SEFA was the first project using sound engineering practices to define optimum aircraft noise shapes (target sounds). It is a unique and innovative approach for exterior aircraft noise control as it considers human factors as well as technical factors.
The project objectives are:
- To establish scientifically how noise annoyance caused by aircraft can be reduced, not just by lowering noise levels but also by improving the characteristics of aircraft noise signatures.
- To develop optimum aircraft noise shapes based on a sound quality approach, together with simulation tools capable of adjusting to future technology developments and improvements in the understanding of noise impact.
- To use these findings and tools in setting up sound engineering design criteria for optimum exploitation of novel noise reduction technologies, innovative aircraft and engine architectures, and extended operational capabilities.
Aiming at the development of sound engineering design criteria for future aircraft, the project went through successive phases that can be summarily described as specification (Work Package 1), understanding (Work Package 2), optimisation (Work Packages 3-5) and definition of design criteria (Work Package 6).
Work Package 2 played a major role in performing the required listening tests. The test results provided baseline information for the design of an optimised target sound and the formulation of the related design criteria for aircraft.
On top of the Work Package 2 tests, Work Package 5 analysed the notion of annoyance and its links to aircraft noise and is developing a tool simulating the subjective perception of residents regarding aircraft noise.
Work Package 3 analysed current aircraft sound shapes and generating optimised target sounds. These target sounds are validated again by psychometric tests (Work Package 2).
Work Package 4 analysed the aircraft sound shapes with respect to individual source characteristics and aimed at developing a tool, which provides audible sound tracks for virtual aircraft configurations. This tool will provide a feedback loop from the optimised target sounds to the aircraft sources and flight procedures. It was applied by Work Package 6 in the final definition of design criteria for aircraft.
Achievements and resultsof SEFA project:
- Validated methods for sound quality listening tests
- Validated aircraft flyover sound synthesizing methods for target sound design
- Validated tools estimating sound quality effects (VLIS) and the corresponding aircraft configuration
- A number of sound modifications have been identified which have an effect on sound quality additional to the sound level.
The project involved the development and deployment of innovative simulation tools:
- The virtual aircraft tool, which provides realistic, audible sounds for overflying virtual aircraft configurations, so generating a feedback link from target sounds to aircraft configurations.
- The virtual listener tool, which provides the average subjective evaluation of the sound from an overflying aircraft with respect to a preference scaling and is therefore simulating the very cost-intensive tests described above.
Finally, SEFA has provided valuable information on how noise annoyance caused by aircraft can be reduced, not only by lowering noise levels, but also by improving the characteristics of aircraft noise signatures.
The sound design metrics were derived by the subjective assessment of overflying aircraft noise events within extensive psychometric listening tests in eigth different laboratories (seven countries). For this purpose, the Semantic Differential Test and the Paired Comparison Test were chosen and the following was organised:
- General questionnaire (personal data, such as age, gender, housing, occupation)
- Noise Sensitivity questionnaire NoiSeQ (individual noise sensitivity of each subject)
- Mood questionnaire (current mood of each subject)
- Audiometric pre-screening (hearing ability of each subject)
- Translation of the questionnaires into 7 languages
- Integration of one common software tool for a standardised procedure in 8 different laboratories
- Definition of common hardware for testing.
The Paired Comparison Test and Semantic Differential Test methods have been shown to be acceptable to describe the human perception of current aircraft sounds. The laboratory standardisation with common software and instructions was very successful. Cultural effects - age, gender, noise sensitivity and the status of being an airport resident - were shown to have no relevance to the judgment of aircraft sounds.
From these extensive tests, the following lessons have been learned:
- The differentiation and scaling of aircraft sounds are very difficult for a typical listener. One of the reasons is that full overflying events are continuously changing typically over a period of about 40 seconds.
- The importance as a disturbing feature of any particular sound characteristics (e.g. fan tones) is largely dependent on the entire sound composition of an overflying event, i.e., on a number of other tonal and broadband components.
- Characterising target sound generally has been shown to have more dimensions than anticipated at the beginning of SEFA.