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Focusing on the issues of sound propagation in a free field condition and on the concept of uniform sound energy in an outdoor performance environment, our research aimed to develop a computer-aided process for the generation of reflective acoustic surfaces to be used as concert-shells, a computational design tool for acoustic form-finding. The project is ultimately aimed to investigate the acoustic potential of complex and doublycurved surfaces through the analysis of the Total Relative Sound Level / Strength parameter (G), with reference to the proposed values set by M.Barron, based upon the source-receiver distance and the subsequent subjective judgments on loudness. A simplified and fast ray-tracing acoustic simulation algorithm was developed in combination with parametrically controlled shape variations of the reflective surfaces. Sound energy uniformity evaluation function considering the direct and reflected sound components was written in order to define and evaluate the rate of distribution uniformity of sonic energy over an audience. This evaluation function was used in a genetic algorithm that enabled us to explore a wide set of surface morphologies which allowed us to isolate the fittest one to our specific uniformity requirements. At the end of the genetic search, an acoustic simulation plug-in called Pachyderm was employed with both NURBS and mesh-based acoustic simulations in order to validate the genetically selected surfaces with specific reference to G values. A further step of resultant data visualization and human selection was necessary to compare the output data and to evaluate the final surfaces from an architectural perspective. 32 Sound-Strength Driven Parametric Design of an Acoustic Shell in a Free Field Environment

We present a novel computational fabrication method for the production of Functionally Graded Structures (FGS) via robotic control of Viscous Thread Instability (VTI). Of interest in several fields and at different scales of application, the fabrication of FGS is often relying on offline fabrication workflows and on stable material conditions. By introducing partial control in the process of spatial deposition of an extruded clay thread in a state of instability, our method extends the design and fabrication possibilities of VTI to the production of FGS. Traditionally exploited for the industrial production of not-graded two-dimensional nonwoven textiles or for surface treatments in design-related 3d printing applications, we frame VTI as the main design and fabrication driver for the computational fabrication of functionally graded clay volumetric structures. Without relying on predictive physical simulation models, our method relies on feedback information provided by sensing equipment in combination with an industrial 6 axis robotic manipulator integrated with a numerically controlled clay extruder. The sensed information is used to retroactively update the inputs of a computational model programmed to guide the robotic additive fabrication of user-defined functional volumetric gradients. We illustrate the main design- and fabrication-related parameters and a set of material experiments designed to validate the accuracy of our model. We present a set of fabricated outputs to illustrate the flexibility of the model to accommodate a variety of design intentions and, finally, we discuss its potential for further research involving cross-scalar and trans-disciplinary applications.