'Lives are being lost'


You are on a First World War battlefield. An enemy artillery gun is firing at your lines. Lives are being lost. You need to stop that gun – but how can you stop it if you don’t know where it is? This was a constant problem faced by the British Army in the First World War. A team of scientists solved it by perfecting a technique called sound ranging. This is how it works. 

Six microphones were placed in a curve behind the front line trenches. A soldier near the front line was given a button which – like all six microphones – was connected to a string galvanometer. The button switches it on. Inside the string galvanometer, each microphone is connected to a thin wire that moves when that microphone picks up a sound. As each wire cast a shadow onto a moving piece of photographic film, a record is made of the sound received by each microphone. When the gun fires, it produces a flash. When he sees the flash, the soldier with the button switches the string galvanometer on, starting the film moving. 

Because light travels faster than sound, he has time to switch it on before the sound reaches the microphones. The sound wave produced by the gun moves at a constant speed across the battlefield. Because the microphones are all at slightly different distances from the gun, they pick up the sound at slightly different times. When each microphone picks up the sound, the corresponding wire moves inside the string galvanometer and its shadow leaves a “blip” on the photographic film. Once the film has been developed, you can measure the time delay between each pair of “blips”. But how will you use this information to find the gun. 

In front of you there is a map mounted on a board showing the locations of the microphones. Between each pair of microphones is a pin. At the other end of the board, are five time delay scales, one for each pair of microphones. All you have to do is connect the pin between each pair of microphones with the appropriate point along their time delay scale with a piece of string. The enemy gun is located where your five pieces of string cross. 

The main problem with using sound to find guns is that each firing produces several different sounds. The gun makee a noise when it fires, the shell makes a noise when it breaks the sound barrier in the air, and the shell makes a noise when it explodes on the ground. In the early years of the First World War, they found it impossible to isolate the low frequency sound of the gun firing, as it was swamped by the other higher frequency sounds. Unexpectedly, a clue to solving this problem was found by William Lawrence Bragg, a British scientist, while he was sitting on a toilet! He noticed that every time a gun fired nearby he was lifted slightly off the seat. This was caused by the pressure wave from the firing gun entering the pipe connected to the toilet. Another British scientist William Sansome Tucker had a similar experience while trying to sleep in his tent in the winter. He realised that the cold puffs of air that were making him shiver were caused by the pressure waves from nearby guns. Tucker designed a microphone that could detect the cooling effects of the puffs of air on a heated piece of wire. This new low frequency microphone worked brilliantly. The sound of a gun firing produced a large “blip” on the photographic film, while the other sounds barely registered at all. 

Sound ranging played an important part in British operations from 1917 onwards including at Passchendaele and Cambrai. It was used to disable as many enemy guns as possible before the infantry arrived, giving the troops the best possible chance of success. When they captured German gun positions, less than 5% of them were unknown to the British. An artillery sound ranging film provides a poignant image of the end of the First World War. You can see the guns firing just one minute before the armistice took effect at 11am on 11 November 1918. You can also see the peace which followed the armistice as the guns fell silent after four years of brutal fighting.

The minutes before and after the Armistice that ended the First World War at 11am on 11 November 1918 were recorded in an image showing artillery activity on the American front near the River Moselle.

It's a fascinating record of one moment at the close of the First World War but how did the technology behind this image actually work?

A re-imagining of what the end of the First World War sounded like, based on the sound ranging tape featured in this video, was part of the Making a New World season at IWM London.


E Company Liverpool Scottish Regiment after vaccination, c. 1914.
First World War

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A group of happy girls in an American automobile in London on the day the Armistice was signed, 11 November 1918.
First World War

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