Patent Publication Number: US-7913813-B1

Title: Noise shield for a launch vehicle

Description:
BACKGROUND INFORMATION 
     1. Field 
     The present disclosure relates generally to noise and, in particular, to reducing noise radiated by a structure. Still more particularly, the present disclosure relates to a method and apparatus for reducing noise radiated by composite and metallic structures of a launch vehicle. 
     2. Background 
     A launch vehicle is used to carry a payload from the surface of the earth into outer space. A launch vehicle may take the form of a rocket, a space shuttle, or some other suitable vehicle that is capable of carrying the payload into outer space. The payload may be, for example, a satellite or some other object that may be carried into outer space. This object may be, for example, an electronics system, a person, or some other suitable object. 
     With respect to operating a launch vehicle, noise generated by the launch vehicle is a concern in carrying a payload. When the launch vehicle takes the form of a rocket, a fairing may be used to enclose and/or protect the payload. For example, a fairing protects the payload against the impact of the atmosphere and may maintain a desired environment for the payload. Once outside of the atmosphere, the fairing is jettisoned or moved to expose the payload. The payload may then be separated from the launch vehicle into an orbit. 
     Noise shields may be employed in fairings or payloads to reduce the noise and vibrations that the payload may be exposed to during the flight of the launch vehicle. 
     The noise levels generated by launch vehicles may exceed 160 decibels during a launch, such as a satellite. Currently, noise control treatments may be attached to the fairings to reduce the noise inside of the fairings where the payload is located. Currently used systems include placing sound-absorbing materials on the walls of the fairings. These sound-absorbing materials may be, for example, insulating panels or mats. Additionally, some fairings may involve multiple layers of shells in which damping materials are located between the shells of the fairing. Additionally, foam blankets also may be used within the fairing. 
     These types of components, however, require space and increase the weight of the launch vehicle. As a result, the size and/or weight of the payload may be reduced. Additionally, these systems also increase the cost of a launch vehicle. 
     Therefore, it would be advantageous to have a method and apparatus that takes into account one or more of the issues discussed above, as well as possibly other issues. 
     SUMMARY 
     In one advantageous embodiment, an apparatus comprises a core having a first surface configured for attachment to a surface of a structure, a face sheet located over a second surface of the core, a number of cavities within an interior of the core, and a number of ports for the number of cavities. The number of ports provides communication between the number of cavities within the interior of the core and the exterior of the core. The number of cavities and the number of ports are configured to reduce noise traveling through the core. 
     In another advantageous embodiment, a noise reduction system for a launch vehicle comprises a core, a face sheet, and a number of acoustic resonators in the interior of the core. The core has a first surface configured for attachment to a surface of a fairing for the launch vehicle. The face sheet is located over a second surface of the core. The number of acoustic resonators is configured to reduce noise traveling through the core. 
     In yet another advantageous embodiment, a method is present for reducing noise. A structure is identified for noise reduction. A number of noise reduction devices is attached to the structure in which each noise reduction device in the number of noise reduction devices comprises a core, a face sheet, a number of cavities within an interior of the core, and a number of ports for the number of cavities. The core has a first surface configured for attachment to a surface of the structure. The face sheet is located over a second surface of the core. The number of ports provides communication between the number of cavities within the interior of the core and the exterior of the core. The number of cavities and the number of ports are configured to reduce the noise traveling through the core. 
     The features, functions, and advantages can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the advantageous embodiments are set forth in the appended claims. The advantageous embodiments, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an advantageous embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is an illustration of a spacecraft manufacturing and service method in accordance with an advantageous embodiment; 
         FIG. 2  is an illustration of a spacecraft in which an advantageous embodiment may be implemented; 
         FIG. 3  is an illustration of a noise reduction environment in accordance with an advantageous embodiment; 
         FIG. 4  is an illustration of a platform in accordance with an advantageous embodiment; 
         FIG. 5  is an illustration of a portion of a launch vehicle in accordance with an advantageous embodiment; 
         FIG. 6  is an illustration of a fairing with a noise reduction system in accordance with an advantageous embodiment; 
         FIG. 7  is an illustration of a portion of a noise reduction environment in accordance with an advantageous embodiment; 
         FIG. 8  is an illustration of a cross-sectional view of a noise reduction device in accordance with an advantageous embodiment; 
         FIG. 9  is an illustration of a cross-sectional view of a noise reduction device in accordance with an advantageous embodiment; 
         FIG. 10  is an illustration of a side view of a noise reduction device in accordance with an advantageous embodiment; 
         FIG. 11  is an illustration of a structure having a cavity and a port in accordance with an advantageous embodiment; and 
         FIG. 12  is an illustration of a flowchart of a process for designing a noise reduction system in accordance with an advantageous embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring more particularly to the drawings, embodiments of the disclosure may be described in the context of spacecraft manufacturing and service method  100  as shown in  FIG. 1  and spacecraft  200  as shown in  FIG. 2 . Turning first to  FIG. 1 , an illustration of a spacecraft manufacturing and service method is depicted in accordance with an advantageous embodiment. During pre-production, spacecraft manufacturing and service method  100  may include specification and design  102  of spacecraft  200  in  FIG. 2  and material procurement  104 . 
