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The engineers Michael Plitzner (front) and Denis Spieß ensure with clever technology for the preservation of church bells. (Photo: Ralf Baumgarten)

Die Ingenieure Michael Plitzner (vorne) und Denis Spieß sorgen mit pfiffiger Technik für den Erhalt von Kirchenglocken. (Foto: Ralf Baumgarten)

The engineers Michael Plitzner (front) and Denis Spieß ensure with clever technology for the preservation of church bells. (Photo: Ralf Baumgarten)

After a huge "Wums" the St. Peter's bell in Cologne Cathedral remained silent. What happened? An engineering team went in search of clues, made a musical fingerprint and was able to repair the bell. bild the science photographer Ralf Baumgarten accompanied the bell doctors to a check-up in Cologne Cathedral.

On 5 January 2011, there was a tremendous noise in the south tower of the Cologne Cathedral. Then the "Fat Pitter", the largest church bell in the world, fell silent. He had lost his 850 kg and three meters long clapper. Fortunately, the steel bars in the tower caught the clappers - nobody was hurt. But why did the metal rod fall off after almost 90 years of ringing? The engineers from the European Competence Center for Bells (ECC-ProBell) at Kempten University of Applied Sciences determined - and now every five years - the St. Peter's bell, as it is officially called. picture of the science writer Nikolaus Fecht reported in January on the work of the bell specialists . The photographer Ralf Baumgarten was at a check-up and photographed the bell-doctors at work.

Musical fingerprint

In order to examine bell and clapper, the CEO of ECC-ProBell Michael Plitzner and his team first record a musical fingerprint of the bell. To do this, they attach a small sensor to their surface, exactly where the clapper beats on the bellcage. The sensor detects the magnitude of the vibrations and reveals how much the bell is deforming. Another sensor on the bobbin also registers power and acceleration on impact. A third sensor measures the impact angle of the clapper. Because it depends on how much energy the bell absorbs. There is an optimal serve angle for each bell. If he is too small, the clapper does not strike. If it is too big, the bell sounds too loud and is heavily loaded.

Search for the cause of the bobbin case

Together, the sensor data gives a musical fingerprint that the bell doctors can evaluate in the lab. On the basis of sound and vibration behavior, for example, they can detect invisible cracks to the naked eye. "With increasing depth and width of the cracks, the different partials of the bell drift apart more and more, " explains Michael Plitzner. This will split the audio frequency. In the case of the thickness Pitter, the laboratory analysis provided a typical cause of the clapper - bell and clapper were not perfectly matched. The 850 kilogram clapper was also too heavy and too fast, so that the impact intensity was very high on the bell.

Healing for the fat pitter

In order to keep the thickness of pitter as long as possible and prevent it from cracking, the bell doctors recommended a new clapper to the people of Cologne. This optimally shaped and with 640 kilograms much lighter clapper now hangs exactly in the middle, so that he touches both sides of the bell evenly. And the control measurement in 2016 shows: The work was worthwhile, the bell is now charged less. display

© science.de - Xenia El Mourabit
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