Now all roads lead to France and heavy is the tread
Of the living; but the dead returning lightly dance.
Edward Thomas, Roads

Friday, May 11, 2018

The Advent of Sound-Ranging Technology

Artillery Was the Greatest Killer of the War

By James Patton

By early 1915 the war of movement in the west was over, the German armies had settled into well-chosen defensive positions, and the Western Front was born. This was the ideal domain of the artillerymen, and both sides quickly realized that they needed counter-battery tactics.

Early on, everyone tried flash detection to get the direction of incoming fire, and they attempted to measure distance by calculating the time lapse between the flash and the audible sound, but this proved unsuccessful because the audible sound wave was a rumble rather than a sharp report, which led to unacceptable inaccuracy, as the battery could only be sited in a fairly large area.  A better method was urgently needed.

The Germans pursued an engineering solution using linked listening posts, called the early warning, the main and two or more secondaries. Alerted by the early warning post, stop watches were switched on when the sound reached the main and then the secondaries. The difference in times was converted to a distance and circles plotted, another circle was then derived that touched these two circles and the main, the center of this circle was the source of the sound. Corrections were attempted for conditions affecting the speed of sound. The biggest drawback was reliance on human hearing, so they added "objective devices," including directional galvanometers, oscillographs, and modified seismographs, with results recorded onto paper or photographic film.

William Bragg
The British sought instead to find a scientific solution. Their artillery command center for the Ypres Salient was located atop Mont Kemmel, a hill about 400 feet above sea level and a mile east of the German positions on Messines Ridge. In mid-1915 a Territorial Force 2nd Lieut. named William Lawrence Bragg (1890–1971) was attached for general duties there, as his Royal Horse Artillery battery had been laid up. No ordinary officer, he was a theoretical physicist with a Nobel Prize. 

Bragg was born in Australia. In 1909 his physicist father, Lawrence, became the Cavendish Chair in Physics at the University of Leeds. William, already a graduate from the University of Adelaide, moved with the family and took another First from Trinity College, Cambridge, in 1911. He was elected a Fellow of Trinity College in 1914, while he was working under Sir Joseph J. Thomson (Nobel Laureate in Physics for 1906) at the famed Cavendish Laboratory, which Bragg himself would subsequently head (1938–1953).

Bragg’s most famous discovery was his law regarding the diffraction of X-rays by crystals, by which the positions of the atoms within a crystal can be calculated from the way in which an X-ray beam is diffracted by the crystal lattice. He made this discovery in 1912 and discussed his findings with his father, who then developed the X-ray spectrometer at Leeds, which enabled the analysis of many different types of crystals.

For this work, father and son shared the Nobel Prize in Physics for 1915. At age 25, William is the youngest person ever to receive the physics prize.

At Mont Kemmel, Bragg went to work on the sound ranging problem. He postulated that there were two distinct disturbances resulting from the firing of a heavy artillery gun: the "shell wave" and the "gun wave." Furthermore, the shell wave would be audible, but the gun wave would be sub-sonic. Almost unbelievably, Bragg is said to have gotten the breakthrough by observing the WC (toilet) at his billet (how many had such a thing on the Western Front?), he detected the pressure differences of shell waves and gun waves.

William Tucker
Bragg found an invaluable assistant, L. Cpl. William S. Tucker (1877–1955), a Territorial with the London Electrical Engineers (a searchlight unit), who was assigned to Mont Kemmel for general duty. Tucker, an engineer and a lecturer at Imperial College (London), specialized in acoustics.

Both the Germans and the French had worked to adapt galvanometers coupled to microphones as a way to improve the accuracy of the sound recording process. The French, led by chrono-photographer Lucien Bull (1876–1972) and astronomer Charles Nordmann (1881–1940), recorded these micro-electric signals on photographic film. This enabled accuracy to hundredths of seconds, but the device could not run continuously because of the wastage of film, which meant that the recording couldn’t start until it was certain that every post was measuring the sound from the same flash, and they were still measuring the shell wave.

This problem was solved in mid-1916 when the new 2nd Lieut. Tucker invented a low-frequency microphone that enabled the isolation of the gun wave from the shell wave and eliminated error caused by the sonic boom as well.

Tucker’s device was highly sensitive; it used an ultra-thin heated platinum wire that was cooled by the gun wave. He had noticed that whenever a gun wave arrived there was a draft of cold air that came through mouse holes near his bed, so he devised a microphone consisting of a thin, electrically heated wire, stretched over a small hole in a container (he used rum jars). The decrease in the electrical resistance of the wire caused by the air pressure of the gun wave was recorded by a galvanometer. This microphone worked well, as the rapid oscillations of the shell wave had almost no effect on the wire, while the gun wave produced well-defined "breaks" on cine film.

By September 1916 all British sound ranging sections in Belgium and France had these devices.  Tucker returned to London to perfect techniques for correcting the sound data to compensate for meteorological conditions and to determine the optimum layout of the "sound ranging base"—an array of Tucker’s microphones. It was found that a shallow curve and relatively short  base was best. Over time, the attacking gun site could be located to within 25 to 50 meters under normal circumstances. Later refinements could even estimate the size of the gun and find the direction of fire, eliminating the need for theodolite flash bearings.

By war’s end Tucker was a major, outranking Bragg. Both men received the Order of the British Empire (OBE) in the 1918 Honours List. Additionally, Bragg was Mentioned in Despatches three times (1916, 1917, and 1919) and awarded the Military Cross, unusual for an officer not actually serving in combat.

Sound Ranging Equipment

After the war both Bragg and Tucker returned to their old lives. Bragg succeeded Sir Ernest Rutherford at Manchester when the latter became the head of the Cavendish Lab. Bragg founded the Unit for the Study of the Molecular Structure of Biological Systems, from which came the groundbreaking work on the structures of proteins and then DNA. He was elected a Fellow of the Royal Society in 1921, knighted in the 1941 Honours List, and shortly before his death became a Companion of Honour (CH), of which there can only be 64 living members.

In 1938 Tucker became Director of Acoustical Research for the RAF. His group constructed a number of large concrete parabolic sound wave mirrors along England’s south coast, which were designed to pick up the sound of aircraft engines from a long distance. This project was superseded by improvements to the Chain Home System.

1 comment:

  1. Great article on a profound advancement of WW I technology. Looking into WW II, much of the sound ranging tactics was based on these WW I techniques. Those that may be interested in the WW II counterbattery era may be interested in a 2004 book I published "Patton's Forward Observers: History of the 7th Field Artillery Observation Battalion." (Brandylane Press) Thanks James for this fine article