Consideration of aircraft noise annoyance during conceptual aircraft design

This dissertation focuses on the ‘annoyance’ aspect of aircraft noise and how it could be minimized during early aircraft design. The annoyance in this case corresponds to the acoustic annoyance, which is directly related to the quality of an aircraft sound. A new approach is followed rather than the current common practice of assessing aircraft noise solely via A-weighted level (dBA) or Effective Perceived Noise Level (EPNL) values during aircraft design optimization. Use is made of the advanced current knowledge on psychoacoustics to optimize aircraft designs for optimal sound quality. To consider aircraft noise impact during conceptual aircraft design, a detailed parametric aircraft noise simulation and assessment software INSTANT was developed during the course of this dissertation’s research. To determine the annoyance aspect, INSTANT was appended with a capability to assess aircraft noise in the psychoacoustics based sound quality metrics of loudness, tonality and sharpness. These metrics capture several characteristics of aircraft noise such as the intensity of overall noise and individual noise components, ratio of high frequency to low frequency content and the perceived unmasked prominence of tonal components. Design sensitivity studies were performed for a representative short-range commercial aircraft by integrating INSTANT into the aircraft design environment MICADO. It was observed that a larger wing can increase the loudness of aircraft noise and at the same time reduce the tonality, by reducing tonal prominence and increasing masking effects. It was also seen that a larger engine can be significantly quieter than a smaller engine, although with a much higher tonality. To explore the optimization possibilities, optimized designs for the EPNL, loudness and tonality metrics were produced. It was found that the minimal EPNL and minimal loudness designs captured the loudness aspect of aircraft noise, via a larger engine and a smaller wing, but didn’t sufficiently capture the tonal content. The minimal tonality design minimized the tonality very effectively by reducing the engine size and increasing the wing size. A conversion to synthesized audio of the simulated aircraft noise of the optimized aircraft designs showed that reductions in tonal intensity could be perceived audibly. The fundamental fan tone for the minimal tonality design was observed to have a reduced intensity in excess of 10 dB in the spectrograms, created at a sample ground location during a standard approach procedure. The results indicate that it is possible to modify the sounds of current aircraft towards less annoying or more acceptable sounds, by knowing beforehand which design parameter can change the aircraft sound quality in which way and form