Abstract:
An aircraft window configuration utilizes a laminate build-up of the primary pane to increase damping and reduce the structural response to the turbulent boundary layer outside the aircraft. The laminate may consist of several acrylic layers or a combination of acrylic and glass layers. Noise dampening results from the introduction of a transparent visco-elastic material or a urethane. A vacuum layer may be introduced between the primary pane and a middle, or fail-safe pane. The vacuum layer decouples the panes over a broad frequency range resulting in a lower response of the inner pane that radiates noise into the passenger cabin. Such a window configuration reduces weight and improves noise performance. A damped laminate also reduces pane deflections into the air stream and improves aerodynamic performance of the aircraft.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an airborne mobile platform laminate window that reduces vibration and sound transmissions to the airborne mobile platform fuselage interior.  
       BACKGROUND OF THE INVENTION  
       [0002]     The reduction of sound transmissions to the fuselage interior of an airborne mobile platform (e.g. a modern jet aircraft) is becoming more of a concern for commercial aircraft manufacturers and their customers in an increasingly-competitive international marketplace. Commercial aircraft manufacturers and their customers are interested in reducing the level of noise inside their aircraft. More specifically, they are interested in reducing the amount of noise that is transferred from the aircraft exterior to the aircraft interior. Noise is typically created by the turbulent flow along the fuselage and radiated from the engine exhaust plume. An area of the aircraft through which noise is typically transferred is the fuselage sidewall, including the aircraft windows and its surrounding window belt area. Although interior noise is considered undesirable in commercial aircraft, aircraft manufacturers and their customers are simultaneously demanding aircraft that are lighter in order to reduce costs, and aircraft that have larger windows in order to increase outside visibility and permit larger amounts of light to enter the aircraft cabin.  
         [0003]     While current aircraft windows are generally satisfactory for their applications, each is associated with its share of limitations. Historically, aircraft manufacturers used relatively dense materials to reduce the amount of noise that entered the cabin through the windows and window beltline. This meant using thick, transparent window materials or multiple pieces of a transparent material to reduce noise transmission. The problem with the prior art solutions to interior noise is that noise levels inside the cabin remained at undesirable levels, the aircraft weight was not being reduced, and the window size, and thus the amount of natural interior light, remained relatively small.  
         [0004]     A need remains in the art for an airborne mobile platform window that overcomes the limitations associated with the prior art, including, but not limited to those limitations discussed above. This in turn, will result in an aircraft window that reduces interior noise relative to existing aircraft windows, remains relatively lightweight, and that is larger in size compared to traditional aircraft windows to permit higher quantities of light to enter the aircraft cabin.  
       SUMMARY OF THE INVENTION  
       [0005]     A window for an airborne mobile platform is disclosed. More specifically, combinations of various window layers for use in an airborne mobile platform are disclosed. A window for an airborne mobile platform has an interior layer of transparent material and an exterior layer of transparent materials that together with the interior layer, define a space. The space may be a layer of air or a vacuum layer. The exterior layer may further be a multi-layer of transparent materials, such as an acrylic layer, a viscous noise-absorbing layer of transparent material, and a glass layer. A rubber seal, that is, a visco-elastic rubber, around the perimeter of the layers of the window provides a vibration and noise-absorbing frame that is further surrounded by a c-channel that peripherally bounds the rubber seal on three of its sides. The c-channel provides additional structural integrity to the window and acts as a structural member to provide support to the fuselage.  
         [0006]     The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0008]      FIG. 1  is a side view of an airborne mobile platform depicting a passenger window beltline;  
         [0009]      FIG. 2A  is a perspective view of an aircraft window having a c-ring frame;  
         [0010]      FIG. 2B  is a cross-sectional view of the c-ring frame of the aircraft window of  FIG. 2A ;  
         [0011]      FIG. 3A  is cross-sectional view of an aircraft window configuration of the prior art;  
         [0012]      FIG. 3B  is a cross-sectional view of an aircraft window configuration according to a first embodiment of the present invention;  
         [0013]      FIG. 3C  is a cross-sectional view of an aircraft window configuration according to a second embodiment of the present invention;  
         [0014]      FIG. 3D  is a cross-sectional view of an aircraft window configuration according to a third embodiment of the present invention;  
         [0015]      FIG. 3E  is a cross-sectional view of an aircraft window configuration according to a fourth embodiment of the present invention;  
         [0016]      FIG. 4  is a graph of the average velocity power spectral density (PSD) over a broadband frequency for the window configurations according to the teachings of the present invention;  
         [0017]      FIG. 5  is a graph of the reduction of vibration (db) over a broadband frequency range for the window configuration employing a layer of visco-elastic material, relative to the prior art configuration;  
         [0018]      FIG. 6  is a graph of the average velocity power spectral density (PSD) over a frequency broadband for various window configurations; and  
         [0019]      FIG. 7  is a graph of the average velocity power spectral density (PSD) over a frequency broadband for various window configurations employing a stiffer c-ring. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Turning now to  FIG. 1 , an airborne mobile platform  10  (e.g. an aircraft) is depicted. The aircraft has a fuselage  12 , a wing  14  attached to the fuselage  12 , an engine  16  attached to the wing  14 , an engine exhaust area  18 , and a window beltline  20 , located just above the wing  14 , having a multitude of passenger windows  22 . In an aircraft  10  having the window configuration depicted, noise from a variety of sources is able to penetrate through the passenger windows  22  and their surrounding window frames  23 .  
