FIELD OF THE INVENTION
- BACKGROUND OF THE INVENTION
The present invention relates generally to noise reduction apparati and methods and, more particularly, to an apparatus and method for aircraft cabin noise attenuation via Non-Obstructive Particle Damping.
Currently, viscoelastic, or rubber-like, materials are frequently used in the construction of floor panels and other structural elements in aircraft for both structural vibration and noise energy attenuation, as well as in other vehicles or structures where so desired. Normally, when used, these materials come in the form of an adhesive, such as a tape, that is adhered to the surface of the floor panel (or other structural element); this adhesive then acts as a structural vibration and noise energy absorbing medium. Thus, structural vibration and noise energy is absorbed via the flexure (i.e., bending) of the viscoelastic materials; this dissipates the mechanical (vibration) energy by converting it into heat.
Aircraft manufacturers have recently come to utilize honeycomb structures, i.e., structural elements comprising a substantially hollow interior portion formed by a web of hollow cells or cavities (a full description of honeycomb structural elements is presented below). Due to their substantially hollow interior, these honeycomb structural elements are low in both weight and mass, parameters of great importance in the design and manufacture of aircraft. However, honeycomb structural elements are also very stiff. Thus, the degree of any flexure of these honeycomb structural elements is small as compared with solid, but heavier, structural elements, such as the floor panels with attenuating adhesive, as described above. Therefore, the viscoelastic materials described above are not very effective structural vibration and noise energy attenuators when used with regards to honeycomb structural elements.
Moreover, the effectiveness of structural vibration and noise energy attenuation by viscoelastic materials is highly dependent on both the frequency of the vibration and the ambient temperature. For example, attenuation by viscoelastic materials does not work well at low frequencies. Additionally, viscoelastic materials not only lose their effectiveness in both low and high temperature environments, but also degrade over time, even in ambient conditions.
- SUMMARY OF THE INVENTION
Thus, there exists a need to develop an adequate structural vibration and noise energy attenuation apparatus for honeycomb structural elements that overcomes the above-stated disadvantages.
Generally speaking, Non-Obstructive Particle Damping (“NOPD”) is a form of damping in which particles of various materials collide with both one another and with the structure in which the particles are located, exchanging momentum and converting vibration energy to heat via friction between the particles. Thus, energy dissipation occurs due to both frictional losses (i.e., when the particles either rub against each other or against the structure) and inelastic particle-to-particle collisions. In contrast to the viscoelastic materials, which dissipate the stored elastic energy, NOPD focuses on energy dissipation by a combination of collision, friction and shear damping. NOPD further involves energy absorption and dissipation through momentum exchange between both the moving particles and vibrating walls, as well as friction, impact restitution and shear deformation.
One advantageous aspect of NOPD is that a high level of damping may be is achieved by actually absorbing the energy of the structure, as opposed to the more traditional methods of damping wherein elastic strain energy stored in the structure is converted to heat. Thus, with a proper choice of particle size, including density and material, NOPD provides a very durable and reliable technique of structural vibration and noise energy attenuation that is essentially independent of temperature.
Studies have been conducted relating to the general effectiveness of NOPD in attenuating undesirable structural vibrations and noise energy. As an example, references is made to “Response of Impact Dampens with Granular Materials under Random Excitation” by A. Papalou and S. F. Masri (“Papalou”), the contents of which are hereby incorporated by reference herein in its entirety, which studied the behavior of particles in a horizontally vibrating, single-degree-of-freedom (i.e., one-dimensional motion) system under random excitation. In particular, the Papalou study focused on the influence of mass ratio, particle size, container box dimensions, excitation levels and direction of excitation on various NOPD methods. Design criteria were provided for optimal efficiency based upon reduction in system response.
