Abstract:
A capsule contains fluid and a solid inertial mass that is free to move within the capsule. The capsule is embedded in a foam panel. A plurality of such foam panels are attached to the internal wall of a launch vehicle fairing. This device augments acoustic energy dissipation with damping the resonant frequency of the fairing to reduce the amount of energy that is transmitted into the acoustic volume contained within the wall. Incorporating a plurality of capsules respectively tuned to many frequencies provides broadband structural attenuation. This abstract is provided to comply with the rules requiring an abstract, and is intended to allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to mitigating vibro-acoustic energy and, more particularly, mitigating such energy in a launch vehicle payload fairing to reduce the resulting load on the payload and thereby decrease the probability of damage to the payload. 
     Vibration and acoustic energy generated by rocket engines are transmitted through the launch vehicle fairing and create severe vibro-acoustic loads that can damage payloads. The magnitude of vibration transmitted into the payload volume largely depends on the external noise levels, the mass, stiffness, and damping characteristics of the fairing, and the acoustic damping within the fairing volume. Composite fairings currently being developed are much lighter and stiffer than their metal-alloy predecessors. However, composite fairings typically have little inherent damping. The lack of damping results in structural resonances, which are very efficient paths for noise transmission, and are thus deleterious to a fairing payload. 
     Acoustic blankets are often attached to the inside of fairings to absorb acoustic energy, thereby reducing the overall response in the fairing. Acoustic blankets are typically fabricated using low-density fiber material or foam. Current acoustic blankets for launch vehicles are usually less than six inches thick because there are strict constraints on their mass and volume. As a result, such blankets do not provide much sound absorption below 250 Hz. Although acoustic blankets can damp acoustic resonances in the fairing, they do little to alter the structural dynamics of the fairing. 
     The physics of sound transmission through walls and panels, and the effect of sound attenuating blankets, have been well documented in the relevant technical literature. Fairing noise transmission can be considered as the coupling of two dynamic systems, the fairing structure and the acoustic volume enclosed by the structure, which is excited by an external distributed broadband acoustic field. The responses of both dynamic systems exhibit resonant behavior, which is determined by physical properties such as air density, structural stiffness, structural mass, structural damping, acoustic damping, the speed of sound, and geometry. A mathematical model of fluid-structure coupling and a further discussion of the physics is reviewed in L. D. Pope, “On the Transmission of Sound through Finite Closed Shells: Statistical Energy Analysis, Modal Coupling, and Nonresonant Transmission”,  J. of the Acoustical Society of America , Vol. 50, No. 3 (Part 2), pp. 1004-1018 (1971); and in P. Gardonio, N. S. Ferguson and F. J. Fahy, “A Modal Expansion Analysis of Noise Transmission Through Circular Cylindrical Shell Structures with Blocking Masses”  J. of Sound and Vibration , Vol. 244, Number 2, pp. 259-297 (Academic Press, 2001). 
     It has been observed that the vibration loads transmitted to the payload by the interior acoustic response are comparable in level to the vibrations transmitted by mechanical truss mounts. There has been much work to reduce the mechanical path transmission using isolation systems, but little progress has been made to reduce the acoustic path transmission. For the Cassini spacecraft launched using a Titan-IV booster, special acoustic blankets were designed, tested, and implemented to meet acoustic load requirements. This work is documented in W. O. Hughes, A. M. McNelis, and H. Himelblau, “Investigation of Acoustic Fields for the Cassini Spacecraft: Reverberant Versus Launch Environments”, AIAA-99-1985, (American Institute of Aeronautics and Astronautics, 1999). As noted by Hughes et al., there has been little development in aerospace acoustic blankets since the 1970&#39;s. They also found that acoustic blankets are typically effective only above 400 Hz. 
     Acoustic blankets provide absorption (or damping) of pressure waves, which means that acoustic energy is converted to heat by interacting with the blankets. The amount of interaction between the blanket and the sound field is frequency dependent. In order to damp low frequency sound, thicker blankets are required. The amount of damping added to an acoustic resonance is determined by impedance matching between the material and fluid waves, material thickness, surface area coverage, and the ability of the material to dissipate energy, with the latter being ultimately limited by mass and volume constraints. 
