US Patent 10502134 Reduction of turbofan noise

The invention relates to an aircraft turbofan type engine (1) comprising an air intake duct (2) having an inner face provided with an acoustic attenuation shroud (11) located upstream from a fan (3) of this engine (1). This acoustic attenuation shroud (11) comprises a regular alternation of first portions and second portions with different first and second thicknesses along the radial direction, the number of alternations being greater than twice the number of passing azimuth acoustic vibration modes occurring in the engine (1) without the acoustic attenuation shroud (11) operating under conditions corresponding to a landing phase and a take-off phase.