     During production, component and subassembly manufacturing  106  and system integration  108  of spacecraft  200  in  FIG. 2  takes place. Thereafter, spacecraft  200  in  FIG. 2  may go through certification and delivery  110  in order to be placed in service  112 . While in service  112  by a customer, spacecraft  200  in  FIG. 2  is scheduled for maintenance and service  114 , which may include modification, reconfiguration, refurbishment, and/or other maintenance or service. 
     Each of the processes of spacecraft manufacturing and service method  100  may be performed or carried out by a system integrator, a third party, and/or an operator. In these examples, the operator may be a customer. For the purposes of this description, a system integrator may include, without limitation, any number of spacecraft manufacturers and major-system subcontractors; a third party may include, without limitation, any number of venders, subcontractors, and suppliers; and an operator may be a company, a military entity, a service organization, and so on. 
     With reference now to  FIG. 2 , an illustration of a spacecraft is depicted in which an advantageous embodiment may be implemented. In this illustrative example, spacecraft  200  is produced by spacecraft manufacturing and service method  100  in  FIG. 1 . Spacecraft  200  may include frame  202  with a plurality of systems  204  and interior  206 . 
     Examples of plurality of systems  204  include one or more of propulsion system  208 , electrical system  210 , hydraulic system  212 , environmental system  214 , and thermal protection system  216 . Although an aerospace example is shown, different advantageous embodiments may be applied to other industries, such as the automotive industry. 
     Apparatus and methods embodied herein may be employed during at least one of the stages of spacecraft manufacturing and service method  100  in  FIG. 1 . As used herein, the phrase “at least one of”, when used with a list of items, means that different combinations of one or more of the listed items may be used and only one of each item in the list may be needed. For example, “at least one of item A, item B, and item C” may include, for example, without limitation, item A or item A and item B. This example also may include item A, item B, and item C or item B and item C. 
     In one illustrative example, components or subassemblies produced in component and subassembly manufacturing  106  in  FIG. 1  may be fabricated or manufactured in a manner similar to components or subassemblies produced while spacecraft  200  is in service  112  in  FIG. 1 . As yet another example, number of apparatus embodiments, method embodiments, or a combination thereof may be utilized during production stages, such as component and subassembly manufacturing  106  and system integration  108  in  FIG. 1 . A number, when referring to items, means one or more items. For example, a number of apparatus embodiments is one or more apparatus embodiments. A number of apparatus embodiments, method embodiments, or a combination thereof may be utilized while spacecraft  200  is in service  112  and/or during maintenance and service  114  in  FIG. 1 . The use of a number of the different advantageous embodiments may substantially expedite the assembly of and/or reduce the cost of spacecraft  200 . 
     The different advantageous embodiments recognize and take into account a number of different considerations. For example, the different advantageous embodiments recognize and take into account that the noise and vibrations generated by a launch vehicle may cause various electronic devices and/or structures to operate incorrectly. These factors may be a primary cause of a payload operating incorrectly during or after the launch vehicle takes off. 
     The different advantageous embodiments recognize and take into account that composite sandwich structures have been used in fairings for launch vehicles, as well as other components for launch vehicles and other types of platforms. These types of components may reduce the weight of the fairing. 
     The different advantageous embodiments recognize and take into account that the composite structures may reduce the weight of the launch vehicle but have a higher stiffness as compared to using metallic structures. As a result, these composite structures generate more noise as compared to metallic structures. 
     The different advantageous embodiments also recognize and take into account that one solution to noise is to change the physics of the waves traveling through structures. The different advantageous embodiments recognize and take into account that reducing the speed of the waves propagating in these structures reduces the generation of noise. 
     For example, waves traveling in structures include bending waves and shear waves. At least one of bending waves and shear waves propagating through a structure may be reduced in speed by attaching an apparatus or other device to the structure. 
