The famous chime of Britain’s most famous bell is one of the most unmistakable sounds of our culture. But until now, the physics behind its iconic ring has been a mystery.
A team from the Advanced Structural Dynamics Centre at the University of Leicester have now had a once in a lifetime opportunity to create a sound map of Big Ben. They wanted to measure the distinct vibrations of different regions on the surface of the bell as it rang, which all contribute to a unique sound that no other bell can create.
The technique they used, called ‘laser Doppler vibrometry’, pointed hundreds of lasers at the bell, and measured how much the light beams were disrupted by vibrations at different points. “Many of the vibrations in the metal of Big Ben are too tiny to be seen by the naked eye,” says Martin Cockrill, from the Department of Engineering at the University of Leicester. “But this is what we were able to map using the lasers…we (got) over 500 measurements across the surface, which just wouldn’t have been possible with previous technologies”.
The cutting-edge method ensured that the map could be created without damaging the bell. “You cannot just glue sensors to a national treasure such as Big Ben. Our ability to do the whole thing without touching the bell was key to the whole project”, adds Martin.
But the experiment didn’t come without hardships. Setting up the technology at the top of Big Ben’s home, the Elizabeth Tower (often mistakenly called Big Ben itself), was no mean feat. “One of the most challenging parts of the job was carrying all of our equipment up the 334 seps of the spiral staircase to the belfry. Then to get everything set up before the first chime, we were literally working against the clock”.
In the end, the team managed to make an intricately detailed 3D map of the Big Ben, which clearly shows different frequencies of vibrations, or vibrational modes, at different regions of the bell. The distinct patterns of the vibrations all contribute to producing Big Ben’s iconic peal.
The project is now the subject of a documentary called Sound Waves: The Symphony of Physics, presented by BBC Focus magazine columnist Helen Czerski. You can catch it now on BBC iPlayer.
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