New acoustic analysis techniques help separate components of aircraft noise
Aircraft noise is an important issue, concerning both environmental pollution and passenger comfort. The noise results from turbulent airflow over the fuselage (the aircraft body) and from external aircraft components. Reducing noise requires advanced study of its various sources. Yet, such study is challenging. One problem is that the physics of aircraft noise is incompletely understood. Researching that alone requires the best current research methods, including computer modelling and physical experimentation. A further difficulty is that teasing out the individual components of aircraft noise is very complicated. Therefore, aircraft engineers have been studying noise sources in isolation, using laboratory testing, since the 1950s. Although acoustic analysis has since advanced to where all sources of aircraft noise can be traced simultaneously, it is not yet possible to conduct wind tunnel testing for a full-size passenger aircraft under cruising conditions. Furthermore, despite advances in computer modelling, it is still impossible to simulate real conditions of a complete aircraft in flight. Therefore, for large aircraft, separating out individual components of noise remains a problem.
Taking the next step
To progress further in the study of aircraft acoustics, it is now necessary to study complex phenomena in flight. Doing that would normally require an array of thousands of microphones externally mounted on the aircraft. Yet, such microphones create their own noise that interferes with aircraft noise. Instead of shielding the microphones, the EU funded ADAPT project used advanced signal processing techniques to separate aircraft and microphone noises. ADAPT was a member of the Clean Sky programme. “The challenge was to develop signal processing methods able to extract acoustic components from the noise radiated in an airflow,” explains project coordinator, Dr Christophe Picard, “based on an array of a limited number of microphones.” The microphones permit isolation of the in-flight noise effect of an engine mounted under a wing.
Successful testing and adoption
The new methods have been developed and validated, via laboratory wind tunnel tests in controlled conditions, on a basic test case. The methods were subsequently applied to acoustic signals measured on more complex test cases. Finally, they were applied to real flight tests. The driving goal of the ADAPT project, beyond developing the acoustic techniques, was to bundle them as software tools for use by Airbus. These tools were successfully completed. They consist of prototype code, accompanied by user manuals about the project’s data. “Airbus can now use the tools to further analyse the project’s test databases, including the flight test data which were difficult to exploit without the tools,” adds Picard. The tools also have other aeronautics and automotive sound analysis applications, for which MicrodB, the lead partner company of the ADAPT project, plans to market the products in those industries. The team will be pursuing such opportunities, looking for early-adopter companies able to collaborate on further development of the technology.
Keywords
ADAPT, aircraft, noise, aircraft noise, Airbus, signal processing, acoustic techniques, software tools, sound analysis