     Thus, the different advantageous embodiments provide a method and apparatus for reducing noise. In one advantageous embodiment, an apparatus comprises a core having a first surface configured for attachment to a surface of a structure. A face sheet is located over a second surface of the core. A number of cavities is present within an interior of the core. Also, a number of ports is present for the number of cavities. The number of ports provides communication between the number of cavities within the interior of the core and the exterior of the core. The number of cavities and the number of ports are configured to reduce noise traveling through the core. 
     With reference now to  FIG. 3 , an illustration of a noise reduction environment is depicted in accordance with an advantageous embodiment. In this illustrative example, noise reduction environment  300  is an example of an environment that may be implemented with spacecraft  200  in  FIG. 2 . 
     Noise reduction environment  300  includes platform  302 , which may take the form of spacecraft  200  in  FIG. 2 . Platform  302  is comprised of structures  304 . In these illustrative examples, structures  304  include at least one of composite structures  306  and metallic structures  308 . 
     Noise reduction system  310  is associated with platform  302  to reduce noise  312  within interior  314  of platform  302 . A first component may be considered to be associated with a second component by being secured to the second component, bonded to the second component, fastened to the second component, and/or connected to the second component in some other suitable manner. The first component also may be connected to the second component through using one or more additional components. The first component also may be considered to be associated with the second component by being formed as part of and/or an extension of the second component. 
     Interior  314  may be payload area  316  in which payload  318  is located. Payload  318  may be, for example, without limitation, a satellite, a spacecraft, a part of a space station, or some other suitable type of payload. Fairing  320  may be a structure within structures  304  that protects payload area  316 . 
     In these illustrative examples, noise reduction system  310  may be associated with fairing  320  to reduce noise  312  within payload area  316 . 
     Noise reduction system  310  may include number of noise reduction devices  322 . Number of noise reduction devices  322  may be attached to composite structures  306  and/or metallic structures  308  to reduce noise  312  generated by sound waves  324 . Number of noise reduction devices  322  may be associated with structures  304 . In other words, number of noise reduction devices  322  may be attached directly or indirectly to structures  304 . 
     In these depicted examples, sound waves  324  radiate from structures  304  and/or platform  302 . Sound waves  324  are formed when waves  326  pass through surface  328  of structures  304  and radiate into interior  314 . Waves  326  include shear waves  330  and bending waves  332 . In the illustrative examples, noise  312  is generated by the coupling of energy present in bending waves  332  with the energy present in shear waves  330  when sound waves  324  are formed. 
     In these illustrative examples, noise reduction device  334  in number of noise reduction devices  322  comprises core  336 , face sheet  338 , number of cavities  340 , and number of ports  342 . 
     In these illustrative examples, face sheet  338  is a layer that covers core  336 . Face sheet  338  may be directly attached to core  336  or indirectly attached with some other number of layers between core  336  and face sheet  338  in these illustrative examples. 
     Core  336  is configured to reduce speed  344  of shear waves  330  traveling through core  336 . Core  336  has first surface  346 , which is configured for attachment to surface  328  in structures  304 . Face sheet  338  is located over second surface  348  of core  336 . Face sheet  338  is configured to reduce speed  350  of bending waves  332  traveling through face sheet  338 . First surface  346  is substantially parallel to second surface  348  in these illustrative examples. 
     In these illustrative examples, number of cavities  340  and number of ports  342  may be formed in walls  349  in interior  352  of core  336 . Walls  349  may be used when walls  349  are comprised of and/or coated with a material that may resist or prevent air from moving through walls  349 . In other advantageous embodiments, number of cavities  340  and number of ports  342  may be formed using number of structures  351 . In these illustrative examples, number of ports  342  may, in part, be formed by number of openings  353  in face sheet  338 . 
     In these illustrative examples, number of ports  342  absorbs noise  312 . Number of ports  342  may act as a resistor, while number of cavities  340  may function as a capacitor. As air or gases are pushed in and out of number of ports  342 , number of ports  342  absorbs noise  312  at different frequencies. 
     Number of ports  342  provides communication between number of cavities  340  in interior  352  of core  336  and exterior  354  of core  336 . For example, if number of ports  342  is located on second surface  348 , second surface  348  is part of exterior  354  of core  336 . 
     Additionally, a cavity within number of cavities  340  may have more than one port to provide communication. In these examples, number of ports  342  provides communication between number of cavities  340  within interior  352  of core  336  and exterior  354  of core  336  by providing an opening or channel from number of cavities  340  to exterior  354 . In these examples, number of cavities  340  and number of ports  342  are configured to reduce noise  312  traveling through core  336 . 