As a further example, “Structural Damping Enhancement Via Non-Obstructive Particle Damping Technique,” by Panossian (“Panossian”), studied NOPD in the modal analysis of structures with a frequency range of 30 Hz to 5,000 Hz. The method described in Panossian, the contents of which are also hereby incorporated by reference herein in its entirety, consisted of making small cavities at appropriate locations in a structure and partially filling an optimized configuration of these cavities with particles of different materials and sizes. Significant decrease in structural vibrations was observed.
To further the strides achieved by the above studies, as well as to develop a novel and more effective noise reduction apparatus, the present invention discloses an apparatus, and a method for constructing and utilizing such an apparatus by a unique application of NOPD. The apparatus comprises a structure portion and filler material. The structure portion includes an internal member defining at least one cavity. Each of the at least one cavities of the internal member of the structure portion is filled with the filler material of a shape, size and density appropriate to achieve the desired damping.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the objects, advantages, features, properties and relationships of the present invention will be obtained from the following detailed description and accompanying drawings, which set forth an illustrative embodiment and which are indicative of the various ways in which the principles of the present invention may be employed.
For a better understanding of the invention, reference may be had to one embodiment, as shown in the following drawings, in which:
FIG. 1 illustrates a perspective view of a section of an apparatus for reducing noise, made in accordance with one advantageous embodiment of the present invention;
FIG. 1A illustrates a perspective view of one cavity of the section of the apparatus for reducing noise, as illustrated in FIG. 1;
FIGS. 2A-2D illustrate various types and sizes of filler material deposited within one cavity of the section of the apparatus for reducing noise, as illustrated in FIG. 1.
FIG. 3 illustrates various levels of flexural activity, at various frequencies, of the apparatus for reducing noise, as illustrated in FIG. 1;
FIG. 4 illustrates an amplitude-of-acceleration v. frequency graph comparing an unfilled structure and various filled noise reduction structures made in accordance with one advantageous embodiment of the present invention; and
DETAILED DESCRIPTION OF THE PRESENTLY-PREFERRED EMBODIMENTS
FIG. 5 illustrates a model test of noise reduction apparatus made according to one advantageous embodiment of the present invention.
The materials an aircraft manufacturer may use during construction are subject to various design parameters. For example, in addition to desiring that the various structural materials used in the aircraft possess a high degree of strength, such materials also need a low degree of flexibility and a relatively low mass. To successfully optimize these design parameters, manufacturers have come to utilize structural elements commonly referred to as “honeycombs” or “honeycomb structures.” For purposes of the present invention described herein, “honeycomb structures” are structural units (i.e., walls, floors, ceilings, etc) of an aircraft comprising a substantially hollow middle portion, generally formed of hexagonal (or other similar shape) rows of hollow cells or cavities, resembling the structure of a honeycomb in a beehive.
One advantage of honeycomb structures is that they can be made of types of materials both preferred and desired by aircraft manufacturers, while providing a sturdy and lightweight alternative to solid structures. However, due to the fact that honeycomb structures comprise a substantially hollow middle portion, they have a tendency to allow the passage of structural vibration and noise energy.
Flexural wave velocities, such as those induced by a turbulent boundary layer located proximate to a structure, such as an aircraft cabin, are generally faster than the speed of sound. Unfortunately, honeycomb structures are efficient radiators of these flexural wave velocities at various frequencies, including the low end of the spectrum, i.e., those frequencies detectable by the human ear. However, it has been found that Non-Obstructive Particle Damping (“NOPD”) can help reduce this radiated noise at most of these frequencies. Furthermore, NOPD methods can make insulation preferences much easier to achieve and the noise internal to the cabin much more bearable to passengers. For example, some of the insulation preferences, such as providing noise amplitudes below 65 dB inside the cabin, are based on the highest noise level that is safe for the human ear to be exposed to for a period of time.