         [0021]     One such source is the exhaust plume that originates in the engine exhaust area  18 , wherefrom noise radiates outwardly from the plume for a number of engine diameters aft of the engine  16 . Engine noise is a key concern in the aft cabin of the aircraft during take-off, climb, and at cruising altitudes. In addition, noise is generated at the fluid boundary layer of the aircraft as it moves through the air during flight. This noise source is apparent throughout the aircraft at cruising altitudes. The boundary layer is that layer of fluid in the immediate vicinity of a bounding surface. For an aircraft wing, the boundary layer is the part of the flow immediately adjacent to the wing, and for the fuselage, the part of the flow immediately adjacent to the fuselage. The boundary layer effect occurs at the region in which all changes occur in the flow pattern, for example, where the boundary layer causes distortion in the surrounding nonviscous flow.  
         [0022]     To compound noise generation, the boundary layer also adds to the effective thickness of the aircraft, through the displacement thickness, which increases the pressure drag of the aircraft. Also, the shear forces at the surface of the aircraft wing create skin friction drag. Larger wings generally create a larger amount of drag. Since the engines are used to overcome the accumulated drag in order to move the aircraft through the air, as the drag increases, the engines must work harder to overcome the drag, which increases noise. Also, as the size of the aircraft increases, the engine size usually increases, which increases the noise generated. This highlights the strong dependence of acoustic design of an aircraft on aerodynamics and propulsion. Ultimately, the presence of noise within aircraft interiors is undesirable, and the present invention may be used to reduce an undesirable level of noise, such as a level that is created by a large aircraft, to a level that is desirable or at least acceptable.  
         [0023]     To reduce the level of noise detectable in a fuselage interior for a given aircraft, various window material panel configurations, according to the present invention, have been developed. Window panel material configurations are also known as window or laminate buildups, window or laminate layups, or simply as layups. Turning now to  FIGS. 2A and 2B , window parts that make up a portion of an aircraft window  22  will be explained.  FIG. 2A  depicts a window frame  23  of a window  22  of the window beltline  20 , and  FIG. 2B  depicts a structural c-ring  24 , or c-channel, of the window perimeter that forms the window frame  23 . The window frame  23  is defined by a c-ring  24  that is generally formed by an outside flange  30  and a web  32 . The c-ring  24  has a mounting flange  26 , which traverses the perimeter of the window  22 , and has a mounting flange inside surface  48  and a mounting flange outside surface  46  that are used for window alignment and mounting purposes. Continuing with the c-ring  24 , the outside flange  30  is referred to as the outside flange because it generally faces the fuselage exterior when the window  22  is installed. The outside flange  30  has an outside flange inside surface  38  and an outside flange outside surface  40 .  
         [0024]     The web  32  not only blends, or joins, the mounting flange  26  and the outside flange  30 , but it provides rigidity, support and strength for the resulting c-ring  24 . The web  32  and outside flange  30  provide a partial enclosure for a visco-elastic rubber seal, to be discussed later, that abuts against the web inside surface  34  and the outside flange inside surface  38 . The web  32  has a web inside surface  34  and a web outside surface  36 . The rigidity or stiffness of the c-ring  24  is cumulatively provided by the outside flange  30 , web  32 , and mounting flange  26 . The c-ring  24  may be manufactured from a rigid, lightweight material such as aluminum or titanium, or other metal or non-metal material. With respect to weights, the specific material will be less dense than most metals or non-metals in its respective category. As will be discussed later, making the c-ring  24  stiffer may provide benefits in terms of noise reduction. To make the c-ring  24  relatively stiffer, a different aircraft aluminum or non-aluminum material could be used. Alternatively, a thicker cross-section of a given material could be used for stiffening purposes.  
