Abstract:
A noise attenuator for attaching a side wall panel to a frame that is exposed to vibratory loads and which includes a rigid core having a sleeve with two radially disposed end plates. A bracket encircles a portion of said sleeve between the end plates and is in non-contiguous relationship with the core. An elastomeric bushing is bonded to a portion of the bracket and the core with the exception of the outer faces of the end plates, which remain exposed. Voids are passed through the rear plate and extend axially between the bracket and the sleeve some length into the sleeve. Due to the geometry of the unit, the non-voided sections of the bushing are loaded in either a combination of tension and shear or compression and shear when the core is connected to a vibrating frame and the bracket is connected to a side wall panel.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to a side panel noise attenuator for reducing the transmission of noise producing vibrations from a supporting frame or frame member to a side wall panel that is mounted upon the supporting frame or frame member. 
     BACKGROUND OF THE INVENTION 
     Elastomeric attenuators have been used for some time for joining wall panels to supporting frames that are subject to vibratory loads in an effort to reduce the amount of noise producing vibrations that are transmitted to the panel. One such application involves the mounting of the interior wall panels of an aircraft to the superstructure of the aircraft. There is an increased demand for reduced cabin noise levels and reduced aircraft weight. Accordingly, there is now a need for more compliant attenuator units that can replace those presently in use, but without having to increase the size and weight of these units. 
     Cylindrical elastomeric shock isolators have also been used for some time in the automobile industry to reduce the effect of road-induced load upon a vehicle&#39;s suspension system. Typically, these devices involve an inner tubular sleeve and an outer tubular sleeve with the sleeves being superimposed one over the other along a common longitudinal axis. An elastomeric bushing is mounted in the space between the two sleeves and the bushing in most cases is bonded to one or both sleeves. As explained in greater detail in U.S. Pat. No. 6,446,993, this type of bushing is generally mounted between the suspension system of the vehicle and its frame with the outer sleeve press fitted in the suspension system and the inner sleeve being secured to the frame by some type of fastener. A pair of opposed voids are sometimes placed longitudinally in the bushing to soften the spring rate of the bushing along the reaction axis of the system along which road induced forces are transmitted back into the suspension system. Forces that are transmitted into the bushing due to steering inputs act perpendicular to the reaction axis and are attenuated in the unvoided areas of the bushing, which have an increased spring rate thus providing the driver with a better feel of the road. 
     Although the above-described voided elastomeric isolators, as used in the automotive industry perform quite well to reduce the effect of road induced forces, these devices do not perform as well when dealing with vibratory induced noise and, in particular, noise in the mid and high range frequencies. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to improve noise attenuators and, in particular, to improve attenuator units dealing with noise produced by a vibratory load. 
     A further object of the present invention is to provide a voided elastomeric noise attenuator that more effectively reduces the transmission of noise produced by vibratory loads, particularly at the mid and high range frequencies. 
     A still further object of the present invention is to provide a voided elastomeric noise attenuator for reducing the amount of noise transmitted from a support frame that is subjected to vibratory load into a side wall panel that is mounted upon the frame. 
     Another object of the present invention is to reduce the amount of noise transmitted to an interior side wall panel of an aircraft mounted upon the airframe of an aircraft. 