     Fairing noise control for launch vehicles is similar to general aircraft noise control, which has been thoroughly researched. In one solution for aircraft, vibration absorbers are attached to the fuselage skin to reduce the structural vibration and sound radiation by the aircraft&#39;s skin. C. R. Fuller and J. P. Maillard, “Control of Aircraft Interior Noise Using Globally Detuned Vibration Absorbers”,  J. of Sound and Vibration , Vol. 203, No. 5, pp. 745-761 (Academic Press Limited, 1997). As noted therein, interior noise response is reduced in two ways: first, the devices can couple to structural resonances and add structural damping, thereby reducing the amplitude of the resonant response (classical tuned mass damper approach); and secondly, by interacting with and breaking up the response of structural modes that radiate acoustic energy, often referred to as “modal reconfiguration”. A similar approach was discussed in the previously referenced paper by Gardonio et al., supra, wherein it is shown that discrete masses added to the fairing shell could reduce coupling between structural modes and the acoustic modes. It has also been shown that passive dynamic absorbers can couple to the fairing structure and significantly reduce interior noise transmission. S. Griffin, S. Lane, C. Hansen, and B. Cazzolato, “Active Structural-Acoustic Control of a Rocket Fairing Using Proof-Mass Actuators,”  Journal of Spacecraft and Rockets , Vol. 38, No. 2, pp. 219-225 (American Institute of Aeronautics and Astronautics, Inc., March-April 2001). 
     There are a number of devices that enhance noise reduction by increasing sound absorption through a panel. For example, U.S. Pat. No. 4,384,634, titled “Sound Absorbing Structures,” uses a perforated face sheet attached to a honeycomb panel to provide acoustic damping, and a viscous elastic layer between the panel and vibrating structure, e.g., a jet engine, to damp structural motion. This treatment, however, is generally too heavy for a fairing and would not provide as much broadband acoustic absorption as a conventional foam blanket. Furthermore, viscous elastic treatments are not effective in coupling with low frequency structural resonances and in providing damping. 
     U.S. Pat. No. 4,560,028, titled “Sound Absorbing Wall Lining,” discloses a fluid-filled honeycomb panel where wave motion in the fluid provides acoustic damping. This invention is intended for waves in fluid mediums, such as underwater. Such a fluid-filled structure would weigh too much for use in launch vehicle fairings. 
     U.S. Pat. No. 4,667,768, titled “Sound Absorbing Panel,” shows a variation of the honeycomb panel. This patent teaches the reactive versus resistive resonator approach, and incorporates panel drains to release liquid build-up, and spheres to fill the honeycomb cell volumes in order to create varying acoustic resonances. This apparatus was designed for jet engine exhaust nacelles, and is not directed to increasing the transmission loss through a structure into an enclosed acoustic volume. 
     U.S. Pat. No. 4,842,097, titled “Sound Absorbing Structure,” also teaches reactive and resistive acoustic resonators bored into sound insulation material. Again, only acoustic damping, without providing any structural damping, is addressed. While the patent claims “substantial sound absorption at frequencies of less than about 1000 Hz”, it is a fact that as the target frequencies become lower, the volume of the resonators&#39; cavities must become larger to reduce the effective air spring. Enlarging the volume of the resonators&#39; cavities in accordance with the teachings of this patent would require geometries impractical for fairing applications intended to attenuate acoustic frequencies below 200 Hz. 
     U.S. Pat. No. 5,024,290, titled “Sound Absorbing Panel for Interior Walls,” uses grooves, funnels and holes in an attempt to augment the acoustic damping provided by the panel. The holes create a reactive and resistive acoustic resonator effect. What is not discussed in these reactive and resistive resonator patents is that as the resonator damping (the resistive part) increases, the coupling of the device to the acoustic volume decreases. Also, such resonators are essentially wasted volume at high frequency because they provide no acoustic absorption. Thus, it is often preferable in practice to maximize the amount of acoustic foam in order to optimize high frequency attenuation instead of implementing resonators. 
     In U.S. Pat. No. 5,824,973, titled “Method of Making Sound Absorbing Laminates and Laminates having Maximized Sound Absorbing Characteristics,” provides an informative discussion on the role of acoustic impedance and impedance mismatching in designing optimal blanket treatments. In addition to discussing the physics and dynamics of multiplayer porous acoustic absorption laminates, it provides plots of normal incidence sound absorption for a variety of configurations. In each case, it is observed that blanket performance is nearly non-existent at frequencies below 200 Hz. Thus, no benefit would be observed in a structural-acoustic system where resonances occur below 200 Hz. As is the case for nearly all acoustic blanket approaches, no consideration has been given to the structural input path. 