     Number of cavities  340  and number of ports  342  may take various forms. For example, number of cavities  340  and number of ports  342  may form number of acoustic resonators  356 . Number of acoustic resonators  356  may include at least one of number of Helmholtz resonators  358 , number of T-shaped resonators  360 , and/or other suitable types of resonators that reduce noise. Number of Helmholtz resonators  358  and/or number of T-shaped resonators  360  may be used to control or reduce noise at frequencies from about 20 hertz to about 80 hertz. These resonators may be used to reduce noise at other frequencies, depending on the particular implementation and configuration of the resonators. In some advantageous embodiments, number of cavities  340  may comprise an absorptive material, such as foam. The absorptive material may be used to increase the range of frequencies at which noise is reduced by number of acoustic resonators  356 . 
     These types of resonators have a cavity with a volume that is larger than compared to the volume of the port. The volume of the cavity relative to the volume of the port may vary, depending on the particular frequency for which noise reduction is desired. 
     The length of these types of resonators may be selected based on the wavelength for which noise control may be desired. In addition to controlling frequencies from about 20 hertz to about 80 hertz, these types of acoustic resonators also may control noise at other frequencies as a result of modal coupling. 
     Number of noise reduction devices  322  may have various shapes and sizes. In one illustrative example, noise reduction device  334  may conform to interior surface  362  of fairing  320 . Noise reduction device  334  may take the form of a single layer that conforms to all of interior surface  362  for each part of fairing  320 . In other advantageous embodiments, noise reduction device  334  may take the form of a tile having various shapes, such as square, rectangular, hexagonal, irregular, or some other suitable shape. 
     In these illustrative examples, shear waves  330  are waves that move in a direction substantially perpendicular to surface  328  of structures  304 , first surface  346  of core  336 , second surface  348  of core  336 , and other surfaces that may be present in platform  302 . Shear waves  330  cause vibrations in core  336  that are substantially perpendicular to the movement of shear waves  330 . The movement of shear waves  330  through structures  304  and/or core  336  is taken into account in determining the configuration for core  336  in these illustrative examples. Bending waves  332  are waves that move substantially parallel to or substantially in the direction of surface  328  of structures  304 , first surface  346  of core  336 , second surface  348  of core  336 , and/or other surfaces in platform  302 . 
     In these illustrative examples, noise reduction device  334  reduces noise  312  in interior  314  of platform  302 . In particular, reduction of noise  312  may be performed using noise reduction device  334  for payload area  316  to reduce noise  312  that reaches payload  318 . 
     This noise reduction is performed in a different manner from other currently used devices, which employ absorptive materials, such as foam. The absorptive materials convert energy produced by sound waves radiating from the structure into heat. This conversion may be achieved in the different advantageous embodiments through the deformation of shear waves through the structure. 
     In the different advantageous embodiments, number of noise reduction devices  322  reduces the speed of waves  326  by an amount that reduces noise  312  within interior  314  of platform  302 . 
     The reduction in the speed of waves  326  may be to less than the speed of sound in these examples. For example, the speed of waves  326  may be reduced to a speed that is substantially two thirds the speed of sound. This reduction in speed may be desired, because number of noise reduction devices  322  is comprised of materials that are less stiff as compared to materials in structures  304 . As a result, waves  326  slow down in speed when propagating from structures  304  to number of noise reduction devices  322 . 
     With composite structures  306 , core  336  may be configured to reduce the speed of waves  326  with respect to resonant modes of bending waves  332 . With metallic structures  308 , core  336  may be configured to reduce the speed of waves  326  with respect to non-resonant modes of bending waves  332 . In this manner, number of noise reduction devices  322  is configured to transfer the energy in bending waves  332  traveling through structures  304  into energy in shear waves  330 . More specifically, the energy in bending waves  332  traveling at supersonic speed  364  may be transferred into energy in shear waves  330  traveling at subsonic speed  366 . 
     In these illustrative examples, supersonic speed  364  is a speed that is greater than the speed of sound. This speed is about 767 miles per hour in dry air at about 68 degrees Fahrenheit or about 343 meters per second at about 20 degrees Celsius. A speed lower than about 767 miles per hour is subsonic speed  366 . One result of this type of transfer of energy from bending waves  332  into energy in shear waves  330  is a reduction in noise  312  in interior  314 . 
     In the different advantageous embodiments, core  336  may take a number of different forms. For example, core  336  may be, without limitation, solid core  368 , honeycomb core  370 , or some other suitable type of core. In some examples, core  336  may be a truss core, an X-core, a K-core, a Kevlar™ core, a Nomex™ core, or some other suitable type of core. These different cores may be comprised of materials such as, for example, without limitation, foam, paper, melamine foam, polymeric compound foam, aramid fiber paper, para-aramid fiber paper, and/or some other suitable material. Further, the different cores may be comprised of materials that allow shear waves  330  to travel through core  336  with subsonic speed  366 . 
