Sound Concentration caused by Curved Surfaces
In 2004 the author was involved, as the acoustical consultant of Peutz, in the renovation of the Tonhalle concert hall in Düsseldorf, Germany.
This concert hall had basically a spherical (cupola) shape with a minimal diffusing structure and suffered a severe echo (see par.14.3). The first solution concept proposed included a high degree of diffusion on this cupola. However, scale model research showed that this would not sufficiently remove the echo. This conclusion was at that time rather surprising and disappointing. In the search for an explanation and better alternatives it became clear that there is hardly any information in room acoustics literature on the topic of sound concentration. After many tests, including scale model diffusing tests, the solution was found to be redirecting the reflected sound. This solution was realized in the real hall, which opened after renovation in 2005 with a concert work for percussion. The echo was not heard and the problem was solved.
Despite the fact that the problem was solved for this particular case, the lack of information concerning sound focusing was still there. So in 2006 the work on this dissertation began and it became clear why the Tonhalle echo problem could not be solved with diffusion. Hopefully the results described in this work will enhance understanding of focusing in room acoustics and will help to solve this kinds of problems, preferably in the design stage.