大象传媒

My new squad was 鈥楳2鈥� which contained quite a number of the old squad members. The theory and use of Sound Ranging requires a knowledge of electricity, magnetism, radio, meteorology and even astronomy.

To deal with these subjects in a brief way I would explain the setting up and working of a sound ranging base, when in action.

On arriving at a new location where a base is to be set-up a recci is done to select suitable points for microphones positions, five or six in number, stretching along the front. This is roughly mapped and a triangulation scheme provisionally worked out to incorporate these points.

A RO is selected, maybe a trig point or even a beacon set-up, Foreign reference points must be considered as suspect and checked. Possibly the best way to obtain a true fix is to be an 鈥楢zimuth鈥�, namely observing a heavenly body, usually the sun, but the moon or known stars can be used. This required a series of shots, at least six, over timed periods, GMT. The Nautical Almanack is consulted to obtain the angles of declination for the readings. Adjustments are made for parallax and refraction being made a complex computation will give the position in latitude and longitude which can then be converted to grid.

The surveyors set out in pairs and observe the angles involved in the triangulation scheme, making alterations to any base points which would prove difficult to operate.

A fighting HQ is wet-up some distance behind the base, selected to give as much protection to personally as possible.

Angle sheets are brought back to HQ and calculations take place to determine the grid co-ords of the base points, accurate to the nearest second place of decimals.

The draughtsman begins to prepare his board, a large gridded one metal faced, plotting and checking angles, and then plotting the base points.

Two 鈥楢P鈥檚 (Advance Posts) are set-up well in advance of the base, 10,000 to 15,000 metres in front. These are surveyed and go onto the board.

The 鈥榃irers鈥� set out to dig in a 鈥榤icrophone鈥� at each base point, wiring back to HQ, reeling out from a truck, over fields, by sides of roads and poling over roads as required. The wire used is insulated double core. A surveyor has to check each installation.

The microphone is similar in size to a dustbin, but double skinned. A fine wire grid is insulated towards the bottom and wired towards the top, positive and negative.

The microphone is dug in with just the lid protruding, covered with coco matting and earth sprinkled on top. There is a space between the lid and body of the 鈥榙ustbin鈥�.

Back at HQ 鈥榮etting up鈥� is progressing. The lines from the microphones and AP鈥檚 are connect4ed to the 鈥楻ecorder鈥�/

The Recorder is designed to record time intervals of gun sounds on film.

The main parts of the recorder are:

路 Film container
路 Camera Box (clutch mechanism, knife and fixed lense0
路 Developer and Fixer Baths. (automatic action)
路 Galvanometer (permanent magnet and harp)
路 Optical System (12v. lamp, 3 focussing lenses, one fixed lense, 3 mirrors and ground glass viewing screen)
路 Timing System (tuning fork, 12v. timing motor and time wheel)
路 Driving System (12v. D.C. motor and gearing)
路 Test and Control Panel

All this contained in a metal box, approx. 40鈥� long, 22鈥� wide by 24鈥� deep.

Batteries are connected and the mic lines balanced to between 25 and 30 m/amps. The film, 200ft. of sensitised paper is put in situ and the tanks filled with developer and fixer solutions.

In the meantime the draughtsman permanently fixes the mic positions on his board, numbered M1 to M5 and then bisects at right angles between the mics. Time scales are selected to give readings each side of the bisectors. Adjustable point fittings are incorporated at the top of the board, suspending fine cords with clips to fix at the bottom.

Sound travels spherically. A shell moves faster than sound, therefore it is impossible to hear one coming straight for you. The speed of sound is 337.6 metres per second, in still air, at 50 degrees Fahrenheit and humidity 50. A sound metre is the time it takes for sound to travel one metre.

Regarding the timing system of the recorder the tuning fork for middle C has a frequency of 50.64 cycles per second. The timing motor rotor has 15 magnetic poles, therefore the motor will revolve 50.64 divided by 15 = 3.376 times per second. The timing wheel has 10 vanes and revolves with the motor making 33.76. each line reproduced on the film represents 10 sound metres, which brings us back to the speed of sound 鈥� 337.6 metres/sec. Two opposite vanes are thicker than the rest and mark out every 50 and 100 sound metres on the film. The time scales used on the draughtsman鈥檚 board are to the same scale.

The AP hears the sound wave of the hostile shell coming over and sets the Recorder working back at HQ. The air disturbance and pressure of the wave affects the grid in each microphone in turn, depending on direction. The disturbance slightly raises the voltage of the line charge. This affects the conductor wires of the harp in the Recorder, suspended in the electrical field or the galvanometer, producing a kick in each of the 5 grid wires in turn, as the sound wave passes over each Mic.

The optical system magnifies the kick 16 times.

While the film is running the AP gives a serial number and general baring on the direction from which the shell came, and the number of rounds.

The film automatically goes through the developer and fixer and is cut when emerging from the machine.

The film reader looks for likely sequences on the film and given each as found to the film booker who records the values and then abstracts the time intervals calling each out to the draughtsman, in order TA to TE.

The cords on the asymptote board are arranged to correspond with the time values. If the cords give a good intersection then a co-ord spot is made and recorded. A clear-cut intersection is not always possible, then a judgement is made. Subsequent firings of the gun will confirm the 鈥榮pot鈥� and make closer the accuracy. For ease of identification a girls name is given to each 鈥榮pot鈥� leading to such a remark as 鈥淔anny is firing again鈥�.

The records go to Divisional HQ for action.

One thing I have forgotten. The film booker in his working out makes adjustments for prevailing winds, temperature and humidity, for data recorded by our RAF met section for the time.

Each of the two AP鈥檚 handles the guns firing in their particular section of the front. The AP鈥檚 also report general intelligence, anything that moves on the enemy side.

Conversation in plain language over the lines is kept to an absolute minimum, in case of enemy tapping. A code is used, the key being changed every day.

Radio link can also be used for communication and base operation, but there is the risk of enemy listening in and locating sources. Radio link has only one advantage, to bring the base in action quickly, before the lines are laid.

Other items on the course were:

路 Calibration 鈥� correcting our own guns by survey and sound ranging means
路 The use of ciphers and codes
路 General information on guns, firing methods. Types of guns used by the enemy and characteristics of shell sounds.
路 Ranging by our own guns to knock-out an enemy gun position, previously located by the troop. During the shoot the surveyors give the range, direction and elevation, and necessary corrections following plotting on the board. The surveyors take over the firing orders, all this accomplished from the HQ and over the field telephone.

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