         [0025]     Before turning to the structure and operative workings of the window layer configurations of the present invention, a review of the construction of a prior art aircraft window will be briefly examined.  FIG. 3A  depicts an aircraft window  50  of the prior art having transparent layers as laminate pieces that make-up the transparent area  52 . The window  50  is bounded about its perimeter by the c-channel frame  24 . Against the c-channel  24  is a first rubber seal  58 , and a second rubber seal  60 . The rubber seals  58 ,  60  are similarly situated in that the seals  58 ,  60  together make up a single continuous seal that traverses the interior portion of the window  50  and bounds the window&#39;s laminate pieces, which will now be described.  
         [0026]     The transparent area  52  of the prior art window  50  is comprised of a mid acrylic layer  62 , a center airspace  64 , and an outer acrylic layer  66 . The layers of material are held in place by a retainer clip. The outer acrylic layer  66  is generally the layer that may be exposed to the elements on the aircraft exterior, while the mid acrylic layer  62  is the layer that lies adjacent to a transparent dust pane (not shown). A passenger may touch the transparent dust pane when a non-transparent, retractable dust cover (not shown) is in its retracted position adjacent a passenger. The acrylic layers  62 ,  66  are bounded about their peripheries by the rubber seals  58 ,  60 , which define the air space  64  in conjunction with the acrylic layers  62 ,  66 . The rubber seals  58 ,  60 , to some degree, seal out noise that may propagate into the window layup and act as a dampener to dampen noise that is able to initially propagate to and into the seal.  
         [0027]     In the prior art of  FIG. 3A , the outer acrylic layer of 0.35″, the air layer of 0.27″, and the mid acrylic layer of 0.22″ were the respective measurements taken on a test window, the results of such testing to be discussed later. A limitation of the prior art window of  FIG. 3A  is the level of noise transmitted through its panes. The embodiments of the present invention will be compared to the prior art  FIG. 3A , also known as the baseline window or layup.  
         [0028]     Turning now to the operative workings of the present invention,  FIGS. 3B-3E  depict various window layer configurations. Before detailed discussion of  FIGS. 3B-3E , it should be noted that the cross-sectional views depict a c-ring  24  and a rubber seal  102  on each side of the window. Although depicted as such, each c-ring  24  and rubber seal  102  is actually a single continuous piece of material that traverses the entire periphery of the laminate-formed window  22 . Additionally, while the rubber seal  102  is shown abutting the c-ring  24  in places, a gap of about 0.03″ to more than 0.1″ between the rubber seal and c-ring  24  may exist in some applications. An oval-shaped window and a rectangular-shaped window with rounded corners are examples of windows according to embodiments of the present invention; however, the invention is not limited to such shapes and other window shapes may be utilized.  
         [0029]      FIG. 3B  depicts a cross-sectional view of an aircraft window  100  according to a first embodiment of the present invention. The aircraft window  100  has multiple transparent layers sandwiched between a c-ring  24  that forms a frame that is lined with a rubber seal  102 . The layers of material are held in place by a retainer clip  25 , and not necessarily any force provided by the c-ring  24 . Throughout the embodiments of the invention, acrylic is used as an example of a transparent material used as panes in the window; however, the acrylic could be any suitable transparent plastic, for example, polycarbonate. The arrangement of transparent layers from the aircraft fuselage exterior  104  to the aircraft fuselage interior  106  is: a first, outer acrylic layer  112 , an air space or layer  110 , and a second, inner acrylic layer  108 . In the aircraft industry, the inner layer is sometimes referred to as a mid-layer. For the purposes of testing with regard to the present invention according to  FIG. 3B , the outer acrylic layer  112  is 0.51″, the air layer  110  is 0.27″, and the inner acrylic layer  108  is 0.22″. The layers  108 ,  110 ,  112  are secured within the c-ring  24  and against the rubber seal  102 . The layers  108 ,  110 ,  112  are designed such that noise waves traveling in the path indicated by arrow  114 , may either be attenuated to some degree or completely stopped before reaching the inside area  106 . More specific advantages of the first embodiment, in terms of the broadband frequency response, will be discussed later.  
