The HEXENOR proposal aimed to study, design and manufacture three items ( short, mean, and long) each made of two innovative concepts, a quiet plug and a quiet diffuser which will be mounted on a turboshaft engine to reduce the noise radiated by the exhaust. A preliminary study was concerned with the capability of two materials a Ferritic stainless steel (low cost material) and TiAl alloy (light but fragile material) to satisfy the design constraints and the engine hard conditions. High cycle fatigue, thermo-mechanical and metallurgical characterisation tests were conducted before the design step. Also an academic work was achieved to determine an optimal configuration of a new quiet diffuser concept made of angular and axial cavities surrounding an internal perforated wall of the duct lined section.

This approach was based on the measurement with a flow duct facility developed during the Ducat European program without and with cold flow of the liner configurations efficiency for a higher order mode incident pressure vector and the development of an analytical model of the acoustic propagation through the lined barrel leading to the calculation of the attenuation of each configuration with and without flow. The choice of the optimal configuration to design the quiet diffuser was the result of a trade off between acoustic efficiency provided by the academic study, mass and cost specifications.