     U.S. Pat. No. 5,910,082, titled “Sound Absorbing Building Panel,” teaches a panel for use as a wall or ceiling tile. The innovation lies in the use of “sound absorbing” granules, which are adhered to one another to form matrix. Such an approach may be beneficial for high frequencies, but it will not damp low frequency acoustic resonances. At low frequency, the sound waves will see the matrix layer as a single, lumped panel and propagate across the panel without attenuation. This treatment is thus ineffective for low frequency absorption or for mitigating structural-acoustic transmission in lightly damped structures. 
     U.S. Pat. No. 6,090,478, titled “Sound Absorbing/Shielding and Electric Wave Absorbing Plastic Sheet Containing Encapsulated Magnetic Fluid, and Sound Absorbing/Shielding and Electric Wave Absorbing Plastic Panel,” discloses a lightweight sound absorbing material with insulating effects. The invention comprises a “sound absorbing and insulating” plastic panel, with glass “balloons” or magnetic fluid-filled capsules. The fluid-filled capsules provide electromagnetic shielding at high frequencies (800 MHz). The use of microcapsules of fluid dispersed randomly throughout a plastic composite, however, cannot significantly mitigate low frequency structural-acoustic transmission outside of mass loading benefits, which could be better achieved with a softer and higher density material. Moreover, even of one were to overlook the aforementioned shortcomings, the density of such a panel would result in a weight that would far exceed allowable weight limits for launch vehicles. In launch vehicles, the acoustic treatment must be lightweight and compact. 
     U.S. Pat. No. 6,109,388, titled “Sound Absorbing Mechanisms Using a Porous Material,” presents another panel that incorporates imbedded acoustic resonators. As with other efforts for sound absorbing panels, this invention focuses only on absorbing or damping the acoustic space without addressing structural transmission. 
     There are some devices that seek noise reduction by preventing or reducing the transmission of noise, as opposed to enhancing acoustic absorption. For example, U.S. Pat. No. 5,504,282, titled “Sound Transmission and Absorption Control Media,” teaches a sound transmission barrier for boats and motor vehicles using multiple “high-mass” layers. In launch vehicles, it is desirable to couple to and damp low frequency structural modes with minimal added mass. 
     U.S. Pat. No. 5,907,932, titled “Wall Structure Having Enhanced Sound Transmission Loss,” discloses two walls separated by an isolating barrier. To extend the transmission loss to low frequencies, however, the walls must be de-coupled, which requires that the combined structure be relatively thick. This is often impractical, as in the case of launch vehicle fairings. 
     There are also many apparatus that address both sound and vibration transmission. Typically, the panel absorption is enhanced if the panel is fabricated from a non-homogeneous material, and vibration damping is provided through molecular friction resulting from wave propagation through a viscous elastic layer. U.S. Pat. No. 5,400,296, titled “Acoustic Attenuation and Vibration Damping Materials,” issued to Cushman and Thomas, discloses a material or barrier intended to provide both acoustic absorption and vibration damping. Particles such as glass spheres, steel pellets, brass pellets, or chunks of lead or cork, are arbitrarily or randomly dispersed in a matrix of some type of urethane, silicon rubber, elastomer, polymer, gypsum, or petroleum extract. There is no tailoring or designing of the vibration damping mechanism to the resonance frequencies of a structural application, e.g., an aircraft fuselage or launch vehicle fairing. The only vibration attenuation mechanism taught by the foregoing device is the damping of waves as they propagate through the material, which results from internal molecular friction. This type of approach yields weak coupling and provides little dissipation of the structural resonances of the host structure. 
     The key innovation of the &#39;296 patent is the use of very small (≈100-micron diameter) particles in the elastomeric matrix to interact with wave energy propagating in the material in order to convert the wave energy from one type of wave, e.g., bending, to another type of wave e.g., torsional. It is asserted that mechanical energy is converted to heat through the wave transformations that are expected to occur, thereby reducing the transmission of energy through the material. Propagating energy is diffused. The efficiency of this mechanism is a function of the wavelength of the energy being propagated, the wave speed of the matrix material, the directionality of the incident acoustic energy, and the material density. At low frequency, sound waves would pass through the material with little to no effect unless the thickness of the material was on the order of ¼-wavelength of the incident sound wave. 
