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.
It worked using a system of microphones and a device called a string galvanometer.
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.
Inside the string galvanometer, each microphone was connected to a thin wire that moved when that microphone picked up a sound.
As each wire cast a shadow onto a moving piece of photographic film, a record was made of the sound received by each microphone.
When an enemy gun fired, it produced a flash. When he saw the flash, the soldier with the button would press it, switching the string galvanometer on and starting the film moving.
Because light travels faster than sound, he had time to switch it on before the sound reached the microphones.
The sound wave produced by the gun moved at a constant speed across the battlefield. Because the microphones were all at slightly different distances from the gun, they picked up the sound at slightly different times.
When each microphone picked up the sound, the corresponding wire moved inside the string galvanometer and its shadow left a “blip” on the photographic film.
Once the film had been developed, it was possible to measure the time delay between each pair of “blips”.
Now they had information – but still needed to find the gun. To do this, sound rangers would use a map mounted on a board which showed the locations of the microphones. Between each pair of microphones would be a pin.
At the other end of the board, there were five time delay scales, one for each pair of microphones.
All they had 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 would be located where the five pieces of string crossed.
The main problem with using sound to find guns is that each firing produced several different sounds. The gun made a noise when it was fired, the shell made a noise when it broke the sound barrier in the air and again when it exploded.
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 experienced 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 effect of these 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 advanced, giving the troops the best possible chance of success.
An artillery sound ranging film – seen at the top of this story – 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.
Visit Making a New World at IWM London to experience a re-imagining of what the end of the First World War sounded like, based on the sound ranging tape featured in this story.