Acoustic shells for outdoor concerts are a specific subclass of passive acoustic systems aimed to amplify and equally distribute sound over an audience area in outdoor conditions. The objective of this paper is to describe an automated design methodology that takes advantages of Computational Morphogenesis to generatively inform an efficient acoustic shape. Numerical predictions combine geometrical acoustic principles, descriptive geometry and Evolutionary Algorithms to explore a multivariate topology and achieve convergence to a series of viable and performing solutions. The case of Resonant String Shells – ReS – is used to show a sample workflow where acoustical, computational and construction problems are addressed under a unique, comprehensive and holistic investigation. The formulation of the problem is general and adaptable to different cases and objectives. The design targets a highly portable structure that results from the aggregation of very simple geometrical elements to support small orchestras over an open-air audience area of up to 400 people. Three acoustic objectives, measured on the audience area, are considered to deliver the project – the maximisation of the sound pressure level, minimisation of the deviation between sound pressure levels and minimization of the sound pressure level difference between different sources. Two technological objectives are pursued in the form of hard constraints of the generative process: the planarity of every reflective panel and adjacency between their edges to realize a watertight enclosed shell. The final, definitive solution is subject to detailed acoustic measurements.