Page 33 - Summer 2020
P. 33

The Geographical Section of the General Staff in London had learned about the French efforts to locate artillery by sound. The head of the topographical subsection at the General Headquarters (GHQ) in Flanders, Belgium, Lieutenant. Colonel Ewan Maclean Jack, Royal Engineers (RE), recruited Second Lieutenant William Lawrence Bragg to put a system in operation.
Bragg was a student of mathematics and physics at Trin- ity College Cambridge in 1909. After taking a First in Part II Physics in 1911, he started working at the Cavendish Laboratory. In November 1915, he and his father, Wil- liam Henry Bragg, were jointly awarded the Nobel Prize in physics “for their services in the analysis of crystal structure by means of Röntgen rays.” W. L. Bragg, who was 29 years old at the time, is still the youngest laureate to have won the Nobel Prize in a scientific category.
Bragg had enlisted shortly after the war began and was commissioned as a Second Lieutenant. In the fall of 1915, Lieutenant Colonel Jack offered Bragg the opportunity to put SR into operation. Bragg accepted, happy to have a scientific job in the war, and recruited an assistant, newly commissioned Lieutenant Harold Roper Robinson, a lec- turer in physics at London University where he worked with Ernest Rutherford (MacLeod, 2000; Van der Kloot, 2005).
Bragg and Robinson traveled to a section of the front in the Vosges Mountains in the Alsace region of France where the French had set up their SR apparatus. The French sound rangers instructed the British officers over the next couple of weeks. However, the front during that time was very quiet and they didn’t see much action. Bragg and Robinson left to establish the first SR section on the British front in Flanders, five miles southwest of Ypres, Belgium, which was being held by the Canadian Corps of the British Second Army. Bragg and his team struggled through 1915 and the first part of 1916, pro- ducing inconsistent results due mainly to the type of microphone they were using (discussed in Microphones).
Despite the lack of progress, Lieutenant Colonel Jack agreed to start up additional SR sections. Bragg recruited new sound rangers by attending unit parades, where he would order “all Bachelors of Science step forward” (Van der Kloot, 2005). By June 1916, 16 SR sections had been recruited, trained, and deployed to areas along the front (one of these recruits was Lance Corporal William Tucker).
During this period, Bragg promoted the exchange of ideas: “At intervals of two months or so, we had a meeting at some central point such as Doullens to which each section sent an expert. They swopped stories, schemes, and boasts of
their achievements and I am sure emulation made every- thing go much faster. The meeting generally ended with a binge of heroic magnitude” (Bragg et al., 1971, p. 38).
Method and Apparatus
Hyperbolas
As mentioned in Beginnings of Sound Ranging, acoustic location was based on determining the differences in the times of arrival of the sound from a cannon to different observation positions or microphones. The difference in time was used to determine the direction of travel of the sound by considering that the gun was located on the asymptote of a hyperbola while the pair of microphones that detected the sound was located at its foci. The time difference would be constant for any gun located along this asymptote. Thus, the time difference established the direction of arrival of the gun wave. Time differences from other pairs of microphones produced their own asymptotes or bearings. The intersection of these bearings determined the location of the gun.
Thus, the sound rangers devised graphical techniques to determine the gun locations. After being surveyed,
 Figure 2. Plotting board used with sound ranging. Nos. 1-6, microphone positions. The timescales between adjacent microphones are within the border that runs from the top left, underneath, and to the right of the array (although difficult to read at this scale). From Mitchell, 2012. See text for detailed explanation.
 Summer 2020 • Acoustics Today 33






















































































   31   32   33   34   35