         [0030]     The rubber seal  102  provides damping of vibration in both the outer pane  112  and middle pane  108 . This reduces the noise transmitted through the transparent area  110 . It also minimizes vibration, which originates as noise outside of the fuselage  12 , from passing from the transparent layers of material into the c-ring  24  and subsequently into the fuselage interior  106 . When a layer of window material protrudes into the rubber seal  102 , the advantage is that the more rubber that is able to protrude around and contact the individual layers of material  108 ,  112 , the more noise and vibration dampening the rubber is able to provide to the respective layer of material. That is, for vibrations that propagate to the edge of the material, the rubber seal  102  may dampen such vibrations since the rubber seal  102  contacts the edge of the material. Such a path through the c-ring  24 , into the rubber seal  102 , into the outer acrylic  112  and into the rubber seal  102  is noted by arrow  111 . Because the rubber seal  102  is arranged in such a fashion, dampening of noise and vibration may occur.  
         [0031]     Another path of noise propagation, from the aircraft exterior  104  to the rubber seal  102  is noted by arrow  116 . The rubber seal  102  lies within the c-ring  24 . The flange  30  and web  32  provide support to the rubber seal  102 , which helps secure the window layers  108 ,  110 ,  112 . The advantage of the window  100  of the first embodiment over the baseline window of  FIG. 3A , is that increased sound dampening is achieved, at least because of its thicker outer layer  112  and its greater edge amount that abuts against the rubber seal  102 . By dampening or eliminating noise, passenger comfort inside the aircraft is increased. In the case of noise path  116  the noise may propagate through the outer layer  112 , but will then be partially or completely dampened by the rubber seal  102 .  
         [0032]     Turning now to  FIG. 3C , a second embodiment of the present invention will be explained. The window  120  of the second embodiment has an increased number of layers, from three to four, over the second embodiment window  100 . The layers and as-tested thicknesses of the second embodiment,  FIG. 3C , from the fuselage exterior  104  to the fuselage interior  106  are: an acrylic layer  122  that is 0.35″ thick, a glass layer  124  that is 0.025″ thick, an air layer  126  that is 0.27″ thick, and a second acrylic layer  128  that is 0.22″ thick. An advantage of the second embodiment window  120  over the first embodiment window  100  is a decrease in broadband frequency response over some frequencies and very similar responses over the balance of frequencies, which will be discussed later. In short, there is increased sound dampening with the second embodiment.  
         [0033]     Further comparing the first and second embodiments, one can see that the window  100  has a 0.51″ thick outer acrylic pane, while the window  120  has an outer acrylic pane  122  that is 0.35″ thick and a glass pane  124  that is 0.025″ thick. The advantage is that the combination of these latter two panes, for a total thickness of 0.375″, provides the same amount of structural stiffness as the first embodiment acrylic pane that is 0.51″ thick. The overall difference in window thickness is 0.135″, so the window  120  is thinner and provides comparable noise reduction as the first embodiment, as will be discussed later. Furthermore, because the window  120  of the second embodiment maintains the same level of structural stiffness and integrity as the first embodiment  100 , the decreased thickness is an advantage.  
         [0034]      FIG. 3D  depicts a window  140  of a third embodiment of the present invention. The third embodiment window  140  has five transparent layers situated adjacent to one another to form the see-through area of the window. From the fuselage exterior  104  to the fuselage interior  106 , specific thickness of the transparent layers were tested for their sound dampening and structural advantages. The layers tested consisted of an acrylic layer  142  that is 0.22″ thick, a urethane layer  144  that is 0.05″ thick, a glass layer  146  that is 0.12″ thick, an air layer  148  that is 0.27″ thick, and a second acrylic layer  150  that is 0.22″ thick. As in the prior embodiments, these layers were placed between a rubber seal  102 , which surrounded the layers and abutted against the inside perimeter of the c-ring  24 . The rubber seal  102  fits within the outside flange  30  and the web  32 . At specific frequencies, this window  140  provides sound dampening advantages over the prior embodiments, as will be discussed later. Urethane, such as the urethane used in this embodiment, is generally a material whose properties are dependent upon time, temperature, and frequency. Additionally, as an interlayer material, instead of urethane, a vinyl or silicon material could be used to bond its adjacent materials and provide sound dampening advantages. Furthermore, for the present embodiment, urethane typically possesses a loss factor around 0.06, but with a constant modulus of say, 1000 psi, and a constant overall damping ratio. As previously stated, and applicable to each embodiment, the rubber seal may actually form a slight gap with the c-ring  24 , as opposed to the rubber seal firmly abutting the c-ring  25 , since the retainer clip  25  secures the window layers.  
