Impact sound insulation of floors is one of the main concerns when designing multi-story buildings. While conventional floor systems use flat or regularly ribbed configurations, interesting opportunities to improve performance are given by enhancing the design of the load-bearing slab. In this work, we propose a methodology to maximize broadband impact sound insulation by designing slabs with a non-uniform material thickness distribution, which is numerically optimized. In the design process, efficient and accurate sound insulation predictions are employed through a detailed finite element floor model coupled with a diffuse room model. In particular, we minimize a Single Number Quantity (SNQ) that represents the aggregate broadband sound pressure level in the receiving room under point-load excitation. The design of single slab floors is considered first. Results indicate that, even if the optimization effectively minimizes the sound pressure level in the different third-octave bands, it does not lead to noteworthy decreases in the SNQ. The design of floating floors is then considered. In this case, a better overall performance is obtained, corresponding to significant improvements at low frequencies and a slight degradation at high frequencies, where, nevertheless, floating floors already perform well. This slight high-frequency decline is linked with the resonances of the thin sub-plate regions present between the optimized ribs, akin to what is observed in waffle ribbed floors. Finally, the robustness of the optimized results against changes in the initial guess is assessed by showing that no SNQ improvements are obtained when transitioning from flat to waffle ribbed initial design.
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