The Tonhalle is Düsseldorf’s most famous concert hall. Originally built in 1926 as the largest planetarium in the world for the exhibition “Gesolei” by architect Wilhelm Kreis. After the destruction of the old Tonhalle in World War II the building was used as a multi-purpose hall and rebuilt in 1977 to a concert hall. The rebuilt geometrical shape, which is close to a hemisphere of approximately 15,000 m3 – 40 m diameter and 24 m height, resulted in grave acoustical problems. The concept for an acoustic solution for the new Tonhalle is based on a visually closed, but “acoustically transparent” inner dome, behind which specifically aligned deflecting surfaces break the hemispherical shape acoustically, subsequently prevent echoing. MCKNHM worked together with the engineering firm Peutz on the implementation of this concept for the architecture firm HPP. We created all digital models and geometric adjustments to the deflection body, which were needed for acoustic measurements in a large scale digitally fabricated physical model, and later for implementation and detailed construction document planning in the realization of this project.