What makes Big Ben's bong so special? Researchers from the University of Leicester have attempted to find out by creating a vibration map of the famous giant bell. The BBC specially commissioned the study, which for the first time uses laser beams to precisely map how the bell generates sound, as part of a documentary that aired on Thursday.

The University's Advanced Structural Dynamics Evaluation Centre (ASDEC) team was given exclusive access to the tower that holds Big Ben in order to measure the bell chimes at 9:00 am, 10:00 am, 11:00 am, and 12:00 noon. While the bell sounded, the team used laser Doppler vibrometry – a contactless technique that maps vibrations by reflecting lasers off the metal surface – in what ASDEC calls unprecedented level of detail without any loss of accuracy or precision.

"Many of the vibrations in the metal of Big Ben are too tiny to be seen by the naked eye," says Martin Cockrill, a Technical Specialist from the Department of Engineering at the University of Leicester. "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."

The teams say that the laser measurements revealed that because Big Ben is thicker than other bells of a similar size and weighs more, this results in an unexpectedly higher pitch for its diameter and the generation of a number of different vibrations or modes, the frequency and intensity of which are affected by the bell's profile.

Formally called the Great Bell, Big Ben tolls the hours over the roofs of Westminster and in quieter times could be heard as far away as Richmond to the west. It is also heard live around the world as the time signal of the BBC World Service during news broadcast.

The huge bronze bell was completed in 1858 and its distinct tone is due to a crack that appeared the first time it was struck and has never been repaired. There are different stories as to how it got its nickname, though one is that it was named after Sir Benjamin Hall, who kept demanding that Parliament give the bell a proper name until someone suggested in exasperation, "Let's just name it after Ben." It weighs 13.5 tons (13.76 tonnes), stands 7.5 ft (2.29 m) tall, is 9 ft (2.74 m) wide, and is sounded by a 441 lb (200 kg) striker. It is also very difficult to access because the tower has no lift.

"Aside from the technical aspects one of the most challenging parts of the job was carrying all of our equipment up the 334 steps of the spiral staircase to the belfry," says Cockrill. "Then to get everything set up before the first chime, we were literally working against the clock."

The BBC documentary "Sound Waves: The Symphony of Physics" was broadcast on March 2 on BBC4.

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