     Face sheet  338  may be constructed from a number of different types of materials. The particular material is selected as one that reduces bending waves  332  traveling through face sheet  338 . These materials may include materials such as, for example, without limitation, aluminum, other types of metals, glass, carbon fiber, graphite fiber, Kevlar® fiber, composite materials, and/or other suitable types of materials. The thickness of face sheet  338  may be selected based on the type of material, weight, environmental factors, vibration frequencies, and/or other suitable factors. 
     Number of structures  351  for number of cavities  340  and number of ports  342  may be comprised of a number of different types of materials. In these examples, the materials may be selected from a material that resists or prevents air from moving through the material. In one example, number of structures  351  may be comprised of a material selected for face sheet  338 . In other advantageous embodiments, number of structures  351  may be comprised of polyvinyl chloride, plastic, and/or other suitable materials. The materials selected for number of structures  351  may not need to be rigid if the material is associated with or secured to the walls of core  336 . 
     Further, in addition to number of noise reduction devices  322 , noise reduction system  310  may include other components. These components may include, for example, foam blankets, fiberglass blankets, and/or other suitable components. 
     In some advantageous embodiments, noise reduction device  334  may include additional layers. For example, damping layer  372  may be located between face sheet  338  and core  336 . Damping layer  372  may be, for example, without limitation, a layer of viscoelastic material. A viscoelastic material is a material that exhibits both viscous and elastic characteristics when undergoing deformation. In these illustrative examples, damping layer  372  may be implemented using, for example, layer of viscoelastic material  374 . Layer of viscoelastic material  374  may cover all of second surface  348  of core  336 . In this illustrative example, layer of viscoelastic material  374  may be damping layer of foam  375 . Damping layer of foam  375  is a layer of foam in which the foam has viscoelastic properties. Damping layer of foam  375  reduces noise  312  and vibrations from structures  304 . 
     In some advantageous embodiments, damping layer of foam  375  may take the form of strip  376  that may cover edges  378  around second surface  348 . In these examples, damping layer of foam  375  may provide contact between core  336  and surface  328  of structures  304 . Damping layer of foam  375  may be implemented using, for example, a foam in the ISODAMP® C-3000 Series of energy-absorbing foams, which is available from Aearo Technologies, Inc., a 3M Company. 
     In addition, in other advantageous embodiments, additional cores and face sheets may be placed in layers in addition to core  336  and face sheet  338  to form noise reduction device  334 . For example, without limitation, structures  304  may have a thickness or stiffness such that an additional core may be used in noise reduction device  334  to reduce noise  312 . In this depicted example, core  382  may be located over face sheet  338  with face sheet  384  located over core  382 . 
     The illustration of noise reduction environment  300  in  FIG. 3  is not meant to imply physical or architectural limitations to the manner in which different advantageous embodiments may be implemented. Other components in addition to and/or in place of the ones illustrated may be used. Some components may be unnecessary in some advantageous embodiments. Also, the blocks are presented to illustrate some functional components. One or more of these blocks may be combined and/or divided into different blocks when implemented in different advantageous embodiments. 
     For example, in some advantageous embodiments, noise reduction system  310  may include other devices in addition to the ones illustrated to reduce noise  312 . For example, active noise reduction devices may be employed in noise reduction system  310  to cancel waves  326  to reduce noise  312 . 
     In some advantageous embodiments, number of noise reduction devices  322  may be used in other platforms other than spacecraft. For example, these platforms may be selected from one of a mobile platform, a stationary platform, a land-based structure, an aquatic-based structure, a space-based structure, a surface ship, a tank, a personnel carrier, a train, a space station, a submarine, an automobile, a power plant, a house, a manufacturing facility, an office building, and/or some other suitable type of platform. 
     With reference now to  FIG. 4 , an illustration of a platform is depicted in accordance with an advantageous embodiment. In this illustrative example, platform  400  takes the form of launch vehicle  402 . Launch vehicle  402  is an example of one implementation for spacecraft  200  in  FIG. 2 . In this illustrative example, launch vehicle  402  has fairing  404 . One or more advantageous embodiments may be implemented in fairing  404  to reduce noise that may be exposed to a payload inside of fairing  404 . 
     With reference now to  FIG. 5 , an illustration of a portion of launch vehicle  402  is depicted in accordance with an advantageous embodiment. In this illustrative example, an exposed view of fairing  404  is depicted. In this exposed view, payload  500  may be located inside of fairing  404 . Payload  500 , in this example, may take the form of satellite  502 . As can be seen in this illustration, fairing  404  may have part  504  and part  506 , which may separate to expose payload  500 . This separation may occur when payload  500  is to be separated from launch vehicle  402 . In this illustrative example, noise reduction system  508  may be located on each part of fairing  404 . Noise reduction system  508  is an example of one implementation of noise reduction system  310  in  FIG. 3 . 