     These and other objects of the present invention are attained according to one version by a noise attenuator for attaching a wall panel to a supporting frame that is subjected to noise producing vibrations. The attenuator contains a rigid core element having an elongated tubular sleeve and front and rear end plates radially extended from each end of the sleeve. A bracket encircles a portion of the sleeve between the end plates and is in non-contiguous relationship with the core element. An elastomeric bushing is bonded to both the core element and a portion of the bracket such that the bushing fills the space between the end plates. A series of voids are passed longitudinally through one of the end plates and extend into the bushing between the bracket and the sleeve. The core element is affixed to the frame and the bracket to the side panel. 
     According to another version, a noise attenuator is provided that comprises a rigid core that further includes a tubular sleeve containing a front face plate that is secured to one end of the sleeve and a rear face plate that is secured to the opposite side of the sleeve. The front and rear face plates extend radially beyond the sleeve to establish a space therebetween. An elastomeric bushing surrounds the sleeve and fills the space between the end places. A series of circumferentially spaced voids pass through one end of the end plates and extend substantially through the bushing, the attenuator further including mounting means for connecting the core to a first member and the bushing to a second member such that noise producing vibrations in one member are attenuated before said vibrations reach the second member. 
     According to one variation, four voids are equally spaced about the sleeve with a first pair of opposed voids being axially aligned along a first load axis of the attenuator and a second pair of opposed voids axially aligned along a second load axis normal to the first load axis. 
     According to yet another version, there is provided a method for manufacturing a noise attenuator, the method comprising the steps of providing a rigid core that includes an elongated tubular sleeve having respective plates at opposite ends of the sleeve, each plate having an end face that is normal to the longitudinal axis of the sleeve. An elastomeric bushing is provided that is bonded to the rigid core and to a bracket that encircles the sleeve in which the bushing fills the space between the end plates. A series of voids are created that longitudinally pass through at least one of the plates and axially substantially through the bushing. One of the plates is secured against one of a first member or a second member subjected to vibratory loads and the bracket is secured to the other of the first or second member. 
     These and other objects as well as features and advantage aspects will be discussed in the following Detailed Description, which should be read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a bracket for supporting an attenuator unit that embodies one version of the present invention; 
         FIG. 2  is a front view of the bracket shown in  FIG. 1  having a noise attenuator that according to one embodiment is mounted therein; 
         FIG. 3  is a rear view of the bracket assembly shown in  FIG. 2 ; 
         FIG. 4  is a side elevation of the bracket assembly shown in  FIG. 2 ; 
         FIG. 5  is a sectional view taken along lines  5 - 5  in  FIG. 2  further illustrating a noise attenuator according to the present invention; 
         FIG. 6  is a side view of a bracket assembly similar to that shown in  FIG. 2  illustrating a noise attenuator unit being employed to support a side panel upon a frame that is subject to noise producing vibrations; 
         FIG. 7  is a side view illustrating a bracket assembly similar to that shown in  FIG. 2  mounted in a four-pole test stand in a free configuration; 
         FIG. 8  is a view similar to  FIG. 7 , illustrating the bracket assembly mounted in the test stand in a blocked configuration; and 
         FIG. 9  is a graphic representation showing noise attenuation plotted against input frequency in which the response of the present unit is compared to that of a prior art unit. 
     
