U.S. patent application number 10/795707 was filed with the patent office on 2005-09-08 for apparatus and method for aircraft cabin noise attenuation via non-obstructive particle damping.
This patent application is currently assigned to The Boeing Company. Invention is credited to Panossian, Hagop.
Application Number | 20050194210 10/795707 |
Document ID | / |
Family ID | 34912509 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050194210 |
Kind Code |
A1 |
Panossian, Hagop |
September 8, 2005 |
APPARATUS AND METHOD FOR AIRCRAFT CABIN NOISE ATTENUATION VIA
NON-OBSTRUCTIVE PARTICLE DAMPING
Abstract
An apparatus for reducing noise in an aircraft cabin is
disclosed. The apparatus comprises a structure portion and filler
material. The structure portion further comprises an internal
member having at least one cavity disposed therein. Each of the at
least one cavity of the structure portion are filled with the
filler material.
Inventors: |
Panossian, Hagop; (Tarzana,
CA) |
Correspondence
Address: |
NEAL, GERBER, & EISENBERG
SUITE 2200
2 NORTH LASALLE STREET
CHICAGO
IL
60602
US
|
Assignee: |
The Boeing Company
Chicago
IL
|
Family ID: |
34912509 |
Appl. No.: |
10/795707 |
Filed: |
March 8, 2004 |
Current U.S.
Class: |
181/293 ;
181/292 |
Current CPC
Class: |
B32B 2250/40 20130101;
B32B 2605/18 20130101; B32B 2307/102 20130101; B32B 3/266 20130101;
G10K 11/172 20130101; B64C 1/40 20130101; B32B 2264/104 20130101;
B32B 3/12 20130101 |
Class at
Publication: |
181/293 ;
181/292 |
International
Class: |
E04B 001/82 |
Claims
What is claimed is:
1. An apparatus for reducing noise, comprising: a structure
portion, the structure portion having an internal member, the
internal member defining at least one cavity; and a filler
material, the filler material being disposed within the at least
one cavity.
2. The apparatus of claim 1, wherein the internal member comprises
at least one sidewall, the at least one sidewall defining the at
least one cavity.
3. The apparatus of claim 1, wherein the internal member defines a
plurality is of cavities.
4. The apparatus of claim 3, wherein the filler material is
disposed within at least one of the plurality of cavities.
5. The apparatus of claim 4, wherein the filler material comprises
a plurality of particles.
6. The apparatus of claim 5, wherein the filler material partially
fills each of the cavities in which it is disposed.
7. The apparatus of claim 1, wherein the filler material comprises
a plurality of particles.
8. The apparatus of claim 7, wherein the filler material partially
fills the at least one cavity.
9. The apparatus of claim 1, wherein the filler material comprises
a silicate.
10. The apparatus of claim 1, wherein the structure portion further
comprises a front member and a rear member.
11. The apparatus of claim 10, wherein the front member defines a
top of the at least one cavity.
12. The apparatus of claim 10, wherein the rear member defines a
bottom of the at least one cavity.
13. The apparatus of claim 1, wherein the rear member is affixed to
a first side of the internal member.
14. The apparatus of claim 13, wherein the front member is affixed
to a second side of the internal member.
15. The apparatus of claim 14, wherein the second side of the
internal member is opposite the first side of the middle
member.
16. The apparatus of claim 15, wherein the front member and the
rear member combine to bound the at least one cavity defined within
the internal member of the structure portion.
17. A method for reducing noise, comprising: providing a structure,
the structure having an internal member; forming at least one
cavity within the internal member; and; depositing a filler
material within the at least one cavity.
18. The method of claim 17, further comprising affixing a front
member and a rear member to the internal member.
19. The method of claim 18, further comprising permitting the
random movement of the filler material within the at least one
cavity.
20. An aircraft noise reduction apparatus, comprising: an aircraft
cabin, the aircraft cabin comprising at least one structure
portion, the structure portion comprising a honeycomb network, the
honeycomb network having a plurality of cavities; filler material
disposed within at least one of the plurality of cavities; and
wherein the filler material is free to move within the at least one
of the plurality of cavities to absorb structural vibration and
noise energy.
21. A noise reducing panel, comprising: a first outside portion; a
second outside portion; a honeycomb portion, the honeycomb portion
being disposed between the first outside portion and the second
outside portion, the honeycomb portion having a plurality of
cavities; a filler material disposed within the plurality of
cavities of the honeycomb portion; and a means for affixing the
first outside portion and the second outside portion to the
honeycomb portion; wherein the filler material is free to move
within at least one of the plurality of cavities to absorb
structural vibration and noise energy.
Description
FIELD OF THE INVENTION
[0001] 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.
BACKGROUND OF THE INVENTION
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
SUMMARY OF THE INVENTION
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For a better understanding of the invention, reference may
be had to one embodiment, as shown in the following drawings, in
which:
[0013] 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;
[0014] FIG. 1A illustrates a perspective view of one cavity of the
section of the apparatus for reducing noise, as illustrated in FIG.
1;
[0015] 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.
[0016] FIG. 3 illustrates various levels of flexural activity, at
various frequencies, of the apparatus for reducing noise, as
illustrated in FIG. 1;
[0017] 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
[0018] FIG. 5 illustrates a model test of noise reduction apparatus
made according to one advantageous embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PRESENTLY-PREFERRED EMBODIMENTS
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.TM.." Nomex.TM. 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.TM. 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] In one test, nine forms of apparatus 10 (each having
structure portion 12 of approximately 2 ft..times.2 ft..times.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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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;
1TABLE 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
[0040] 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.
2TABLE 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
[0041] 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.
[0042] 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.
* * * * *