     U.S. Pat. No. 5,526,324, titled “Acoustic Absorption and Damping Material with Piezoelectric Energy Dissipation,” issued to Cushman, discloses piezoelectric particles embedded within a matrix material. It is asserted that the particles act as electrical short circuits and are able to convert acoustic and vibration energy into heat through electrical discharge. This approach would be significantly heavier that traditional blankets since it requires electrically conductive or active materials and piezoelectric particles. Its impedance matching to the acoustic space and broadband attenuation would be inferior to that provided by foam. The particles are said to be randomly distributed. This neglects transmission loss at low frequency. U.S. Pat. No. 5,706,249, titled “Panel Spacer with Acoustic and Vibration Damping,” also issued to Cushman, teaches an acoustic and vibration dampening spacer to hold apart and transmit loads between multi-panel walls. Mechanical waves propagating in the panel, whether resulting from acoustic loads or structural vibration, are dissipated by an elastomeric material. The disclosed device is simply an elastomer spacer placed between panels, and is not related to a sound absorbing or sound barrier treatment suitable for fairings. 
     U.S. Pat. No. 5,745,434, titled “Acoustic Absorption and Damping Material with Integral Viscous Damping,” also issued to Cushman, is basically the same concept originally presented in U.S. Pat. No. 5,400,296, but employs a material composed of discarded tire rubber and has channels and passage ways through a matrix to allow for more effective acoustic coupling. Obviously, such a product would be very dense, have poor acoustic coupling ability, and would reduce structural transmission primarily by increasing the mass and thus the impedance load. It would also be subject to the same bandwidth limitations that were noted in the &#39;296 patent. U.S. Pat. No. 5,754,491, titled “Multi-technology Acoustic Energy Barrier and Absorber,” yet another patent issued to Cushman, also uses the concept disclosed in the &#39;296 patent, with the added innovation of using multiple layers, with each layer being separated by a viscous elastic layer intended to act as a constrained-layer damper. 
     U.S. Pat. No. 6,237,302, titled “Low Sound Speed Damping Materials and Methods of Use,” teaches a vibration and acoustic treatment that utilizes granular fill. In this invention, a granular material, e.g., lead shot, sand, or rice, is used to damp structural vibrations. The granular material is placed in “intimate contact” with the structural member. The invention asserts that structurally radiated noise can be reduced by the granular damping treatment. However, the patent describes only means to damp structural vibration; it provides no means to absorb acoustic energy. The process taught therein would significantly increase the mass of the structure. Furthermore, the foregoing approach is not designed to target those frequencies that most contribute to noise transmission. 
     As previously noted, the fairing noise problem is similar to the aircraft cabin noise problem. U.S. Pat. No. 5,904,318, titled “Passive Reduction of Aircraft Fuselage Noise,” presents the concept of using a reinforced skin structure to mitigate interior noise. A combination of air barriers, insulating layers, and dampening rods is incorporated into double or multiple layer panels of a fuselage, which is impractical for a fairing. The effect is to decouple the various panel layers and increase the damping of structural resonances. This patent is noteworthy because it provides a good explanation of the structural-acoustic coupling between the external loads, fuselage skin, and the enclosed acoustic space. It is pointed out that “vibrating skin panels can often act as an efficient loud speaker, radiating noise into the interior panels and into the cabin of the aircraft.” The patent discusses the use of acoustic blankets and their limitations. It also discusses the use of viscous elastic treatments that are applied directly to the vibrating fuselage in an attempt to add damping to structural resonances. At low frequency, however, there is little strain energy transferred to the viscous elastic material, and thus little damping is added to the low frequency dynamic response. 
     Several references directly address the vibro-acoustic response in launch vehicle fairings. U.S. Pat. No. 5,670,758, titled “Acoustic Protection on Payload Fairings of Expendable Launch Vehicles,” and U.S. Pat. No. 6,231,710, titled “Method of Making Composite Chambercore Sandwich-type Structure with Inherent Acoustic Attenuation,” both teach the use of Helmholtz resonators imbedded in the fairing wall to couple to and attenuate low frequency acoustic resonances in the fairing volume. However, no test data is presented in either patent that demonstrates that this can actually be achieved. In the &#39;758 patent, horn-shaped or cup-shaped resonators are inserted into tile panels of acoustic foam and adhered to the fairing wall. In the &#39;710 patent, double panel walls of the fairing are used as the resonator volumes, with orifices cut into the fairing structure to permit coupling to the acoustic field. Neither invention addresses the structural transmission path, nor is either designed to reduce the amplitude of structural resonances. 
