Low-frequency acoustic center body

1. A center plug for attenuating noise in a gas turbine engine, comprising:

an inner skin, the inner skin having a substantially cylindrical shape and extending along an axial centerline;

an outer skin positioned radially outside the inner skin;

a forward bulkhead disposed proximate a forward end of the inner skin, the forward bulkhead connected to and extending radially outward from the inner skin;

an aft bulkhead disposed proximate an aft end of the inner skin, the aft bulkhead connected to and extending radially outward from the inner skin; and

an acoustic panel disposed within a volume defined by the inner skin, the outer skin and the forward bulkhead and the aft bulkhead, the acoustic panel including

a first sheet having a first plurality of perforated walls and a first plurality of non-perforated walls together forming a first N-shaped pattern,

a second sheet having a second plurality of perforated walls and a second plurality of non-perforated walls together forming a second N-shaped pattern,

wherein the first sheet is sandwiched together with the second sheet to form a structure, the structure having a combined plurality of perforated walls including one of the first plurality of perforated walls and an adjacent one of the second plurality of perforated walls and having a combined plurality of non-perforated walls including one of the first plurality of non-perforated walls and an adjacent one of the second plurality of non-perforated walls, and

the first plurality of perforated walls extend a first length in a radial direction and the second plurality of perforated walls extend a second length in the radial direction, the second length being greater than the first length.

2. The center plug of claim 1 , wherein the outer skin is connected to and positioned radially outward of the forward bulkhead, the aft bulkhead and the structure.

3. The center plug of claim 2 , wherein the structure defines a resonator cavity having a first sub-cavity, a second sub-cavity and a third sub-cavity and the outer skin includes a perforated surface.

4. The center plug of claim 3 , wherein the one of the first plurality of perforated walls extends in the radial direction between the inner skin and the outer skin separating the first sub-cavity and the second sub-cavity.

5. The center plug of claim 4 , wherein the radial direction is within a first range from minus ten degrees to plus ten degrees of perpendicular to the axial centerline.

6. The center plug of claim 5 , wherein the radial direction is perpendicular to the axial centerline.

7. The center plug of claim 6 , wherein another wall of the second plurality of perforated walls extends in the radial direction between the inner skin and the outer skin separating the second sub-cavity and the third sub-cavity.

8. The center plug of claim 7 , wherein the first sub-cavity, the second sub-cavity and the third sub-cavity define a length, the length being greater than a radial depth of the structure.

9. The center plug of claim 8 , wherein the one of the first plurality of non-perforated walls is oriented at an angle within a second range from forty degrees to eighty degrees with respect to the axial centerline.

10. The center plug of claim 9 , wherein the acoustic panel includes a third sheet having a third plurality of perforated walls and a third plurality of non-perforated walls sandwiched together with the second sheet to form the structure.

11. The center plug of claim 10 , wherein the first sheet defines a first repeating structure including the first N-shaped pattern, the second sheet defines a second repeating structure including the second N-shaped pattern and the third sheet defines a third repeating structure.

12. The center plug of claim 11 , wherein the first repeating structure and the third repeating structure define a first pattern frequency and the second repeating structure defines a second pattern frequency different from the first pattern frequency.