It should be noted that, although the discussion herein is focused on the application of the present invention in relation to various structures within the interior of an aircraft, it is nevertheless contemplated that the teachings of the present invention be applicable to other structures wherein a need exists for the attenuation of either structural vibration and/or noise energy. Further, while the teachings of the invention disclosed herein are focused on the attenuation of both structural vibration and noise energy caused primarily by flexural wave velocities, it is also contemplated that the present invention be equally applicable to such disturbances caused by other means.
Referring now to FIG. 1, there is illustrated a perspective view of noise reduction apparatus 10 for attenuating both structural vibration and noise energy in a variety of situations, such as, for example, an aircraft cabin. Noise reduction apparatus 10 is illustrated as comprising structure portion 12. Structure portion 12 preferably forms a wall, floor panel or other similar element having front member 14, interior member 16 and rear member 18. Preferably, interior member 16 comprises a honeycomb structure, as described above. That is, interior member 16 comprises honeycomb network 20. As illustrated in FIG. 1, honeycomb network 20 is formed by rows of cavities 22. Due to the presence of cavities 22, honeycomb structure 20 acts to significantly reduce the total weight of structure portion 12 from that of a solid panel.
As mentioned above, weight, flexibility and strength are important factors to be considered in the manufacture of adequate aircraft materials. Thus, it is preferable that front member 14 and rear member 18 be comprised of carbon composite, plastic or any other type of light and relatively strong material, while interior member 16 be comprised of extremely light and thin sheets of a substantially paper-like and heat- and flame-retardant material, such as that manufactured by DuPont under the trade name “Nomex™.” Nomex™ is a material uniquely designed for this specific purpose, i.e., it is a strong, lightweight carbon composite designed of substantially paper-like and heat- and flame-retardant material.
Although illustrated in FIG. 1 as having a hexagonal shape, each cavity 22 may be any shape wherein walls 24 extend between and support front member 14 and rear member 18. For example, cavity 22 may comprise a generally circular shape. An example of a hexagonal embodiment of cavity 22 is illustrated in the inset, FIG. 1A, of FIG. 1. Additionally, while not shown in FIG. 1, it is contemplated that a channel may extend between proximate cavities 22, allowing for the transfer of filler material 26 between cavities 22.
To reduce both structural vibration and noise energy in noise reduction apparatus 10, filler material 26 (shown in the inset, FIG. 1A, of FIG. 1) is deposited within cavity 22 defined within honeycomb network 20. Preferably, filler material 26 may comprise separate particles which may be metallic and/or non-metallic, or a mixture thereof. For example, metallic particles may be iron, steel, lead, zinc, magnesium, copper, aluminum, tungsten or nickel. Non-metallic particles may be ceramic—such as zirconium oxide, carbon, and silicon nitride or other silicon-based hollow materials preferably in the form of micro-balloons—or viscoelastic or rubber-like. Alternatively, metals in the form of liquids, such as mercury, may be used. A liquid damping material may be preferred for very low frequencies, while small and light solid particles are preferred for relatively high frequencies.
Preferably, the particles used for filler material 26 should not be larger than about half the diameter of cavity 22. In practice, the dimensional sizes of the particles should be such that a multiple of them should be able to fit within each cavity 22. Although the shape of each individual particle of filler material 26 may vary, it is preferred that the particles be generally spherical hollow micro-balloons.
FIGS. 2A-2D illustrate various types of filler material 26, as deposited within each cavity 22. As illustrated by FIG. 2A, filler material 26 may comprise generally spherical particles. Additionally, these particles may also comprise hollow micro-balloons, as described above. Examples of these materials are Perlite™ micro-balloons. In FIG. 2B, generally cubic, or crystalline, particles are illustrated as comprising filler material 26. In FIG. 2C, slightly imperfect spherical particles, or generally elliptical particles are shown. Finally, in FIG. 2D, filler material 26 is illustrated as comprising irregular-shaped particles. Examples of these particles are sand.
