Reduction of turbofan noise

1. Turbofan type aircraft engine comprising a fan case provided with an acoustic attenuation shroud having regular alternation of first portions and second portions provided with different first and second thicknesses along a radial direction, the number of alternations being greater than twice the number of passing azimuth acoustic vibration modes occurring in an engine without the acoustic attenuation shroud and operating under conditions corresponding to a landing phase and a take-off phase, the acoustic attenuation shroud located in the engine, upstream from a fan of the engine.

2. Engine according to claim 1 , in which the acoustic attenuation shroud is fixed to an engine case surrounding an air intake duct immediately upstream from the fan.

3. Engine according to claim 2 , in which the first portions form grooves on an outer face of the acoustic attenuation shroud, the grooves being oriented parallel to an axis of revolution of the acoustic attenuation shroud.

4. Engine according to claim 2 , wherein at least one of the first portion of the attenuation shroud is fixed to the engine case.

5. Engine according to claim 1 , wherein the acoustic attenuation shroud is made from a composite material with a honeycomb structure in which cells of the honeycomb structure extend in the radial direction and are partially open to form resonators.