         [0035]      FIG. 3E  depicts a cross-sectional view of a window  160  according to a fourth embodiment of the present invention. Like the third embodiment, the fourth embodiment window  160  also has five layers of transparent material that entail the layup structure. From the fuselage exterior  104  to the fuselage interior  106 , the layers are: an acrylic layer  162  that is 0.22″ thick, a viscous layer  164  that is 0.05″ thick, a glass layer  166  that is 0.12″ thick, an air layer  168  that is 0.27″ thick, and a second, inner acrylic layer  170  that is 0.22″ thick. Like the other embodiments, the transparent layers are situated between a rubber seal  102 , which surrounds and abuts against the layers. The rubber seal  102  is then mounted against the inside perimeter of the c-ring  24  within an area bounded by the web  32  and the outside flange  30 . As each of the prior embodiments, the fifth embodiment window also provides advantages related to its sound dampening characteristics.  
         [0036]     Concerning the visco-elastic material used in the present invention, it is a material that exhibits a high damping loss factor, generally greater than one (“1.0”)—and generally possesses a low modulus when compared to metal. When used in the embodiments of the present invention, a visco-elastic material is one in which shear strains due to deflections (e.g. vibrations) are converted to heat, which serves as a loss or damping mechanism.  
         [0037]     Before turning to the advantages of the above structures, an explanation of the evaluation parameters applied to the embodiments of the present invention will be provided. Power Spectral Density (PSD) was the means used to measure and evaluate the sound dampening characteristics of the various structures. PSD is the amount of power per unit (density) of frequency (spectral) as a function of the frequency and describes how the power (or variance) of a time series is distributed with frequency, that is PSD dictates which frequencies contain a signal&#39;s power. Mathematically, it is defined as the Fourier Transform of the autocorrelation sequence of the time series. An equivalent definition of PSD is the squared modulus of the Fourier transform of the time series, scaled by a proper constant term. Being power per unit of frequency, the dimensions are those of a power divided by Herz.  
         [0038]     Now,  FIGS. 4 through 7  will be used to explain the operative workings, performances and advantages of the various embodiments.  FIG. 4  is a graph of the average velocity power spectral density (PSD) over a broadband frequency for the window configurations according to the teachings of the present invention. The results of  FIG. 4  are based upon testing of an isolated window, that is, one window in isolation, using the finite element method (FEM).  FIG. 4  reveals that the baseline model of  FIG. 3A  of the prior art has the highest velocity PSD for most of the frequency band, and the window with a visco-layer in it has the lowest velocity PSD response. The comparative results with respect to the baseline window reveals that the average velocity PSD reduction of the window with the visco-material layer is significant (about 11.3 dB at 160 Hz), see  FIG. 5 . This is due to the improvement of modal damping and the higher stiffness of the window layup, provided by the c-ring  24 .  
         [0039]     As can be seen from  FIG. 4 , the fourth embodiment window  160  of  FIG. 3E  having in part, a 0.22″ acrylic layer  162 , a 0.05″ viscous material layer  164 , and a 0.12″ glass layer  166 , all adjacent the fuselage exterior, dampens the vibration levels across the broadest frequency range most effectively. Also depicted in  FIG. 4  is that the third embodiment window  140  also achieves a high level of dampening the vibration levels across a broad frequency range. The third embodiment window  140 , depicted in  FIG. 3D , has in part, adjacent the fuselage exterior, a 0.22″ acrylic layer  142 , a 0.05″ urethane material layer  144 , and a 0.12″ glass layer  146 . According to the graphical results of  FIG. 4 , while the fourth embodiment window  140  and fifth embodiment window  160  of  FIGS. 3D and 3E , respectively, display excellent vibration reduction across a broad range of frequencies, relative to the other embodiments, the first embodiment window  100  of  FIG. 3B  displays excellent vibration reduction for a narrow frequency range, approximately 300-390 herz. The first embodiment window  100  employs a 0.51″ thick piece of acrylic adjacent the fuselage exterior  104 . As  FIG. 4  depicts, for almost every frequency in the isolated window tests involving different compositions of the outer glass, the embodiments of the present invention performed better than that of the existing baseline window. For the purpose of the present invention, the term “outer glass” is known as those layers of material that lie between the 0.27″ airspace of the window and the fuselage exterior  104 . Therefore, the “outer glass” may have more than one layer of material.  
