The mystery of ‘Bong’ sound of Big Ben

How Big Ben, one of the largest bells in the UK, produces harmonious sound is a mystery right from the date it was commissioned. However, now we have the laser analysis to arrive at a definite conclusion. When the massive 200 Kg hammer strikes the walls of the bell, a chain of vibration modes set in. The vibration of the metal caused pulsation on the entire structure of the bell. This is what is referred to as the ‘bong’ sound.

Five large bells are hanging in the belfry of Elizabeth Tower at the Palace of Westminster. And the Big Ben is the largest and also known as ‘the Great Bell’.

The thickness of the walls of the Big Ben is more than the other four bells and naturally weighs more. The high pitch is a result of this weight and thickness, a study by the University of Leicester revealed. The human ears hear the chime as one big sound. In fact, this single sound is a combination of many sounds of distinct frequencies.

The researchers from the University of Leicester used two lasers to scan Big Ben as it chimed at 9, 10, 11 and 12 O’ clock. The technical name of the process is Laser Doppler Vibrometry, which created a three-dimensional computer model of the Big Ben Bell. Two lasers were deployed to analyse the vibration module of the walls of the bell.

Strangely after each repair, the Big Ben’s bongs could sound differently, as some significant and structural modification takes place at each repair and refurbishing. But alas, the residents and the visitors cannot hear the Big Ben Bong for many months as the bell and Elisabeth tower are going for a three-year repair cycle. The repair cost is mind blogging at £29 million.

The experts feel that the tower clock is in a ‘bad state’ that it may fail if work is not carried out urgently. In fact, the bell and the tower need urgent repair.

The University of Leicester experts says that there could be changes in the sound wave propagation pattern after the repair of this 13.5-ton bell. That would severely alter the sound and length of the bong. There is a crack on the wall of the bell which needs to be fixed by metal stitching or by in situ moulding/casting. The accumulated soot and dust need be removed. The supporting structures and cantilevers are in bad shape now. These need to be repaired and strengthened. The refitting of the frame is tricky enough and can affect the length of travel of the bongs. The broken bearings need to be fixed or replaced.

Big Ben is not tuned so far. Maybe at a later date, the tuning will take place making the sound closer than today. The crack developed at the Whitechapel Bell Foundry produced to its unusual dissonant sound. It is more than one and half century that no one could hear the original sound.

The sound of Big Ben is unique and made up some different frequencies.  Of course, they are working against time!

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