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push a key or button when he heard or saw guns fire. This signal would initiate the film to start recording at Central, which would run for 20-30 seconds. The film record contained the muzzle blast, at times the shell wave, and the explosion from the bomb’s impact. The film was developed, the time differences plotted on the plotting board, and the results delivered to the artillery
section, all within 10 minutes. If the strings on the plot- ting board intersected at a single point, the location was deemed accurate and less accurate if they intersected in more than one point. The SR sections were able to deter- mine the location of the gun, the location of impact, and the time of the round in air. This information aided in determining the caliber of the gun. They would send scouts to scavenge shell fragments to confirm the cali- ber and type (not an enviable assignment!). According to Bragg, “a typical report gave the caliber, number of guns, and target on which the battery had registered” (Trowbridge, 1920; Bragg et al., 1971). The reports were provided to the artillery who used the information to target the enemy batteries.
In the BEF, a “sound-ranging section had 3 officers and 18 others: 1 sergeant, 1 instrument repairer, 1 photogra- pher, 3 linemen, 2 telephonists (telephone switchboard operators), 3 forward observers, 3 batmen and 4 motor transport drivers” (Van der Kloot, 2005). Although
efforts were made to conceal their posts, the observers were often in vulnerable positions, being located close to the front line. In addition to pressing the key to start the film at the time at which a gun was fired, they would esti- mate the location and caliber of the enemy batteries. If possible, they would also provide other intelligence, such as the location of machine guns and troop movements. Linemen were the most vulnerable. They laid out the wire from Central to the observation posts and the six micro- phones. When wires were damaged, they would have to repair and splice or, if beyond repair, replace them, often under adverse conditions. Surveyors established the loca- tions of the microphones so that their positions could be accurately placed on the plotting board. The soldiers at Central performed several functions: they developed film, measured time differences and plotted them on plotting boards, maintained and repaired instrumentation, and reported the results to the artillery. Central was also vulnerable, often setting up in wrecked houses located within the range of enemy artillery.
A report on SR issued by the General Staff in March 1917 claimed that the error from a single good observa- tion was usually within 50 yards, dropping to less than
25 yards when several observations were averaged. The report also claimed that a single SR section obtained 260 locations of German batteries in the two-month period of December through January.
Another report from 1918, prepared by an artillery information officer of the AEF, compared the locations of artillery batteries provided by flash and SR. In the first case, flash ranging outperformed SR during a period of mobility where the allied forces were advancing and the enemy forces were retreating. Out of 425 locations, flash sections reported 79% while sound sections reported 21%. However, during a period when the front was stationary, SR outperformed flash ranging; out of 392 locations, flash reported 46% while sound reported 54%. The flash sections outreported the sound sections during periods of mobility because the flash sections required less equipment, enabling them to pick up and set up more quickly in mobile conditions, while the sound sec- tions performed much better during periods of stationary warfare. The SR locations were also more accurate.
Weather affected SR in several ways. The first is temper- ature because sound travels faster in warm air than in cooler air. The time differences are inversely proportional to the speed of sound: an increase in sound speed makes the gun appear closer.
Wind was also a big factor. When the wind blew toward the enemy lines, the microphones were sometimes unable to detect the muzzle wave. The wind speed is lower near the ground than at higher altitudes, with the result that the effective speed of sound decreases with altitude. This causes the sound to refract upward. Bragg complained, “due to the ‘principle of maximum cussedness’ the wind in Flanders and Artois was generally westerly” (quoted in Van der Kloot, 2005). When the wind direction was from the enemy guns, the effective speed of sound increased with altitude, caus- ing sound propagating in this direction to refract toward the ground, sometimes distorting the signals due to ground reflections but providing measurable signals. SR worked best for the Allies when the winds were light and the tem- perature uniform and in foggy weather, not an uncommon condition in Flanders (Bragg et al., 1971).
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