Introducing sound quality as a design objective in conceptual aircraft design

As aircraft noise becomes an increasingly relevant matter of public health, affecting negatively the quality of life of citizens living near airports, engineers are challenged to develop aircraft concepts that are not only sustainable in terms of energy consumption and gaseous emissions, but also more silent. In this context, a primary approach is to investigate potential retrofit measures that can contribute to reduce the noise of conventional aircraft designs. This can be achieved by considering noise as a key performance indicator during the design process. Recent research activities indicate that conventional metrics based on A-weighting sound pressure level might provide insufficient information to adequately quantify human annoyance. Moreover, the sufficient suitability of the Effective Perceived Noise Level (EPNL), a metric developed in the early 1960’s for this purpose and nowadays mainly considered for aircraft noise certification purposes according to ICAO Annex 16, is still under discussion. The literature suggests that psychoacoustic metrics, which are deeply based on the human perception of sound, may provide a better objective approximation of human annoyance to aircraft noise than conventional metrics. Nevertheless, the design of low-annoyance aircraft using psychoacoustic-based metrics, opposed to the concept of low-noise aircraft optimized by conventional metrics, poses a challenging paradigm change which legitimacy needs to be proven by listening tests. The present study will provide a first insight into the implications of this approach and aims to obtain the necessary knowledge for the aircraft designs used for this purpose. Working towards the development of a framework capable to consider human annoyance during the aircraft design stage, this work focuses on a parametric study of the wing geometry of a medium-range aircraft with design specifications similar to an Airbus A320. The main goal is to identify possible retrofit modifications that can contribute to mitigate not only the sound immission levels, but also short-term annoyance. For this purpose, we employ the modified psychoacoustic annoyance model as a sound performance indicator. Moreover, the assessment shall not be limited to a simplified and non-physical parameter variation but it should capture all the inherent effects of the design modifications on the aircraft’s flight performance and the associated noise generation. To achieve this goal, an existing design process is extended regarding its noise assessment capabilities, allowing the computation of objective psychoacoustic metrics. It is capable to realistically synthesize aircraft designs and predict their associated flight performance and noise emissions along individually tailored flight trajectories. The noise immission is evaluated at dedicated observer positions by means of conventional metrics and the psychoacoustic annoyance model, which is based on the sound quality metrics loudness, sharpness and tonality. Finally the results of the parametric study are discussed in order to get insights about whether the psychoacoustic annoyance model reveals different key performance parameters and design limits than the ones obtained by the conventional metrics