     With reference now to  FIG. 6 , an illustration of a fairing with a noise reduction system is depicted in accordance with an advantageous embodiment. In this illustration, fairing  404  is shown in an exploded view. In this view, noise reduction system  508  has section  600  and section  602 . Section  600  is associated with part  504 , while section  602  is associated with part  506  of fairing  404  in these examples. 
     The illustrations of fairing  404  in  FIGS. 4-6  are not meant to imply physical or architectural limitations to the manner in which different fairings may be implemented. For example, other implementations of fairing  320  in  FIG. 3  may have different shapes and sizes other than that shown for fairing  404 . In addition, in other advantageous embodiments, fairing  320  may have three parts, four parts, or some other number of parts instead of part  504  and part  506  illustrated for fairing  404 . 
     Further, noise reduction system  508  is shown with two sections, section  600  and section  602 . These sections are illustrated as conforming to the shape of part  504  and part  506  of fairing  404 . In other words, section  600  has a shape similar to part  504 , and section  602  has a shape similar to part  506 . These shapes allow for the sections to be placed or secured against the parts. In other advantageous embodiments, noise reduction system  508  may be comprised of a plurality of tiles that are attached to part  504  and part  506 . 
     Turning now to  FIG. 7 , an illustration of a portion of a noise reduction environment is depicted in accordance with an advantageous embodiment. In this illustrative example, a portion of noise reduction environment  700  is illustrated. Noise reduction environment  700  is an example of one implementation for noise reduction environment  300  in  FIG. 3 . 
     In this depicted example, noise reduction system  701  takes the form of noise reduction device  702 . As depicted, noise reduction device  702  is associated with surface  704  of structure  706 . In this illustrative example, structure  706  takes the form of fairing  708 . Fairing  708  is an example of one implementation of fairing  320  in  FIG. 3 . As another example, structure  706  may be for part  504  for fairing  404  in  FIG. 5 . 
     Noise reduction device  702  is shown in a partially exposed perspective view. Noise reduction device  702  is comprised of face sheet  710 , core  712 , and face sheet  714 . Face sheet  710  may be referred to as an outer face sheet, while face sheet  714  may be referred to as an inner face sheet. Face sheet  714  is attached to surface  704  of fairing  708  in these examples. Core  712  is located between face sheet  710  and face sheet  714 . Core  712 , in this example, takes the form of foam core  716 . 
     Foam core  716  contains number of cavities  717  and number of ports  718 . In this example, number of cavities  717  comprises cavity  720  and cavity  722 . Number of ports  718  comprises port  724  and port  726 . Cavities  720  and  722  are located in interior  728  of core  712 . Ports  724  and  726  provide communication between cavity  720  and cavity  722  in interior  728  of core  712  and exterior  730  of core  712 . In these examples, port  724  provides communication for cavity  720 . Port  726  provides communication for cavity  722 . In these examples, these ports provide an opening or access for air and other gases to enter or exit cavities  720  and  722 . Number of cavities  717  and number of ports  718  are configured to reduce noise traveling through core  712 . These components may form acoustic resonators  732 . Acoustic resonators  732  may be tuned to different frequencies, depending on the volume of number of cavities  717  and the size of number of ports  718 . 
     In these illustrative examples, foam core  716  is configured to reduce a speed of shear waves traveling through foam core  716  in the direction of arrow  719 . Face sheet  710  and face sheet  714  are configured to reduce a speed of bending waves traveling through face sheet  710  and face sheet  714  in the direction of arrow  721 . 
     In these illustrative examples, shear waves traveling through core  712  affect the level of noise that radiates from face sheet  710  of noise reduction device  702  and causes noise that may be heard by a passenger. The speed of shear waves in core  712  may be defined as follows: 
               c   s     =       [       G   c       ρ   c       ]       1   /   2             
where C s  isthe speed of shear waves, G c  is the shear modulus, and ρ c  is the density of the core.
 
     The speed of shear waves in core  712  may be reduced by selecting materials in the configurations for core  712 . For example, in this illustrative example, core  712  takes the form of foam core  716 . In other illustrative examples, core  712  may be a solid core. 
     Noise reduction system  701  is not required to carry the structural and dynamic loads in fairing  708 . As a result, core  712  may be constructed with a shear modulus selected to allow shear waves to travel through core  712  at subsonic speeds. The particular material in the configuration selected for core  712  may be made by selecting the shear modulus and density for core  712  such that the speed of shear waves is reduced when the shear waves and/or bending waves travel from structure  706  to core  712  in noise reduction device  702 . 