    
    
     DETAILED DESCRIPTION 
     The following relates to an exemplary embodiment for a noise attenuator for reducing the transmission of noise producing vibrations from a support frame to a side wall panel mounted upon the frame. Certain terms are used throughout in order to provide a sufficient frame of reference with regard to the accompanying drawings. These terms, however, are not intended to be limiting of the present invention, as claimed herein, except where so specifically indicated. 
     Referring initially to  FIGS. 2-6 , there is illustrated a noise isolation assembly or attenuator unit, generally referenced  10 , that is configured to attenuate vibratory induced noises over a wide frequency range of between 100 Hz and 10,000 Hz. In this exemplary embodiment a single attenuator unit  10  is illustrated, the unit being mounted in a single bracket referenced  12 . It should be understood, however, that one or more attenuator units may be mounted within a single bracket without departing from the teachings of the present invention and therefore the present embodiment is intended to be exemplary. 
       FIG. 1  illustrates the geometry of the bracket  12  with the attenuator unit removed. The bracket  12  is fabricated of a rigid material, such as metal or a high strength plastic, that can withstand high loads without deforming or breaking. The bracket  12  includes a central hole  13  sized for accepting the attenuator unit  10 . Also included in the bracket  12  are two mounting holes  14  at opposing ends thereof for mounting the bracket against a flat surface (not shown). 
     Turning more specifically to  FIG. 5 , an attenuator unit  10  is shown in greater detail. The attenuator unit  10  contains a central core element, generally referenced  35 , which preferably is fabricated from metal or other rigid material. The core element  35 , in turn, includes a tubular sleeve  36  and a pair of opposed radially extended end plates  37  and  38  located at either end of the sleeve. The end plates  37 ,  38  according to this embodiment are circular in form and are coaxially aligned with the longitudinal axis  40  of the sleeve  36 . Other forms are possible provided they cover the sleeve ends. The front end plate  37  has a diameter that is slightly less than the diameter of the center bracket hole  13  and thus allows the front end plate of the core element  35  to pass through the hole in the bracket  12 . 
     The attenuator unit  10  may be fabricated in a molding fixture (not shown) that allows the core element  35  of each unit to be passed through the center hole  13  in the bracket  12  wherein the bracket is positioned between the two end plates  37 ,  38  of the core element  35 . The thickness of the bracket wall  15  is less than the longitudinal inside distance between the end plates  37 ,  38  and thus encircles only a portion of the sleeve  36 . As shown in  FIG. 5 , the bracket  12  is located within the fixture so that the receiving hole  13  in the bracket is coaxially aligned with the longitudinal axis  40  of the core element  35 . The mold cavity is then closed and an elastomeric bushing  44  is molded so that it encapsulates the core element  35  and the section of the bracket  12  that surrounds the hole  13 . As molded, according to this specific embodiment, the elastomeric bushing  44  is cylindrical in form with the center of the bushing being coaxially aligned with that of the core element  35 . In addition, the opposed outer faces of the bushing  44  are in coplanar alignment with the outer faces  42  and  43  of the two end plates  37 ,  38  so that the faces of the end plates are exposed when the bracket  12  is removed from the mold. 
     A series of circumferentially spaced voids  49 - 49  are formed in the attenuator unit  10 . In this particular embodiment, four voids at 90° intervals are placed around the sleeve  36 . Each void passes through the rear end plate  38  of the core element  35  and extends axially substantially through the entire width of the elastomeric bushing  44  passing between the bracket  12  and the sleeve  36 . The voids  49  in this embodiment are generally arcuate shaped with two of the voids being centered upon the vertical loading axis  50 ,  FIG. 3 , of the unit  10  and two other voids being centered upon the horizontal loading axis  51 ,  FIG. 3 , of the unit. Although four voids  49  are employed in the present embodiment, more or less voids might be utilized without departing from the teachings of the invention. The voids included in the rear end plate  38  facilitate molding. Accordingly, the stiffness in the voided areas in the elastomeric bushing  44  is considerably reduced thus reducing the amount of noise that is transmitted through the bushing when the unit  10  is experiencing vibratory loads. It should be further noted the bushing  44  in the non-voided regions is constrained between the two end plates  37 ,  38  of the core element  35 . Exerting a vibratory load upon the attenuator unit  10  along the load axis thus causes the non-voided regions of the bushing  44  to be placed under either a combined compression and shear stress or a combined tension and shear stress depending upon the direction of the vibration input. Tests have proven that by establishing this type of compound stress in the bushing  44 , a further increase in attenuation can be realized. 
