of very thick concrete or metal, in order that it did not absorb any of the aircraft sound energy and therefore reduce the maximum detection range. The problem with Prof Mather’s parabolic dish is that it had a single focal point and the dish had to be rotated to face the aircraft in order to find the maximum sound level and hence its bearing. The mechanics of rotating a very large and heavy dish plus preventing any turntable noise being injected into the dish presented difficulties. Therefore, Dr Tucker decided to abandon the parabola in favour of the hemisphere.

A concrete bowl for early sound detection (Credit: Major Tony Abela)

Unlike the parabola, the inner surface of the hemisphere is a constant radius and the bowl can be stationary while the sound pickup, a microphone or horn, can scan the inner surface to find the maximum sound. The final position of the articulated arm holding the detector provided the bearings, both azimuth and elevation, of the sound emitting aircraft.

In 1922 a 20-foot (6,1 m) wide concrete bowl was built on War Office land at Hythe near Folkestone in Kent, where trials were carried out and ranges of 15 miles (24 km) were obtained. By 1927 two more 20-foot bowls had been built, one at Denge near Dungeness and another at Abbot Cliffe west of Hythe. The three bowls were connected to an operations centre where aircraft movements were plotted from the bearings given by operators at each bowl. By 1929, the two 20-foot bowls at Hythe and Denge had been replaced with 30-foot (9 m) bowls that gave improved range.

A passive early warning system located in Denge, Uk. (Credit: Major Tony Abela)

The trials with the 20- and 30- foot bowls indicated that the larger bowl had an horizontal coverage of about 100°, only a small vertical strip around the centre of the bowl was required. This is because the angle above the horizon of an aircraft at 10,000 feet (3,048 m) at a range of 25 miles (40 km), is only 5° and at a 40,000 feet (12,200 m) it’s only 20°. Dr Tucker then introduced the largest and most spectacular of these sound locating dinosaurs, the “Strip Mirror” locator.

The Strip Mirror locator was constructed in the shape of an elongated strip from a sphere 150 feet (46 m) in radius, built in smooth faced concrete measuring 200 feet (61 m) horizontally from end to end and 27 feet (8 m) in height. In front of the mirror is a 55-foot (16,8 m) sloping concrete apron leading to a “listening trench”, a further 2 feet (0,6 m) deep and measuring 20 feet (6,1 m) wide at the narrowest point. A semicircular end wall 100 feet (31 m) wide and side walls contained the earth bank from the trench. An equipment cabin attached at the rear of the mirror also accommodated the operators. There were only two of this type constructed, the first was the prototype and trials model built in 1930 at Denge and the second, an operational mirror, at Maghtab in Malta built and tested in 1934-35.

The “Strip Mirror” at Maghtab, Malta (Credit: Malta Aviation Museum)

The vertical wall of the strip mirror reflected the detected sound in a geometric plane across the listening trench. The sound was picked up by a series of very sensitive microphones placed in the trench, the microphones being tuned in the region of 30 to 100 Hz and particularly sensitive between 60 to 75 HZ, the range of frequencies transmitted by aircraft engines and propellers. Each of the 20 microphone placed in the trench covered a 5° angular horizontal section of the wall giving a nominal 100° of coverage across its width and it is documented that the directional properties between each adjacent section was remarkably sharp.

The electrical output from each microphone was fed back to the equipment and operator building where the signal was amplified to output a loud speaker. There was also provision to switch a relay and put on a lamp to indicate the section of the wall activated by the detected sound, thus giving the bearing of the aircraft. At first, sentries patrolled sectors of the listening trench to alert the operator in the equipment and operations building which section of the mirror an aircraft could be hear. However, the sentries were later superseded by the electronic indication. Optimistically, it was suggested an average detection range of 35 miles (56 km) but 20 to 25 miles (32 to 40 km) was the normal obtained in trials.

At the speed of aircraft in the early 1930s this order of range would give a warning of between 10 to 15 minutes time to accurately lay the anti-aircraft guns and get fighter aircraft airborne.

The operator building (Credit: Malta Aviation Museum)

The intention was to have a series of Strip Mirror sound locators positioned around the east and south coast of Britain with the bearing lamp signals being sent to a central operations centre. In December 1933 approval was given for the construction of an acoustic anti-aircraft early warning system for the protection of London which consisted of nine Strip Mirror locators around the mouth of the Thames Estuary with an operations centre at RAF Biggin Hill.

However, owing to the political situation in Italy and the fact that the Italians had started their Abyssinian campaign, it was decided to immediately strengthen Malta’s defences. The island was under territorial threat by the Italians and at the time was also a major Royal Navy base and Air H.Q Royal Air Force Mediterranean Command. As part of this initiative, the intention was to build a chain of five 200-foot (61 m) Strip Mirror locators around the island. Building the Maghtab installation started in 1934 and completed in 1935.

However, in 1935, the scientist Robert Watson-Watt had proven that radio waves could detect aircraft at a greater range than sound and that radio waves were not prone to weather conditions, unlike sound waves. The Anti-Aircraft Research Sub-Committee, under the aegis of the government’s Imperial Defence Committee set up in the 1920s, decided to finance the radio wave approach for detecting aircraft, known today as Radar. The building of further concrete Strip Mirrors was put on hold and later that year cancelled completely, including the project in Malta.

Additional view of the Strip Mirror at Maghtab, Malta. (Credit: Malta Aviation Museum)

The Maghtab sound locator survived the 1939-1945 war undamaged and is today almost as good structurally as the day it was built; if the special microphones and amplifiers were fitted, it would still locate aircraft sound at ranges in the order of 25 miles (40 km)…that is, if the traffic along the coastal road was stopped and it was accepted that a modern jet aircraft would have crossed the coast before its sound was heard! Unfortunately, the other locators and the Strip Mirror at Denge, are in a very poor state, the severe UK winters, unstable foundations and time having taken its toll. The installation in Malta is now the only complete locator of this type remaining and an important historic air defence artefact.

In conclusion, aircraft sound detection has a fascination but in practice also has many shortcomings. If radar had not been developed and sound location had been the only anti-aircraft air defence system when the war started in 1939, it begs the question, would the outcome of the battle of Britain or indeed the Siege of Malta have been different?

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