     U.S. Pat. No. 6,195,442, titled “Passive Vibroacoustic Attenuator for Structural Acoustic Control,” teaches the use of a combined structural and acoustic approach. However, this device is designed specifically for low frequency acoustic modes and is intended as an add-on to existing foam treatments. The acoustic mitigation is provided by a tuned diaphragm, membrane structure, or even acoustic resonator, and is not intended to provide the broadband acoustic dissipation of an acoustic blanket. 
     There is need in the art for a lightweight acoustic barrier between a structure and an enclosed acoustic volume that can optimally interact and dissipate structural resonances of the enclosing structure that contribute to noise transmission into the enclosed acoustic volume, while simultaneously affording maximum dissipation of the broadband interior acoustic response. The present invention addresses this need in the art. 
     SUMMARY OF THE INVENTION 
     The present invention is a passive acoustic barrier for launch vehicle fairings and other acoustic enclosures contained within flexible wall structures. The acoustic barrier interacts with and dissipates structural resonances of the enclosing structure that contribute to noise transmission into the acoustic volume, while simultaneously dissipating the broadband interior acoustic response. 
     The present invention includes at least one foam panel attached to a wall associated with a vehicle. At least one capsule containing a liquid and an inertial mass is embedded within the foam panel. Each capsule contains at least one spherical mass with a diameter that is less than the minimum inner diameter of the capsule, so that the mass is free to move within the capsule. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, and illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates multiple foam panels of the present invention attached to the interior wall of a fairing, and shown through cutouts in the fairing. 
         FIG. 2  is a cross-sectional view of a single foam panel with an embedded capsule containing liquid and an inertial mass. 
         FIG. 3  is an isometric view of a capsule containing liquid and an inertial mass. 
         FIG. 4  is a cross sectional view of a single foam panel with multiple embedded capsules, each containing liquid and an inertial mass. 
         FIG. 5  is a cross sectional view of two foam panels with multiple embedded capsules respectively located at different depths, in order to target two different structural resonance frequencies. 
         FIG. 6  is an isometric view of a foam panel containing multiple capsules, attached to a fairing wall. 
         FIG. 7  is an isometric view of a panel containing a single capsule device, attached to a fairing wall. 
         FIG. 8  is a schematic drawing of simple one-dimensional model used to approximate the dynamic behavior of a fairing wall having an attached foam panel, where the panel has an embedded rigid, solid mass. 
         FIG. 9  is a plot of the frequency responses for fairing wall models for simulations performed with and without an attached foam panel having an embedded rigid, solid mass. 
         FIG. 10  is a schematic drawing of simple, one-dimensional model used to approximate the dynamic behavior of a fairing wall with an attached foam panel having an embedded fluid-filled capsule of the present invention. 
         FIG. 11  is a plot of the frequency response functions for fairing wall models for simulations performed with and without an attached foam panel having an embedded capsule of the present invention. 
         FIG. 12  is a schematic drawing of the laboratory setup used to test the invention&#39;s performance. 
         FIG. 13  is a plot of the frequency responses in an acoustic cavity measured during laboratory tests to evaluate the effectiveness of the capsule of the present invention, performed using the laboratory setup schematically illustrated in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
     Turning to the drawings,  FIG. 1  illustrates a plurality of acoustic panels  110  of the present invention attached to interior surface of fairing  120  of launch vehicle  130 . Panels  110  are shown through random cutouts in fairing  120 . Each panel  110  is shaped to avoid interfering with instruments, apparatus, and the payload contained within fairing  120 . Panels  110  are comprised of foam adapted to provide broadband dissipation of acoustic energy within volume  140  enclosed within fairing walls  120 . The amount of acoustic dissipation is determined by the material properties of the foam, the thickness of the foam, and the total surface area covered. 
     A cross sectional view of panel  110  is illustrated in  FIG. 2 . Fluid-filled capsule  210  is embedded in panel  110 . Capsule  210  contains fluid  215  and inertial mass  217 . Capsules  210  are adhered to panel  110  to keep them in place, but an adhesive may not be necessary, depending on the constraining force applied by the foam material. Capsules  210  can be made of metal or polymer; their composition has little relevance to the physics of the invention. Fluid  215  can be a viscous or non-viscous liquid depending on the application and the required damping levels. Inertial mass  217 , such as a ball bearing, is sealed inside each capsule  210 . The diameter of mass  217  is less than the inner diameter of capsule  210  and permits mass  217  to move relative to capsule  210 . 