The density of each individual particle of filler material 26 is preferably related to the mass of each individual particle of filler material 26. Because it is not desired to have a substantial mass in cavity 22, lighter density particles, such as, for example, aluminum or aluminum oxide powder, may be used. An additional factor that must be considered is viscosity. The more viscous filler material 26, the lower the frequency which can be damped. Conversely, the less viscous filler material 26, the higher the frequencies which can be abated. Thus, the viscosity of filler material 26 should be selected depending upon the frequency range to be attenuated.
In a preferred form, the mass of all filler material 26 which is to be deposited within cavities 22 of honeycomb network 20 is less than the mass of unfilled structure portion 12. It is also preferred to only partially fill each cavity 22 with filler material 26, such as, for example, filling cavity 22 until cavity 22 is 50% to 90% full. The reason for a partial fill of each cavity 22 with filler material 26 is to reduce the effective noise level as much as the system requirements allow, while not compromising any weight restrictions or preferences.
Initially, the parts of structure portion 12 are separate parts. Preferably, prior to the deposition of filler material 26 in cavity 22 of honeycomb network 20 of interior member 16, rear member 18 is affixed to rear side 28 of interior member 16. Rear member 18 may be affixed to rear side 28 through a variety of means and/or methods, such as, for example by applying a thin coat of strong adhesive on the internal surface of rear member 18 and attaching, or gluing, rear member 18 to rear side 28 of interior member 16. After filling each cavity 22 of honeycomb network 20 with filler material 26, to a desired level, front member 14 is affixed to front side 30 of interior member 16 (i.e., the side of interior member 16 opposite rear side 28) in, preferably, the same manner as rear member 18 is affixed to rear side 28 of interior member 16.
In an effort to arrive at the most optimal solution to overcome the disadvantages of the prior art, it became necessary to study the damping effectiveness of various types of filler material 26 deposited within cavities 22 of honeycomb network 20. These tests were carried out to determine the effectiveness of NOPD on various honeycomb panels, and to develop a prediction and design tool that can be used for future NOPD application on structures. To this end, a test and analysis program was initiated. In this program, Finite Element Model (“FEM”) analyses were carried out to predict the modal characteristics of honeycomb network 20 and to correlate the FEM analyses results with laboratory modal test results. These tests were then repeated, utilizing various types of filler material 26 and various configurations of honeycomb network 20. During testing, front member 14 was removed, various types of filler material 26 were deposited in cavities 22 of honeycomb network 20, and front member 14 was re-affixed. The assembled noise reduction apparatus 10 was then suspended with rubber bungee cords and structurally excited by electromechanical shakers. The acceleration and velocity response amplitudes were measured using a multitude of small accelerometers placed on the suspended apparatus, and damping values were predicted using the measured data. The data was then compared with the same testing procedure using no filler material 26, as well as the procedure using various types of filler material 26. A Statistical Energy Analysis (“SEA”) was then carried out to predict an acoustic attenuation profile in the frequency range of interest.
In one test, nine forms of apparatus 10 (each having structure portion 12 of approximately 2 ft.×2 ft.×0.5 in.) were tested for modal characterization using various filler material 26. The FEM analyses and tests indicated numerous vibration modes, the first starting at around 63 Hz frequency. Vibration modes illustrate the level of flexural activity of a vibrating apparatus 10 at each individual frequency. FIG. 3 illustrates the various levels of flexural activity in the vibrating apparatus 10 at various frequencies. As is FIG. 3 illustrates, the vibration modes begin at around 63 Hz.
After performing the FEM analysis, the results were correlated with the test data to anchor the model such that the model predictions match the test data more closely. That is, to predict the performance of any fill configuration of filler material 26. This FEM analysis was then used for the prediction of the modal characteristics of any configuration and material content of the apparatus, where each cavity 22 was considered as an individual solid element.