         [0040]      FIG. 5  is a comparison graph that depicts the vibration reduction, in decibels (dB), achieved with the fourth embodiment window  160  having an outer pane of 0.22″ acrylic, 0.05″ visco-material, and 0.12″ glass. This particular window, known as the “visco-elastic window”, is depicted in  FIG. 3E . The vibration reduction is calculated using the following formula:
 Reduction (dB)=20 log( X   2   /X   1 ) 
 where X 1  is the velocity PSD of the prior art window of  FIG. 3A  and X 2  is the velocity PSD of the fourth embodiment window  160 . Although the calculation was performed in terms of structural velocity, it is assumed that the normal velocity of the window is directly proportional to the radiated acoustic pressure. Therefore, the noise reduction associated with the fourth embodiment window is also represented by  FIG. 5 . 
 
         [0041]     The results depicted in  FIG. 5  are relative to the baseline window of  FIG. 3A . As an example, the fourth embodiment visco-elastic window dampens 6 decibels more at 200 Hz than the known window of  FIG. 3A . As depicted, for most of the broadband range from approximately 20 Hz to 1,400 Hz, the visco-elastic window  160  of the fourth embodiment responds much more favorably, in terms of dampening vibration, and reducing noise transmission, than the baseline window. In fact, the visco-elastic window analysis of  FIG. 5  depicts the most advantageous vibration reduction below 250 Hz and above 700 Hz.  
         [0042]     In order to improve the benefit above 250 Hz, the present invention introduces a vacuum layer between the outer and middle panes. The effect of evacuating the air from between the two panes effectively decouples the panes over a broad frequency range. The vacuum layer, if utilized, in all embodiments may be either a full or partial evacuation of gas from between the middle and outer panes. Without the vacuum, when the outer pane deflects or vibrates during aircraft flight, it causes the air between the middle and the outer pane to act as a spring and is a medium to transmit vibration noise energy by compressing and expanding accordingly. This exerts a force on the middle pane and causes it to vibrate and transmit noise into the passenger cabin. When the panes are decoupled by a vacuum layer, the transmission of noise energy is effectively decoupled and lessened. There is, however, vibration energy transmitted through the boundary of the window layer panes in the area of the rubber seal.  
         [0043]     Turning to  FIG. 6 , results of testing using the finite element method on an isolated window having a vacuum layer between the outer acrylic and mid-acrylic layers reveals that a window with a vacuum between physical panes has a great advantage over other windows with respect to sound dampening. The advantage is attributed to the result of sound not transmitting through a vacuum. Referring to the window layup of  FIG. 3E , the air layer was made into a vacuum layer. By examining the dashed curves of  FIG. 6 , which are analysis performed on a 787 model aircraft window, a dramatic reduction in noise response is depicted. The dashed plots of FIG.  6  are the results of analysis performed on an isolated window model, while the solid plots are the resulting responses of a full window belt model, that is, a non-isolated window. In actuality, the full window belt model consisted of a three-bay group of windows.  
         [0044]     It was expected that the 787 window with a vacuum layer between the panes would provide an advantage over other windows. This is evident looking at the dashed plots of  FIG. 6 , where the 787 isolated model with vacuum depicts a response that is more desirable than its non-vacuum counterpart. However, when the vacuum layer was incorporated into a full window belt model, the results depicted by the solid plots were obtained. The benefit due to the vacuum was reduced, particularly at low frequencies. In an attempt to reproduce the performance of the 787 isolated model with vacuum in the window belt model, a stiffer c-ring was investigated. The effects of a stiffer c-ring is depicted in  FIG. 7 .  
         [0045]     Further investigation and testing of an isolated window model reveals that the primary path of vibration from the outer pane to the middle pane is through the rubber seal. Further, the majority of the vibration is absorbed by the outer pane boundary, but vibration propagates more efficiently to the middle pane through the c-ring.  FIG. 7  depicts the results of a stiffer c-ring, such that the stiffer c-ring is effective in further decoupling the middle and outer panes, as desired. These results indicate that although a vacuum between window panes is effective for reducing vibration on the middle pane, stiffening the window frame c-ring provides additional vibration and sound reduction benefits.  
         [0046]      FIG. 7  depicts results for a 787 full model, a 787 full model with a vacuum layer between the outer and middle panes, and a 787 model with a vacuum layer and a stiffened c-ring. From  FIG. 7 , the solid plots are examples of full beltline models, again three bay models, whereas the dashed plot indicates testing on what is essentially an isolated window. Although it is a full model, since the c-ring is stiffened in the finite element analysis, each window in the model is further isolated, which results in a more favorable, that is, increased dampening, response.  
         [0047]     While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.