Acoustic panel for a nacelle of an aircraft propulsion unit, and related manufacturing methods

1. An acoustic panel with resonators for a nacelle of an aircraft propulsion unit, the acoustic panel comprising a plurality of adjoining acoustic cells which form a cellular core, each acoustic cell being delimited by a peripheral wall, and each acoustic cell extending along a longitudinal main axis corresponding to an axis of propagation of sound waves, from a front end up to a rear end of the acoustic cell,

wherein at least one acoustic cell among the plurality of acoustic cells includes a first obstacle and a second obstacle extending transversely with respect to the longitudinal main axis of the at least one acoustic cell, from the peripheral wall of the at least one acoustic cell, the obstacles having a free end edge which delimits a passage with a portion of an opposite wall to increase a length of a path traveled by the sound waves through the at least one acoustic cell,

wherein the first obstacle and the second obstacle are shifted along the longitudinal main axis, wherein a passage defined by a free end edge of the first obstacle and a passage defined by a free end edge of the second obstacle is radially shifted so as to form a baffle to increase the length of the path traveled by the sound waves through the at least one acoustic cell, and

wherein at least one of the first and second obstacles comprises a central portion which extends transversely with respect to the longitudinal main axis of the at least one acoustic cell and two lateral portions, each lateral portion extending in an oblique direction with respect to the longitudinal main axis of the at least one acoustic cell, each oblique direction being between 30° and 60° with respect to the longitudinal main axis.

2. The acoustic panel according to claim 1 , wherein a portion of the free end edge of the first obstacle and a portion of the free end edge of the second obstacle are superimposed along the longitudinal main axis of the acoustic cell to inhibit the sound waves from drawing a direct path from the front end up to the rear end of the at least one acoustic cell.

3. The acoustic panel according to claim 1 , wherein the at least one acoustic cell includes at least one acoustically permeable septum which extends across the at least one acoustic cell.

4. The acoustic panel according to claim 3 , wherein the at least one acoustically permeable septum extends from the free end edge of one of the first and second obstacles, up to the peripheral wall of the at least one acoustic cell.

5. The acoustic panel according to claim 3 , wherein the at least one acoustically permeable septum extends from the free end edge of the first obstacle up to the free end edge of the second obstacle.

6. The acoustic panel according to claim 1 , wherein the peripheral wall of the at least one acoustic cell comprises six faces forming a hexagonal cell, at least one of the first and second obstacles being fastened to at least two adjacent faces of the at least one acoustic cell.

7. The acoustic panel according to claim 1 , wherein the peripheral wall of the at least one acoustic cell comprises six faces forming a hexagonal cell, at least one of the first and second obstacles being fastened to at least two non-adjacent faces of the at least one acoustic cell.

8. A nacelle for an aircraft propulsion unit comprising at least one acoustic panel according to claim 1 .

9. A method for manufacturing an acoustic panel according to claim 1 , the method comprising:

fastening at least one ribbon to a first plate and a second plate to the first plate by respective nodal portions; and

forming non-nodal portions of the first plate and the second plate, the non-nodal portions being shaped such that each non-nodal portion of the first plate forms, with a respective non-nodal portion of the second plate, the peripheral wall of each acoustic cell delimiting a corresponding acoustic cell of the cellular core, such that a portion of the at least one ribbon forms one of the first or second obstacle in the at least one acoustic cell.

10. The method according to claim 9 , wherein, during the fastening step, the at least one ribbon extends throughout the nodal portions of the first plate and the second plate.

11. The method according to claim 9 , wherein a strip is fastened to one end of the at least one ribbon during the fastening step, and during the forming step, the non-nodal portion of the at least one ribbon forming the at least one of the first and second obstacles is shaped by pulling the strip.

12. The method according to claim 9 , wherein the peripheral wall of the at least one acoustic cell comprises six faces forming a hexagonal cell, the at least one of the first and second obstacles being fastened to three adjacent faces of the at least one acoustic cell, the three adjacent faces being portions of the first plate or the second plate.

13. A method for manufacturing an acoustic panel according to claim 1 , the acoustic panel further comprising a first plate and a second plate fastened together by nodal portions, wherein non-nodal portions of the first plate and the second plate form walls delimiting cells of the cellular core, the method comprising fastening the first obstacle to a respective non-nodal portion of the first and second plates.

14. The method according to claim 13 , wherein a first fastening leg of the first obstacle is fastened to a non-nodal portion of the first plate, the first fastening leg extending in a direction of a first end of the at least one acoustic cell, and wherein a second fastening leg of the first obstacle is fastened to a non-nodal portion of the second plate, the second fastening leg extending in a direction of a second end of the at least one acoustic cell.

15. The method according to claim 13 , wherein a first fastening leg of the first obstacle is fastened to a non-nodal portion of the first plate, the first fastening leg extending in a direction of one end of the at least one acoustic cell, and wherein a second fastening leg of the first obstacle is fastened to a non-nodal portion of the second plate, the second fastening leg extending in a direction of the same end of the at least one acoustic cell.

16. An acoustic panel with resonators for a nacelle of an aircraft propulsion unit, the acoustic panel comprising a plurality of adjoining acoustic cells which form a cellular core, each acoustic cell being enclosed by a peripheral wall, and each acoustic cell extending along a longitudinal main axis corresponding to an axis of propagation of sound waves, from a front end up to a rear end of the acoustic cell,

wherein at least one acoustic cell among the plurality of acoustic cells includes a first obstacle extending from a first side of the peripheral wall and a second obstacle extending from an opposing second side of the peripheral wall, the first and second obstacles extending transversely with respect to the longitudinal main axis of the at least one acoustic cell, the first and second obstacles each having a respective free end edge which delimits a passage with a portion of the peripheral wall opposing the respective free end edge to increase a length of a path traveled by the sound waves through the at least one acoustic cell,

wherein the first obstacle and the second obstacle are spaced from each other along the longitudinal main axis, wherein the passage defined by the free end edge of the first obstacle and the passage defined by the free end edge of the second obstacle are radially shifted away from each other so as to form a baffle to increase the length of the path traveled by the sound waves through the at least one acoustic cell,

wherein at least one of the first and second obstacles comprises a central portion which extends transversely with respect to the longitudinal main axis of the at least one acoustic cell and two lateral portions, each lateral portion extending in an oblique direction with respect to the longitudinal main axis of the at least one acoustic cell, each oblique direction being between 30° and 60° with respect to the longitudinal main axis.