       FIG. 6  illustrates the noise isolation assembly  10  supporting a side wall panel  57 , shown partially, of an aircraft upon the frame  58 , which is also partially shown and part of the aircraft&#39;s superstructure. As noted above, the bracket  12  contains a noise attenuator unit  10  that embodies the teachings of the present invention which acts in concert to reduce the amount of noise that is transferred from the frame  58  to the side wall panel  57 . The bracket base is seated tightly against the panel  57  and is secured in place using a pair of threaded fasteners  62  that pass into engagement with the panel  57 . Although threaded fasteners are employed in this embodiment, it should be understood that any other suitable fastener that is capable of securing the bracket to the panel under the expected load condition may also be used without departing from the teachings of the invention. The exposed face of the front end plate  37  of the attenuator core  35  is seated securely against the frame  58  of the aircraft and the attenuator unit  10  is secured to the aircraft frame using a threaded fastener  65 . The joint that is established between the attenuator unit  10  and the frame  58  is tight enough so that the attenuator unit moves in unison with the frame as the frame is caused to vibrate. 
     A test stand was constructed to investigate the noise isolation characteristics of the present attenuator unit  10 . The test stand was designed to employ the well known four-pole method of measurement, which provides more accurate data concerning noise attenuation when compared to the more classic mass-spring-damper test method, particularly when dealing with vibratory loads in the mid and high frequency ranges. 
     The test stand, generally referenced  70 , is illustrated in  FIGS. 7 and 8 . The test stand includes an electro-dynamic shaker or vibratory head  71  that is connected to a mounting head  72  by an arm  73 . The shaker  71  imparts a sinusoidal input to the mounting head  72  at desired frequencies in a range of frequencies between about 100 Hz and 10,000 Hz. A mounting block  75  is situated adjacent the mounting head  72  and is stationarily supported upon a substrate  77 , as shown in  FIG. 8 . 
     Bracket  12  illustrated in  FIGS. 1-6  is shown secured to the mounting block  75 . An attenuator unit  10  molded in the bracket  12  is secured to the mounting head  72  by a bolt  81 . 
     The test fixture  70  illustrated in  FIG. 7  is configured to conduct what is generally referred to as a free measurement test. For this test, an accelerometer  84  is secured to the mounting head  72  and a second accelerometer  86  is secured to one side of the bracket  12 . Accordingly, the input acceleration A 1  and the output acceleration A 2  can be recorded when the bracket  12  is free or in an unrestrained posture apart from the mounting block  75 . 
       FIG. 8  illustrates the test stand  70  configured in a blocked condition wherein the bracket  12  is held immobile upon the mounting block  75 . In the blocked position and as shown in this figure, the mounting block  75  is fixed to a load sensor unit  88  that is secured in place to the substrate  77 . The load sensor unit  88  provides a readout of the total force F 2  at the output side of the system at  91 . 
     The aircraft side wall panel mounting arrangement illustrated in  FIG. 6  establishes a linear mechanical system. As such the four-pole test method that uses frequency dependant quantities of acceleration and force can be used to find the transfer matrix T of an attenuator system, such as that illustrated in  FIG. 6 . The vibratory frame input creates both an acceleration A 1  and a force F 1  upon the core rigid element. The input acceleration and the input force are transformed by the attenuator system to an output acceleration A 2  and an output force F 2  due to the transfer matrix T of the attenuator system. As will be explained below, the four-pole test stand allows one to find the attenuation A 1 /A 2  of the system by testing the attenuator bracket in both a blocked and unblocked condition and thus enables a determination to be made as to the effectiveness of the attenuator system. 
     For a linear mechanical attenuator system, such as that illustrated in  FIG. 6  that is subjected to vibratory input load: 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             F 
                             1 
                           
