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Cabin noise Reduction by Experimental and numerical Design Optimization

Final Report Summary - CREDO (Cabin noise Reduction by Experimental and numerical Design Optimization)

Motivated by the aircraft industry's acute need to validate and calibrate prediction models and advanced design tools for the cost-effective design of low-noise cabins, the CREDO project addressed a critical deficiency in the available data by developing technologically viable experimental procedures and analytical tools by which the sound power entering an aircraft cabin can be determined sufficiently quickly, accurately and with the necessary spatial resolution. Owing to the reverberant nature of the sound field in an aircraft cabin, existing methods are categorically insufficient for this task and entirely new methods shall be developed.

Two parallel approaches were pursued. In the first, the sound power entering the cabin was locally extracted from local measurements of the total field. This approach employed a hitherto unavailable microphone array concept: the double layer array, together with purpose-developed processing and procedural algorithms. In the second approach, the sound power entering the cabin was determined globally through numerical inversion of measurements throughout the entire cabin using the new experimental tools. This was to be achieved with pioneering inverse finite element implementations and groundbreaking inverse simplified energy methods, developed in close connection with novel measurement technology and algorithms, extended from the local to the global level.

The project was structured into the following work packages (WPs):
WP 1 - Specification of industrial requirements and definition of benchmarks related to design and calibration
WP 2 - Local measurement and processing procedures
WP 3 - Global measurement and processing procedures (in the low-medium frequency range and in the medium-high frequency range)
WP 4 - Integration and application to aircraft cabins
WP 5 - Integration and application to helicopter cabins
WP 6 - Synthesis of results.

The two techniques are operational and given their advantages and limitation, the project proposed a combination of the two in order to get the information needed for a global reengineering of an aircraft cabin or cockpit. Given the size of a Falcon's cabin and the high frequency assumptions of the MES model this procedure is reliable in the octave bands 0,5, 1, 2 and 4 kHz (SIL4 domain). For the same reasons in a Falcon's cockpit this procedure is reliable in the octave band 1, 2 and 4 kHz.

Similar results have never been obtained with such resolution and furthermore they have never been applied to flight tests before. The techniques allowed identifying the entering intensity into the cabin independently from the reverberant nature of the environment. Nevertheless further developments are needed for the optimisation of cabin internal surfaces absorption in order to design optimal materials able to absorb the entered noise.