     Similarly, the materials in the configurations selected for face sheet  710  and face sheet  714  are made by selecting elastic moduli and densities for face sheet  710  and face sheet  714  such that the speed of bending waves is reduced when the bending waves and/or shear waves travel through face sheet  714  and face sheet  710 . 
     The speed of bending waves traveling through face sheet  710  and face sheet  714  may be given by: 
     
       
         
           
             
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     where c b  is the speed of bending waves, ω is radian frequency or 2π times frequency, E f  is the elastic modulus, ρ f  is the density of material for the face sheet, ν is Poisson&#39;s ratio, and h is the thickness of the face sheet. This equation gives the speed of bending waves in face sheets made out of isotropic materials. Isotropic materials are materials which have the same properties in all directions. 
     The speed of bending waves in face sheets made out of orthotropic materials, such as orthotropic composite materials, may be calculated by determining a bending stiffness using finite element models. Orthotropic materials are materials with properties that may vary, depending on the direction in which the properties are measured. In other words, orthotropic materials are anisotropic materials. 
     In these examples, at low frequencies, the generation of noise is determined by the bending and/or vibrations of substantially all of noise reduction device  702 . At high frequencies, the bending in face sheet  710  determines the level of noise generated. In the mid-frequencies, shear waves determine the level of noise generated. In these examples, low frequencies are frequencies that are lower than about 400 hertz. Mid-frequencies may be from about 400 hertz to about 3,000 hertz. High frequencies are frequencies that are greater than about 3,000 hertz. In other advantageous embodiments, noise reduction device  702  may be designed such that the low frequencies, the mid-frequencies, and/or the high frequencies have different ranges of frequencies. 
     Turning now to  FIG. 8 , an illustration of a cross-sectional view of a noise reduction device is depicted in accordance with an advantageous embodiment. In this illustrative example, noise reduction device  702  is shown in a cross-sectional view taken along lines  8 - 8  in  FIG. 7 . 
     With reference now to  FIG. 9 , an illustration of a cross-sectional view of a noise reduction device is depicted in accordance with an advantageous embodiment. In this illustration, noise reduction device  702  is shown in a cross-sectional view taken along lines  9 - 9  in  FIG. 7 . 
     With reference now to  FIG. 10 , an illustration of a side view of a noise reduction device is depicted in accordance with an advantageous embodiment. In this illustrative example, noise reduction environment  1000  is an example of another implementation for noise reduction environment  300  in  FIG. 3 . As depicted, noise reduction environment  1000  includes noise reduction device  1002 , which may be associated with surface  1004  of structure  1006 . In this illustrative example, structure  1006  takes the form of fairing  1008 . 
     In this illustrative example, noise reduction device  1002  comprises face sheet  1010  and core  1012 . Core  1012  has side  1014  and side  1016 . Additionally, in this illustrative example, side  1016  of core  1012  is directly attached to surface  1004  of fairing  1008 . Side  1014  is attached to face sheet  1010 . In this illustrative example, core  1012  takes the form of solid core  1018 . In this example, solid core  1018  is comprised of material  1020 . 
     Core  1012  has cavities  1022 ,  1024 ,  1026 , and  1028 . In this example, each cavity has a port. For example, cavity  1022  has port  1030 , cavity  1024  has port  1032 , cavity  1026  has port  1034 , and cavity  1028  has port  1036 . Of course, in other advantageous embodiments, these cavities may have more than one port. 
     Additionally, different cavities may have other numbers of ports as compared to other cavities, depending on the particular implementation. These cavities and ports form acoustic resonators  1038 ,  1040 ,  1042 , and  1044 . 
     In this illustrative example, waves traveling through structure  1006  may travel into noise reduction device  1002 . The speed of the waves traveling from structure  1006  into noise reduction device  1002  is reduced. This reduction in speed is caused by bending and/or shear movement within noise reduction device  1002 . In these illustrative examples, face sheet  1010  causes a reduction in the speed of waves traveling through face sheet  1010  in the direction of arrow  1046 . These types of waves are bending waves. Core  1012  reduces the speed of shear waves traveling in the direction of arrow  1048  through core  1012 . As a result, waves traveling through fairing  1008  may be slowed down in speed by noise reduction device  1002 . This reduction in speed reduces the amount of noise that may be generated by or heard from structure  1006 . 