                         
                       
                       
                         
                           
                             A 
                             1 
                           
                         
                       
                     
                     ) 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               T 
                               11 
                             
                           
                           
                             
                               T 
                               12 
                             
                           
                         
                         
                           
                             
                               T 
                               21 
                             
                           
                           
                             
                               T 
                               22 
                             
                           
                         
                       
                       ] 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             
                               F 
                               2 
                             
                           
                         
                         
                           
                             
                               A 
                               2 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where the transfer matrix is broken into four acceleration and force components. The vibrational responses for the input acceleration and input force can be expressed as follows:
 
 F   1   =F   2   T   11   +A   2   T   12    (2)
 
 A   1   =F   2   T   21   +A   2   T   22    (3)
 
     The vibrational responses of the four-pole arrangement described in equation (1) can be solved by the following equations: 
     
       
         
           
             
               
                 
                   
                     T 
                     11 
                   
                   = 
                   
                     
                       
                         F 
                         1 
                       
                       
                         F 
                         2 
                       
                     
                     ⁢ 
                     
                       | 
                       
                         
                           A 
                           2 
                         
                         = 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     T 
                     12 
                   
                   = 
                   
                     
                       
                         F 
                         1 
                       
                       
                         A 
                         2 
                       
                     
                     ⁢ 
                     
                       | 
                       
                         
                           F 
                           2 
                         
                         = 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   
                     T 
                     21 
                   
                   = 
                   
                     
                       
                         A 
                         1 
                       
                       
                         F 
                         2 
                       
                     
                     ⁢ 
                     
                       | 
                       
                         
                           A 
                           2 
                         
                         = 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
             
               
                 
                   
                     T 
                     22 
                   
                   = 
                   
                     
                       
                         A 
                         1 
                       
                       
                         A 
                         2 
                       
                     
                     ⁢ 
                     
                       | 
                       
                         
                           F 
                           2 
                         
                         = 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     As noted above, the subscript A 2 =0 indicates output terminal pairs are measured in the blocked condition and the subscript F 2 =0 indicates they are measured in the unblocked or free state. It can be further assumed that the force and acceleration transmissibilities are equal such that T 22 =T 12 . 
     Attenuation can now be solved as follows: 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       1 
                     
                     
                       A 
                       2 
                     
                   
                   = 
                   
                     1 
                     
                       [ 
                       
                         
                           T 
                           22 
                         
                         + 
                         
                           ( 
                           
                             
                               T 
                               21 
                             
                             
                               C 
                               2 
                             
                           
                           ) 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     where C 2  is the below mount drive point accelerance. 
     The below mount drive point accelerance is typically measured independently on the side panel of interest. 
     As noted above, classical mass-spring-damper models do not accurately predict noise attenuation of isolators in the mid and higher range frequencies. The four-pole method, however, provides a means to better measure an isolators response over a wide range of frequencies. Tests have shown a 15 dB improvement or more in noise attenuation is realized when compared to most noise attenuators that are presently in use having the same size envelope without adversely affecting the attenuators load carrying capability. The increase in attenuation of the present device over the prior art devices is shown graphically in  FIG. 9  wherein the solid line curve  100  represents the attenuation of the present invention and the dotted line curve  101  represents a typical prior art unit. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Parts List for FIGS. 1-9 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 noise isolation assembly or attenuator unit 
               
               
                 12 
                 bracket 
               
               
                 13 
                 central hole 
               
               
                 14 
                 mounting holes 
               
               
                 15 
                 bracket wall 
               
               
                 35 
                 core element 
               
               
                 36 
                 sleeve, tubular 
               
               
                 37 
                 front end plate 
               
               
                 38 
                 rear end plate 
               
               
                 40 
                 longitudinal axis, sleeve 
               
               
                 42 
                 outer face 
               
               
                 43 
                 outer face 
               
               
                 44 
                 elastomeric bushing 
               
               
                 49-49 
                 voids 
               
               
                 50 
                 axis 
               
               
                 51 
                 axis 
               
               
                 57 
                 side wall panel 
               
               
                 58 
                 frame 
               
               
                 62 
                 threaded fasteners 
               
               
                 65 
                 threaded fastener 
               
               
                 70 
                 test stand 
               
               
                 71 
                 vibratory head or electro-dynamic shaker 
               
               
                 72 
                 mounting head 
               
               
                 73 
                 arm 
               
               
                 75 
                 mounting block 
               
               
                 77 
                 substrate 
               
               
                 81 
                 bolt 
               
               
                 84 
                 accelerometer 
               
               
                 86 
                 accelerometer 
               
               
                 88 
                 load sensor unit 
               
               
                 91 
                 output side, system 
               
               
                 100 
                 curve 
               
               
                 101 
                 curve 
               
               
                   
               
             
          
         
       
     
     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof to adapt to particular situations without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope and spirit of the appended claims.