“Sound ranging “ is the locating of enemy guns using the “Sound Waves” made when a gun or guns fire.
Basics comprise a base with Microphone positions all accurately surveyed in on the ground and plotted on what is known as an Asymtote Board situated in HQ. The length of the base depends on the lie of the land etc, Usually 4, 5, or 6 “Mikes” at regular intervals. Also in HQ the “Recorder” and Batteries supply; 70-80 volt to lines to the various “Mikes.” These lines are often laid over rough ground and in adverse conditions. In front of the base, laid as far forward as possible, the A.P. or A.P’s. (advanced posts.) These positions were manned in shifts of 3 surveyors who lived and ate in these dugouts, protected by sand bags.
The “Mikes” consisted of an oil drum set into the ground at the surveyed location. The lines bearing an electrical current are fed into the drum and down to the grid at the bottom. The grid consists of very fine wires set in a cardboard surround and which “Glowed Red” like an element when a current is passed through it. The drum was then covered with a wind shield and a warning notice to “Keep off.” Each line led back to the recorder in HQ, and these lines were often destroyed by enemy shelling, or our own tanks and other vehicles.
These lines were often a source of puzzlement to the Security Police when checking lines (mostly telephone)
The recorder consisted at one end of a camera, and at the other “Harp Strings” fixed in between a permanent magnet; these strings being an extension of the lines bearing the electrical current.
When the man on duty at the AP heard an enemy gun or guns fire (i.e. from a troop or battery) he pressed a “Tit” which through a line to HQ activated the recorder. He also took a compass bearing.
The sound waves in a circular motion hit each of the “Mikes” in turn, and the cooling effect on the grid caused an increase in the electrical current flowing through it. This in turn caused the harp string (corresponding to the “Mike”) to “Kick” within the magnetic field and this “Kick” is recorded on the film which is running. This happens for each of the “Mikes” involved. (To see the film is rather like that showing Earthquake shocks.) The film is marked vertically at “Time Intervals” and the speed of sound is also adjusted for climatic conditions, i.e. Heat and wind. The later work carried out by an RAF attachment of Two men with their Balloons and instruments.
The film when developed and read by the plotters can give distances from the various “Mikes” to the source of the sound, i.e. the gun or guns of the enemy. These distances are plotted on the Asymtote Board and a map reference with co-ordinates is given to the CBO or the CRA (Counter Battery) or (Command Royal) artillery. At the battle of EL Alemein over 300 locations were achieved.
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