     In these illustrative examples, acoustic resonators  1038 ,  1040 ,  1042 , and  1044  also reduce noise that travels through core  1012 . In the illustrative examples, these resonators may reduce or eliminate noise at different frequencies. In the different advantageous embodiments, each of the acoustic resonators may reduce noise in the same frequency or in different frequencies. 
     The illustrations of noise reduction device  702  and noise reduction device  1002  are not meant to imply physical or architectural limitations to the manner in which noise reduction devices may be implemented. Other components, in addition to or in place of the ones illustrated, may be used. Also, some components may be unnecessary in some advantageous embodiments. For example, in some advantageous embodiments, a noise reduction device may have a number of cores and a number of face sheets in addition to the ones illustrated in these examples. 
     With reference now to  FIG. 11 , an illustration of a structure having a cavity and a port is depicted in accordance with an advantageous embodiment. In this illustrative example, structure  1100  has cavity  1102  and port  1104  and may be located within interior  352  of core  336  in  FIG. 3 . In these illustrative examples, structure  1100  may be an acoustic resonator in number of acoustic resonators  356 . In this example, structure  1100  may be a T-shaped acoustic resonator. In these illustrative examples, structure  1100  has a number of different dimensions. Structure  1100  has length  1106 , which is the sum of length L 2   1108  and length L 3   1110 . In this illustrative example, length L 2   1108  and length L 3   1110  are measured from X 1  axis  1112 . These different lengths define cavity  1102 . Port  1104  for structure  1100  has length L 1   1114 . 
     With reference now to  FIG. 12 , an illustration of a flowchart of a process for designing a noise reduction system is depicted in accordance with an advantageous embodiment. The process illustrated in  FIG. 12  may be implemented in a noise reduction system, such as noise reduction system  310  in noise reduction environment  300  in  FIG. 3 . 
     The process begins by identifying a structure for noise reduction (operation  1200 ). This structure may be, for example, a fairing for a launch vehicle or some other type of structure. Thereafter, vibration analysis is performed (operation  1202 ). In operation  1202 , the vibration analysis may include determining characteristics of vibrations that may be formed in the structure or travel through the structure. These vibrations may be caused by waves traveling through the structure. Further, the vibration analysis also may be performed for different components of a noise reduction device. For example, the characteristics of vibrations that may be formed in a face sheet, a cavity, or a core may be identified. 
     The process then configures a noise reduction system (operation  1204 ). The configuration in operation  1204  may take into account a number of different factors. For example, without limitation, these factors are size constraints, weight constraints, a desired level of noise reduction, material constraints, and/or other suitable factors. For example, the noise reduction system may be configured to have a selected number of cores and ports comprised of a particular material. In some advantageous embodiments, a cavity may have more than one port. In yet other advantageous embodiments, a noise reduction system may have different cavities with different numbers of ports that are configured to reduce or absorb noise for different frequencies. Further, the cavities and ports may be formed by the walls of the core and the face sheet in some advantageous embodiments. In yet other advantageous embodiments, a number of structures may be placed inside of the core to provide the desired level of noise reduction. 
     Further, configuring the number of noise reduction devices also may include selecting a particular number of cores, a particular number of face sheets, and/or a particular number of damping layers. Configuring the number of noise reduction devices also may include selecting materials, thicknesses, dimensions, and other suitable factors for different components within the noise reduction system. Thereafter, the process attaches the noise reduction system to the structure (operation  1206 ), with the process terminating thereafter. 
     Thus, the different advantageous embodiments provide a method and apparatus for reducing noise. In one advantageous embodiment, an apparatus comprises a core, a face sheet, a number of cavities, and a number of ports. The core has a first configuration for attachment to a surface of a structure. The face sheet is located over a second surface of the core. The number of cavities is located within the interior of the core and the number of ports provides communication between the number of cavities within the interior of the core, and the exterior of the core. The number of cavities in the number of ports is configured to reduce noise traveling through the core. 
     In this manner, the noise reduction devices in the different advantageous embodiments reduce noise generated by a structure in the platform. Reduction in noise is achieved in a number of different ways. For example, the ports and cavities may reduce the noise traveling through a core. Additionally, the speed of waves traveling through a structure to the noise reduction device may be reduced through the use of materials that are less stiff in the noise reduction device as compared to those in the structure. 
     With one or more of the different advantageous embodiments, the use of other noise reduction devices may be avoided or reduced. The reduction or elimination of other noise reduction devices in a noise reduction system may reduce the weight and/or cost of these systems. Further, the reduction or elimination of other noise reduction devices may increase the payload or performance of the platform. 
     The description of the different advantageous embodiments has been presented for purposes of illustration and description, and it is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different advantageous embodiments may provide different advantages as compared to other advantageous embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.