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article imageThe ringing of Big Ben revealed by lasers

By Tim Sandle     Mar 4, 2017 in Technology
London - It's one of London's most famous 'landmarks', although it can normally only be heard rather than seen. Every hour Big Ben chimes telling Londoners the time. It's a special sound and new technology reveals the physics behind the chime.
Before discussing the secrets of the chime it's important to clear up what Big Ben is: it's a bell and not the iconic clock tower, which is sometimes misnamed Big Ben. The clock tower is the Elizabeth Tower (as named since 2012) and it is part of the Palace of Westminster, where the U.K.'s two-tier parliament sits (the House of Commons, which is elected, and the House of Lords, which is appointed). Big Ben is the common name for the bell housed within the tower. The official name for the bell is the slightly less interesting 'Great Bell.' As to why the bell is more often called 'Big Ben', it was probably named after Sir Benjamin Hall, who oversaw the installation of the Great Bell (although there is some dispute between historians as to the origin of the name).
Here's the iconic sound of Big Ben:
When the bell and tower were completed in 1859, it was, according to clockmaker Ian Westworth, “the prince of timekeepers: the biggest, most accurate four-faced striking and chiming clock in the world.” The Great Bell (Big Ben) is not the only bell in the tower. The belfry houses four quarter bells which play the Westminster Quarters on the quarter hours. The four quarter bells sound according to the musical notes G♯, F♯, E, and B.
As Big Ben moves towards its 160th anniversary, scientists have recently used lasers to analyse how the sound of Big Ben's "bongs" is created. In February 2017, two lasers were used to scan the bell as it chimed at 9, 10, 11 and 12 o'clock. The data from the lasers was used for a measurement technique termed "laser Doppler vibrometry". For this a three-dimensional computer model of Big Ben was created and the vibrations in the metal of the bell were mapped out as the bell chimed. With the method, vibration amplitude and frequency are extracted from the Doppler shift of the reflected laser beam frequency due to the motion of the surface. The Doppler effect is the change in wavelength of a wave for an observer moving relative to its source.
Analyzing the data, physicist Martin Cockrill told BBC News that "many of the vibrations in the metal of Big Ben are too tiny to be seen by the naked eye. But this is what we were able to map using the lasers and not just one or two points on the surface; we were able to get over 500 measurements across the surface which just wouldn't have been possible with previous technologies."
As to the unique and characteristic sound, since the Big Ben bell is thicker than other bells of a similar size, and it weighs more, the effect is higher pitch than expected for its diameter, which means the sound Londoners hear each day would be hard to replicate elsewhere in the world.
The full results of the study have been featured in the first episode of a television series Sound Waves: The Symphony of Physics (which is available to view in some regions, via the BBC).
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