The FEM analysis predictions initially showed a first bending mode at 115 Hz frequency with the uncorrelated model. As illustrated in FIG. 3, and after correlation with test data, the FEM analysis predictions were re-evaluated for the first flexural mode at 63 Hz, as well as at numerous higher frequency modes. The FEM analysis was then slightly modified again to correlate better with the test results and to reflect the mass and damping effects of the various types of filler material 26 on the structure portion 12 and correlated with test data. This correlated FEM was then used for design purposes of future NOPD treatments of structures.
In the second test, modal and vibration tests were carried out to characterize the modal parameters of the nine different structure portion panels. One panel was left unfilled and used as a baseline. The remaining panels were filled with filler material 26 containing various particles and tested under identical suspension and vibration conditions for comparison.
Thus, for example, one of the panels was filled with 3M Scotchlite (i.e., 3M Light), having a mass of 0.12 g/cc, and another panel with “3M Heavy,” having a mass of 0.63 g/cc. 3M Light and 3M Heavy particles are generally spherical hollow micro-balloons, such as is illustrated in FIG. 2A. The weights of structure portion 12 were measured to calculate the particle mass of filler material 26 added in each test. The weight of the empty panel was 2187.7 g. The panel filled with 3M Light was 2358.2 g, while the panel filled with the 3M Heavy was 2546.9 g. Further, another panel, filled with Aluminum Oxide micro-balloons, was 3901.4 g. Thus, the added weight for the 3M Light was only 42.5 g /sq.ft., for the 3M Heavy it was 89 g/sq. ft., and for the Aluminum Oxide it was 428 g/sq. ft. These added weights represent 7.7%, 16% and 78%, respectively, of the total noise reduction apparatus mass.
For purposes of the present invention, “micro-balloons” are, relative to the size of cavity 22, small particles of filler material 26. Preferably, micro-balloons, as used in the present invention, are air-filled. Due to their high volume and low weight, micro balloons may be utilized as a preferred filler material for the present invention. Preferably, these micro-balloons have a range of dimensions varying between 300-600 microns in size.
The overlays of the frequency response functions for the empty panel (i.e., the baseline panel), the panel filled with 3M Light and the panel filled with 3M Heavy particles are illustrated in FIG. 4
. FIG. 4
illustrates the amplitude, as frequency increases, recorded in panels filled with 3M Light and 3M Heavy, as well as a comparison with an empty (i.e., baseline) panel. As illustrated, there are quite a few modes in the range of 50 Hz to 3200 Hz. The lowest mode was measured at around 63 Hz. Damping for this mode was not increased significantly by either of the two lighter particles. However, both the 3M Light and 3M Heavy particles did enhance damping appreciably as frequency increased. Specifically, as the frequency increased from approximately 1000 Hz, both the 3M Light and 3M Heavy panels show a very distinct level of damping. As FIG. 3
shows, both 3M Light and 3M Heavy reduced the amplitude of the vibration to around 40 g/lb. A summary of the damping estimates, as well as the response amplitudes of the structure, are given in Table 1. More specifically, Table 1 illustrates the percentage of damping present, at various frequencies, in 3M Light, 3M Heavy, a third material—Aluminum Oxide, and a baseline panel;
|TABLE 1 |
|Percentages Of Damping Present At Various Frequencies For Various |
|Filler Material (And A Baseline (i.e., Empty) Panel). |
| ||Material ||Frequency ||Percentage Damping |
| || |
| ||Baseline Panel ||152.450 ||0.246 |
| || ||364.805 ||0.321 |
| || ||764.809 ||0.667 |
| || ||779.545 ||0.705 |
| || ||1178.391 ||0.833 |
| || ||1324.300 ||0.842 |
| ||3M Light ||147.363 ||0.387 |
| || ||350.140 ||0.752 |
| || ||727.039 ||1.327 |
| || ||755.470 ||1.440 |
| || ||1053.580 ||1.755 |
| || ||1128.004 ||2.474 |
