Sculpting the sound of aircraft: a novel MDO approach for noise annoyance alleviation

The paper presents a novel approach to include community-noise considerations in a MDO (Multidisciplinary Design Optimization) formulation for the conceptual design of commercial transportation aircraft. The basic idea deals with the introduction of the sound quality concept into the design of aircraft, as an alternative to the classical sound-level-based approach, currently used for the environmental certification of airplanes. In the present work, the noise produced by the configuration under analysis is compared to a target sound, and the design concept is marked as a good one if the matching between the two sounds is satisfactory. This research is part of the European project SEFA (Sound Engineering For Aircraft), whose aim is to provide indications to the designers on how to reduce the annoyance of aircraft noise by improving the quality of the acoustic emissions. The contribution of the authors to SEFA deals with the development of a methodology for including this concepts into an MDO framework. It is evident that such an approach requires an optimization algorithm capable to modify the shape of the noise spectrum, rather than only reduce its level, thus requiring the use of simulation modules dedicated to each noise source. In this respect, the use of the word sculpting in the title of this work is twofold: on one hand, it effectively describes the practical difficulty of shaping the noise spectrum in the whole audible range; at the same time, it evocates, in a metaphorical sense, the aesthetic implication in the struggle for making beauty emerge from a rough block of rock. In order to define a robust method to quantify the matching between the current sound and the target, the Sound Similarity Index (SSI) is introduced and defined as the Lp-norm of the difference of the two sounds spectra. Preliminary results show that the SSI represents a good measure of the distance between the two sounds, and that is capable to effectively drive the optimization process towards those configurations satisfying sound quality requirements.