       FIG. 3  provides an isometric view of capsule  210 . Capsule  210  is positioned within panel  110  at a specific depth that is a function of the stiffness of the foam, the mass of capsule  210 , and the frequency of the target structural resonance that is to be attenuated by capsule  210 . Panel  110  will provide some effective stiffness that is a function of the material properties of the foam and the thickness of panel  110  lying between fairing wall  120  and capsule  210 . The mass of capsule  210  and the stiffness of panel  110  will create a resonant frequency at which capsule  210  will vibrate within (and relative to) panel  110 . The resonance of the capsule  210  is designed to interrupt and attenuate the structural vibration of the walls of fairing  120 , as will be subsequently explained. This approach allows panel  110  to act on low frequency structural dynamics. 
     Targeting low frequency structural resonances can be achieved using a single capsule  210  in each panel  110 , as shown in  FIG. 2 , or using multiple capsules  210  in each panel  410 , as shown in  FIG. 4 . Using multiple capsules  210  in each panel  410  produces multiple resonance frequencies. The dominant resonance frequency is produced by the combined mass of capsules  210 . Other resonances may be respectively determined from the properties of panel  410  and combinations of capsules  210  successively comprised of one capsule  210 , then two, up to the total number of capsules  210  minus one. 
     Different panels can be designed to target different structural resonances where it is desired to attenuate multiple structural resonances.  FIG. 5  shows panels  510  and  520 , each designed for different resonant frequencies. This is accomplished by placing capsules  210  at different depths in panels  510  and  520 , respectively. Alternatively, the mass of capsules  210  can be varied to change the resonant frequencies. Panels  510  and  520  are respectively attached to interior fairing walls  522  and  524 . 
       FIG. 6  illustrates an isometric view of foam panel  610  having multiple capsules  210  embedded therein. Panel  610  is attached to the interior surface  612  of fairing wall  614 . In actual application, capsules  210  would not be visible, since they would be embedded within panel  610 . Capsules  210  are positioned within panel  610  to maximize spatial coupling to the target structural resonances. 
       FIG. 7  is an isometric view of foam panel  710  having a single embedded capsule  210 . Foam panel  710  is attached to interior surface  712  of fairing wall  714 . It is to be understood that the panels of the present invention are not necessarily rectangular, but may have other geometries. When external acoustic loads impinge on the external surfaces of fairing wall  714 , the wall will vibrate. This vibration will cause capsule  210  to vibrate. The entire system will behave like a spring-mass-damper system, where the portion of foam panel  710  lying between interior surface  712  and capsule  210  acts as a spring. 
     As an example, consider the simple, one-dimensional system  810  schematically shown in  FIG. 8 . A fairing wall is modeled as simply supported beam  820 , which has some mass, stiffness, and damping. Damping of launch vehicle fairings is generally light, so for this example one can assume a damping ratio of 0.1%. Attached to the beam  820  is a spring-mass-damper system  830  that represents a rigid, solid mass embedded in a foam panel, such as panel  710  (without embedded capsule  210 ). Spring-mass-damper system  830  includes dashpot  870  in parallel with spring  880 . The structural damping of the foam panel is modeled by the dashpot  870 , and is reasonably assumed slight at 0.5%, and the embedded mass is normalized to the mass of the fairing wall, i.e., beam  820 , such that the ratio of the embedded mass to the panel mass is 1%. The stiffness provided by beam  820  is defined such that the resonance of the spring-mass-damper system  830  is proximal to the resonance of beam  820 , which is a simplifying assumption for this analysis. A disturbance input is shown as F(jω), and the panel&#39;s out-of-plane motion is denoted by X(jω). 
     The frequency responses of beam  820  with and without an embedded mass represented by spring-mass-damper system  830  are shown by the overlaid graphs in  FIG. 9 . The frequency response between an external disturbance input and the out-of-plane displacement of a foam panel, such as panel  710 , is shown on the ordinate as X(jω)/F(jω)). The frequency has been normalized by the panel resonant frequency, and is shown on the abscissa as ω np . A comparison of the graphs shows that the on-resonance response is reduced by the spring-mass-damper system  830 , but that two lightly damped peaks are present at different frequencies. Two sharp peaks occur because the damping was assumed to be slight, 0.5%, a reasonable approximation for acoustic foam, which is lightweight and thus has little structural damping. Since there is little overall reduction across the bandwidth, it is clear that this approach provides little to no reduction of the transmitted noise. 