| ||3M Heavy ||141.863 ||0.498 |
| || ||340.353 ||0.941 |
| || ||694.430 ||1.819 |
| || ||717.478 ||2.557 |
| || ||1031.280 ||3.223 |
| || ||1103.100 ||4.821 |
| ||Aluminum Oxide ||93.600 ||3.500 |
| || ||300.000 ||4.500 |
| || ||530.000 ||3.600 |
| || ||743.000 ||6.000 |
| || |
Referring to FIG. 5
, the modal tests were conducted by suspending each noise reduction apparatus 10
with bungee cords 32
from four points. Four points of contact are illustrated in FIG. 5
; however any number of bungee cords may be used to suspend each noise reduction apparatus 10
, provided bungee cords 32
are very flexible and do not effect the responses of the structure significantly. Accelerometer 34
was placed on noise reduction apparatus 10
and laser vibro-meter was used to measure the velocity profile. Both hammer and shaker inputs (not shown) were used to excite noise reduction apparatus 10
under random and sine dwell excitations with various amplitudes, to study the nonlinear effects. The measurements were then used to identify the mode shapes and frequencies and to calculate the damping ratios. Fifteen specific modes were selected and sine dwell excitations were used for modal characterization. This data helped the correlation with the FEM analyses and the derivation of the mode shapes. The damping ratios in Table 2 show an average increase of 100% for the 3M Light test, over 200% for the 3M Heavy test and over 500% average increase for the Aluminum Oxide test. The relative amplitudes illustrated in Table 2 indicate more amplitude reductions. More specifically, Table 2 illustrates the recorded values of amplitude, at various frequencies, for 3M Light, 3M Heavy, Aluminum Oxide and a baseline panel. As can be seen by Table 2, amplitude for 3M Light Decreased to 5.3 g/lb, while that of 3M Heavy decreased to 2.1 g/lb at an approximate frequency range of 700-750 Hz (as compared with an amplitude of 11.2 g/lb for the baseline panel). At this frequency, Aluminum Oxide's amplitude was reduced to 1 g/lb.
|TABLE 2 |
|Amplitude Values Present At Various Frequencies For Various |
|Filler Material (And A Baseline (i.e., Empty) Panel). |
| ||Material ||Frequency ||Amplitude (g/lb) |
| || |
| ||Baseline Panel ||152.450 ||16.15 |
| || ||364.805 ||20.57 |
| || ||764.809 ||10.89 |
| || ||779.545 ||11.20 |
| ||3M Light ||147.363 ||11.05 |
| || ||350.140 ||7.95 |
| || ||727.039 ||5.46 |
| || ||755.470 ||5.30 |
| ||3M Heavy ||141.863 ||8.65 |
| || ||340.353 ||7.33 |
| || ||694.430 ||2.72 |
| || ||717.478 ||2.10 |
| ||Aluminum Oxide ||93.600 ||0.17 |
| || ||300.000 ||1.45 |
| || ||530.000 ||1.40 |
| || ||743.000 ||1.00 |
| || |
Based on the above, it becomes apparent that filling cavities in a honeycomb structure with micro-balloons provides significant damping of vibration and resulting noise. More specifically, the lightest type of filler material, 3M Light, provides greater than 50% vibration attenuation in low frequency modes, as compared with the application of no filler material. In general, heavier particles provide for a greater degree of damping for very low frequency modes. However, as is always the case when considered in relation to aircraft cabins, the selection of a type of particle may be subject to weight constraints. Lighter, but more flexible particles, such as foam particles, could also provide significant damping when used in a noise reduction apparatus. Moreover, the above-mentioned prediction FEM code is necessary to be able to select the appropriate particles and fill configuration, and even predict the expected responses under excitation. The fundamental procedure for the selection and fill configuration of particles for a noise reduction apparatus entails the use analyses and prediction tools as describes previously. Thus, an optimum configuration and particle selection is possible via the approach described above.
While specific embodiments of the present invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, it will be understood that the particular arrangements and procedures disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any equivalents thereof.