Bell casting: from sound design to the finished bell
The sound of a bell has been optimised and the bell cast on the Garching Campus.
As part of the work on his Masters thesis, Felix Thomann designed a bell sound, which was intended to be very different from a listener's expectations, largely determined by the listener's cultural background. In most cases, church bells in Europe have a minor-third harmonic interval. On the other hand, major-third bells with a brighter character are seldom found and the almost cylindrical Asiatic ones sound exotic in Europe. It was Felix Thomann's aim to create a completely new, unexpected sound from his jazz bell. The desired sound was designed using acoustic command variables and the shape of the bell was fine-tuned in a finite element model using an evolutionary optimisation process.
This interdisciplinary work at the TUM Chair of Metal Forming and Casting (utg,) and Chair of Vibro-Acoustics of Vehicles and Machines (VIB) was supervised by Thomas Greß, Magdalena Scholz und Lennart Moheit. The directors of the two chairs, Prof. Wolfram Volk and Prof. Steffen Marburg got the idea for a possible collaboration at a meeting at Scheyern Abbey. The bell as a resonating form plays an important role in the teaching of VIB while the infrastructure at utg offers an opportunity to cast objects, which have been designed on a computer, from both light and heavy metals using not only sand but also mould-casting methods.
At utg a casting concept was worked out for the virtual model of the bell, which had previously been optimised topologically with regard to sound by VIB. Traditionally bells are produced using a top pouring casting process. A characteristic of this method is fast filling of the mould, which can be an advantage when casting thin-walled objects. However, the top pouring casting process can lead to increased bubble formation, blowholes and jetting. The bell project therefore pursued two different production strategies: top pouring and rising casting. The design of the casting system was carried out using numerical simulation. It was possible to evaluate the system with regard to blowholes, shrinkage cavities, mould filling and solidification, due to simulation of the casting process. The casting moulds were made by the company, voxeljet AG, Friedberg (www.voxeljet.com) using sand bound with furanic resin.
The tin-bronze bell, weighing c. 20 kg, was successfully cast at utg using both top pouring and rising casting methods. A concluding acoustic and structural analysis is intended to provide information on how closely the simulated sound of the bell tallies with the actual sound.
Ringing of the bell with the TUM emblem should take place soon on the Garching Campus.
For TUM press release see:
Metal Forming and Casting (utg) at the Department of Mechanical Engineering:
Thomas Greß, M.Sc.
+49 (0)89 / 289 - 13980
Vibroacoustics of Vehicles and Machines (VIB) at the Department of Mechanical Engineering:
Lennart Moheit, M.Sc.
+49 (0)89 / 289 - 55126
Magdalena Scholz, M.Sc.
+49 (0)89 / 289 - 55133