Micro-perforated acoustic liner
An example liner assembly includes a backing plate that supports a cellular structure covered by a perforated face sheet including a plurality of openings. Each of the plurality of openings is of a size determined to provide desired acoustic performance.
1. A method of fabricating a noise attenuating liner comprising the steps of:
a) supporting a cellular structure on a support backing;
b) covering the cellular structure with a perforated face sheet, where the perforated face sheet includes a plurality of openings for communicating acoustic energy to the cellular structure;
d) determining a range of percent open areas of the perforated face sheet based on selected upper and lower limits of a non-linearity factor and a resistance value;
e) determining an opening size for each of the plurality of openings based on the selected upper and tower limits of the non-linearity factor and the resistance value that is no greater than ¼ th a thickness of the perforated face sheet;
f) generating a design range of percent open areas and hole diameters based on the determined range of percent open area and opening size; and
g) forming the plurality of openings within the perforated sheet that includes openings with an opening size and percent open area within the generated design range.
2. The method as recited in claim 1 , wherein the method includes sizing each of the plurality of openings to provide a desired acoustic resistance.
3. The method as recited in claim 1 , wherein the method includes sizing each of the plurality of openings to provide a desired non-linearity factor.
4. The method as recited in claim 1 , wherein the method includes determining a size of each of the plurality of openings according to a relationship including a selected non-linearity factor, a selected resistance value and the thickness of the perforated sheet.
5. The method as recited in claim 1 , wherein an open area of the perforated face sheet is greater than 5%.
6. The method as recited in claim 1 , wherein the size of the opening comprises a hydraulic diameter within the opening through the perforated face sheet.
7. The method as recited in claim 1 , wherein the percent open area is determined according to the relationship:
POA
=
4.202
(
95
+
85
NLF
R
105
(
NLF
-
1
)
)
0.474
Where: POA is percent open area;
NLF is the selected non-linearity factor; and
R 105 is the selected resistance value in cgs Rayls.
8. The method as recited in claim 1 , wherein the hole diameter is determined according to the relationship:
d
=
9.42
(
19
+
17
NLF
R
105
)
0.263
(
NLF
-
1
)
0.237
μ
t
10
-
NLF
Where d is the diameter of the opening in centimeters;
NLF is the non-linearity factor;
R 105 is the resistance value in cgs Rayls;
t=thickness of the face sheet in centimeters; and μ=the absolute viscosity (1.789×10 −4 gm/cm-sec).
9. The method as recited in claim 1 , wherein the step of generating a design range includes defining outer boundaries for the percent open area and hole diameters based on the upper and lower limits of the non-linearity factor and resistance value.
10. A method of fabricating a perforated face sheet for a noise attenuation liner comprising the steps of:
(a) specifying a desired thickness of the perforated face sheet;
(b) determining a desired percent open area for the perforated face sheet based on a non-linearity factor and a value indicative of resistance to airflow;
(c) determining an opening size based on the non-linearity factor and the value indicative of resistance to airflow;
(d)forming a plurality of holes in the perforated face sheet including the determined opening size and percent open area, wherein a diameter of each of the plurality of openings is not greater than ¼ th a thickness of the perforated face sheet.
11. The method as recited in claim 10 , wherein the diameter of each of the plurality of openings comprises a hydraulic diameter within each opening through the perforated face sheet.
12. The method as recited in claim 10 , wherein the diameter of each of the plurality of holes is no greater than 0.0254 cm (0.0100 inches).
13. The method as recited in claim 10 , wherein the thickness of the perforated sheet is at least 0.051 cm (0.020 inches).
14. A noise attenuation liner assembly comprising
a cellular structure disposed on a support backing;
a perforated metal sheet including a plurality of openings affixed over the cellular structure on a side opposite the support backing, wherein an opening size for each of the plurality of openings and percent open area of the perforated metal sheet is determined based on a desired non-linearity factor and a factor indicative of resistance to flow.
15. The assembly as recited in claim 14 , wherein an upper and lower limit of both the non-linearity factor and the factor resistance to flow are utilized to define a range of desired percent open areas and opening size that provide a desired acoustic performance, and the plurality of openings are formed based on the defined range.
16. The assembly as recited in claim 14 , wherein the opening size defined based on the non-linearity factor and the factor resistant to flow is less then ¼ th the thickness of the perforated metal sheet.
17. The assembly as recited in claim 16 , wherein the opening size comprises a hydraulic diameter.