Rigid sandwich panel acoustic treatment

Details Rigid sandwich panel acoustic treatment Rigid sandwich panel acoustic treatment

1999-01-12

2001-02-06

Dang, Khanh (Department: 2837)

Acoustics

Sound-modifying means

Sound absorbing panels

C181S021000

Reexamination Certificate

active

06182787

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to materials and structures for noise suppression. More particularly, this invention relates to an acoustic treatment panel for suppressing radiated noise in an aircraft engine nacelle, with the panel being configured as a sandwich panel that exhibits acoustic properties similar to that of bulk absorber material.
BACKGROUND OF THE INVENTION
Gas turbine engines operate over a broad range of speeds and thrusts, and as a result generate a broad range of noise frequencies. Acoustic treatments in the form of acoustic liners that line the fan and exhaust ducts of gas turbine engines are widely used to suppress aircraft engine noise beyond those levels that can be achieved by the particular design of the turbo machinery. In view of stringent noise abatement requirements around the world, considerable effort has been directed to designing acoustic liners that are capable of absorbing noise over a broad range of frequencies, while also being durable, relatively low-weight, readily fabricated, and having minimal impact on engine performance.
There are two primary sources of aircraft-generated noise. One source is the viscous shearing that takes place between the rapid exhaust gases and the relatively quiescent surrounding air, while the second source is the rotating blades of the fan, compressor and turbines, and the resulting air flow past the vanes and other stationary objects within the engine air flow path. Acoustic treatments for suppressing noise produced by the latter source can generally be categorized as bulk absorbers or resonator-type absorbers.
A bulk absorber
10
is represented in FIG.
1
. With this type of treatment, a porous material
12
, such as a fibrous or rigid foam material, fills a cavity between two sheets
14
and
16
. The sheet
14
is formed of an air-permeable material that forms the walls of a nacelle flow duct of a gas turbine engine, e.g., the fan inlet and fan exhaust ducts and the turbine exhaust duct. The sheet
14
and the bulk absorber
10
absorb sound waves that impact these walls as the waves propagate through the duct. Examples of suitable materials for the sheet
14
include sheet fabricated from sintered or felted metal, or other porous materials having suitable flow resistances. The back sheet
16
is typically rigid and air-impermeable.
Acoustic treatments referred to as resonator-type absorbers include Helmholtz resonator chambers or compartments. A double-layer resonator absorber
20
of this type is represented in
FIG. 2
as having a compartmented airspace core with an air-permeable facesheet
22
and an air-impermeable back sheet
24
, between which there are a number of compartments or cells
26
. The facesheet
22
typically has perforations
30
within which sound absorption occurs. In the double layer resonator
20
shown in
FIG. 2
, a porous septum
32
is present between and parallel to the facesheet
22
and back sheet
24
. Conventional methods by which the resonator
20
is manufactured typically entail individually forming the resonator layers separated by the septum
32
, and then bonding the layers and the septum
32
together. As a result, misalignment often occurs between the cells
28
of these layers. In a single-layer resonator (not shown), the porous septum
32
is omitted.
As a rule, the cells
26
of resonator-type absorbers have been defined by hard, air-impermeable walls
28
, which are often configured so that the cells
26
have a hexagonal-shaped cross-section that yields a honeycomb cell pattern. Passages between resonator cells
26
have been proposed, as shown in U.S. Pat. Nos. 3,972,383 and 4,189,027. However, the former resonator absorber relies on air being forced through the cells
26
from an exterior source in order to tune the facesheet
22
, while the latter absorber requires adjacent cells
26
to be asymmetric, which causes air pumping between cells
26
when air flows over the perforations
30
in the facesheet
22
.
There are known advantages and shortcomings with each of the acoustic treatments described above. The double-layer resonator-type absorber
20
represented in
FIG. 2
provides good noise attenuation over a relatively wide band of frequencies centered about a particular frequency to which the cells
26
are tuned, based in part on their depth. To achieve a broadband capability, a resonator-type absorber must have a variety of cavity sizes to cover the frequency range of concern, or must be capable of mechanically changing the sizes of the cells. Both of the approaches are mechanically complex and contribute undesirable weight to the engine.
In contrast, bulk absorbers of the type shown in
FIG. 1
offer higher suppression performance than either single-layer or double-layer resonator-type treatments by their ability to absorb noise over a wider frequency range. In spite of this performance advantage, bulk absorbers are not widely used in aircraft engines due to disadvantages inherent in she material properties. Specifically, the conventional concern is that fibrous materials will disintegrate with aging and the high dynamic vibration levels within gas turbine engines, and may wick liquids that could create a fire hazard. Another drawback of bulk absorbers is their poor serviceability.
In view of the above, it can be seen that it would be desirable if an acoustic treatment were available for gas turbine engines, by which a broad band of noise suppression was possible along with structural integrity compatible with air flow conditions of the gas turbine engine environment.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, there is provided an acoustic treatment for the air ducts of a gas turbine engine. The acoustic treatment generally includes a facesheet having a plurality of holes therein, a backplate spaced apart from the facesheet, and a plurality of interconnected cells between the facesheet and backplate. Each of the cells is defined by walls attached to the facesheet and the backplate, and at least some of the walls are formed of a porous material that provides flow resistance therethrough and allows acoustic propagation in a direction parallel to the facesheet and backplate.
A significant advantage of the above construction is that the acoustic treatment of this invention is able to exhibit the suppression performance advantages of bulk absorbers, yet has the structural advantages of a resonator-type absorber. Specifically, the porous walls of the cells allow acoustic waves to travel in a direction parallel to the facesheet, which provides the acoustic treatment with the noise suppression properties of a bulk absorber. On the other hand, the rigid facesheet and backplate provide a sandwich structure that is resistant to the hostile thermal, chemical and mechanical environment of a gas turbine engine. The porous material of the cell walls is also able to contribute to the structural integrity of the treatment without unduly restricting airflow between adjacent cells. With this construction, cell size and cell wall porosity can both be controlled in order to achieve the desired acoustic and structural properties for a particular acoustical environment.
Other objects and advantages of this invention will be better appreciated from the following detailed description.


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