       FIG. 10  is a schematic drawing of spring-mass-damper system  882 , which is a modification of spring-mass-damper system  830  obtained by adding dashpot  886  in parallel with dashpot  870  and spring  880 . Dashpot  886  models the interaction between inertial mass  217  and fluid  215  of capsule  210 . System  882  thus models the effect of embedding capsule  210  in foam panel  710 . A ball bearing will behave like an inertial “reference” mass, and fluid  215  provides additional damping, very similar to the so-called “skyhook damper,” which is described in S. Griffin, J. Gussy, S. Lane, B. Henderson, and D. Sciulli, “Virtual Skyhook Vibration Isolation. System,”  Journal of Vibration and Acoustics, Vol.  124, No. 1, pp. 63-67 (ASME, January 2002). This modification augments the inherent damping of foam panel  710  and, as shown in the graph of  FIG. 11 , the amplitude of vibration of beam  820  is reduced, which will result in a significant decrease of acoustic transmission. Targeting specific structural resonances that are efficient acoustic radiators is the second important feature of this invention, and offers a distinct advantage from prior sound barriers and blankets. Since capsule  210  occupies a very small volume and has a negligible mass relative to the volume and mass of the foam panel in which it is embedded, its presence does not adversly affect the performance of the panel. Combining optimal structural vibration attenuation with the inherent acoustic dissipation of foam panel  710  thus enhances the noise reduction afforded by the foam panel, with a negligible weight penalty. 
     The present invention was tested and demonstrated in an experiment using the setup shown in  FIG. 12 . Acoustic cavity  905  was enclosed by hollow aluminum cylinder  910 . Cylinder  910  was made from ¼-inch aluminum, was 80 inches in length, and had an inside diameter of 24 inches. Cap  911  was formed using medium density particleboard having a thickness of 3 inches. Panel  912  capped the other end of cylinder  910 , and was formed from ⅛-inch thick aluminum. Panel  912  simulated the lightly-damped resonant behavior of a launch vehicle fairing, and had a fundamental resonance at about 130 Hz. Cavity  905  had its first acoustic resonance at 80 Hz, and it second resonance at 160 Hz. Microphone  913  was placed inside cavity  905  to measure the before and after acoustic response. Loudspeaker  914  was placed outside cavity  905  to provide an acoustic disturbance source. Accelerometer  915  was attached to diaphragm  916  of loudspeaker  914  to provide a reference of the disturbance. Spectrum analyzer  920  generated the disturbance signal, measured the reference signal, measured the microphone signal, and computed frequency responses. 
     In the first test case, a small block of foam with a small metal ball bearing was attached by spray adhesive to aluminum panel  912 . A small block was used because it was desired to isolate and compare the effect of the ball bearing with that of capsule  210  of the present invention, and a large foam block would have mass-loaded panel  912  and obscured the results of the experiment. In the second test case, the same foam block was used but, instead of the metal ball bearing of the first test case, capsule  210  having a mass equal to that of the metal ball bearing of the first test case was embedded in the foam block and the foam block was attached to panel  912 . 
     The experimental results are shown in the graph illustrated in  FIG. 13 , which shows the measured frequency responses between the disturbance input, denoted as F(jω), and the microphone measurement, denoted as P(jω). The frequency response functions show peaks at the first two acoustic resonances at 80 Hz and 160 Hz, and at the structural resonance at 130 Hz. The first test case (“added mass only”) split the structural resonance as predicted in  FIG. 9 . The second test case (“with capsule  210 ”) used capsule  210  of the present invention, and behaved as predicted by  FIG. 11 . More particularly, panel  912  having embedded capsule  210  attached thereto was able to optimally couple with and attenuate the structural resonance, which significantly reduced the measured acoustic response over the 120 Hz to 150 Hz bandwidth. This is a relatively broadband reduction for a single structural device. The response was reduced by approximately 12 dB. The presence of capsule  210  had no adverse effect, i.e., spillover, on the noise transmission at other frequencies, which is often a problem with structural devices and active noise control approaches 
     It is to be understood that the preceding is merely a detailed description of several embodiments of this invention and that numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.