Patent Publication Number: US-6658134-B1

Title: Shock improvement for an electroacoustic transducer

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a transducer and, more particularly, to a shock resistant transducer particularly suitable for hearing aids. 
     BACKGROUND OF THE INVENTION 
     Transducers are particularly useful in hearing aids. The transducer may be used as a microphone to convert acoustic energy into electrical energy or as a receiver to convert electrical energy into acoustic energy. Typical transducers suitable for hearing aids comprise a coil having a first air gap, a magnetic member having a second air gap and an armature with an armature leg that extends through both of the air gaps. A diaphragm connects to the armature leg. 
     The operation of the transducer follows. Vibrations of the diaphragm are transmitted to the armature leg, and the vibrating armature leg causes an electric alternating electric current in the coil. Conversely, an alternating current supplied to the coil causes a vibration of the armature leg, which is transmitted to the diaphragm. Under normal conditions the vibrations of the armature leg are relatively small displacements. In extreme cases, however, the armature leg may deflect a large amount and touch the magnetic member. 
     One problem with the conventional transducers is that a shock or impact load exerted on the transducer may cause plastic deformation of the armature leg. For example, when the transducer falls and contacts a solid object, the armature leg deflects or bends so far that undesirable plastic deformation can occur in the armature leg. Once the armature leg is plastically deformed such that it is closer to one side of the magnetic member than the other in a steady-state condition, the transducer does not function properly. 
     Some conventional transducers have attempted to address this shock problem. For example, Knowles Electronics, Inc. produces a transducer (e.g. Model ED1913) with deformations on a central portion of the armature leg that is positioned within the air gap of the coil. When the Knowles transducer suffers a shock, the armature leg deflects until the deformations contact the surface of the coil, thus limiting the freedom of movement of the armature leg. One example of the Knowles transducer is generally disclosed in U.S. Pat. No. 5,647,013. Another example of a conventional transducer with shock resistance is produced by the assignee of the present applicant Microtronic BNV. The Microtronic transducer (2300 series) has a rotated coil with respect to the magnetic member. This rotation forms a stop for the armature leg to inhibit excessive bending of the armature leg in the occurrence of a shock. One example of the Microtronic transducer is generally disclosed in European Patent Application No. 847,226. 
     One disadvantage of the above transducers is that the shock resistance, though improved, does not meet the increasing shock standards of the hearing aid industry. Furthermore, especially for the Knowles transducer, special and/or additional parts must be used to provide the shock resistance which increase the expense of the transducer. 
     It is a general object of the present invention to solve the above problems. More particularly, there is desired a transducer with superior shock resistance, and which can be easily assembled from standard parts at a low cost. 
     SUMMARY OF THE INVENTION 
     According one aspect of the present invention, there is provided a transducer comprising a coil having a first air gap, a magnetic member having a second air gap and an armature. The armature includes an armature leg extending through the first air gap and the second air gap. The armature leg is capable of movement within the air gaps. The magnet member has at least one nub extending into the second air gap that limits the range of motion of the armature leg to inhibit large deflections of the armature leg and plastic deformation. The nubs may be comprised of a drop of adhesive. 
     In another aspect of the present invention, there is provided a transducer suitable for hearing aids comprising a coil having a first air gap, a magnetic assembly having a second air gap and an armature. The armature includes an armature leg that extends through both the first air gap and the second air gap. The armature leg is capable of movement within the second air gap. The magnetic assembly has a cushioning element secured to the magnetic assembly that extends into the second air gap. When the transducer is subjected to a shock, the movement of the armature leg is limited as it engages the cushioning element. Furthermore, the cushioning element may comprise a soft material to absorb a portion of an impact of the armature leg when the armature leg moves into contact with the cushioning element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The forgoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
     FIG. 1 is a cross-sectional side view of a shock resistant transducer according to one embodiment of the present invention; 
     FIG. 2 is a cross-sectional front view of the transducer of FIG. 1; 
     FIG. 3 is a perspective view of the transducer in FIG. 1; 
     FIG. 4 is a perspective view of the armature of the transducer in FIG. 1; 
     FIG. 5 is a schematic diagram of a mechanical shock test apparatus; and 
     FIG. 6 is a graph of shock resistance test results. 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Turning now to the drawings and referring initially to FIG. 1, there is depicted a longitudinal sectional view of a shock resistant transducer  10  according to the present invention. The transducer  10  comprises a magnetic member  12  and a coil  14 . In the illustrated embodiment, the magnetic member  12  comprises a magnet housing  16  and two spaced apart magnetic elements  18  and  20 . The coil  14  has a first air gap  22 . As depicted in FIG. 2, the cross section of the first air gap  22  is substantially rectangular; however, the first air gap may have a different cross sectional shape in other embodiments. The magnetic elements  18  and  20  define a second air gap  24 . The cross section of the second air gap  24  is substantially rectangular; however, the second air gap may have a different cross sectional shape in other embodiments. As shown in FIG. 1, the two air gaps  22  and  24  are substantially aligned with each other. When viewed in the cross section of FIG. 2, the edges of the rectangular first air gap are parallel to the respective edges of the rectangular second air gap  24 . In other embodiments, one of the air gaps may be rotated relative to the other air gap. When the rotated embodiment is viewed in the cross section, the edges of the rectangular first air gap are not parallel to the respective edges of the rectangular second air gap. 
     The transducer  10  further comprises an armature  26 . The armature  26 , as more completely illustrated in FIG. 4, is an E-shaped armature. In other embodiments, the armature may have a U-shape. In general, the E-shaped armature  26  has three legs  28 ,  30  and  32 , lying generally parallel with each other and interconnected at one end by a leg connecting part  34 . As illustrated in FIG. 3, the middle armature leg  30  is positioned within the two aligned air gaps  22  and  24  with the leg connecting part  34  being located on the side of coil  14 . The two outer armature legs  28  and  32  extend on the outer side along the coil  14  and the magnet housing  12 . Although not shown, the two outer armature legs  28  and  32  are affixed to the magnet housing  12 . The free end of the middle armature leg  30  is connected to a diaphragm with a connecting element (not shown). 
     The operation of the transducer  10  follows. When an electrical signal, originating from an amplifier (not shown) is supplied to the coil  14 , the middle armature leg  30  vibrates in cooperation with a magnetic field of the magnetic member  12 . The movement of vibration of the middle armature leg  30  is transmitted via the connecting element to the diaphragm, which causes sound vibrations. Conversely, sound vibrations vibrate the diaphragm causing the middle armature leg  30  to vibrate via the connecting element. This vibration generates an electrical signal in the coil  14 . The electrical signal may then be detected and processed accordingly. 
     Under normal conditions the vibrations of the armature leg are relatively small displacements. However, sometimes the transducer  10  may be subjected to a shock such as the result of an impact after a fall. The shock causes a large acceleration that is exerted on the middle armature leg  30 . The shock deflects the middle armature leg  30  further from its state of equilibrium and beyond the typical vibrations of normal operation. To prevent the middle armature leg  30  from striking the magnetic elements  18  and  20  and potentially becoming plastically deformed, the transducer  10  includes a pair of nubs  36  and  38  secured to the magnetic elements  18  and  20 . As illustrated in FIG. 2, the nubs  36  and  38  protrude into the second air gap  24  to inhibit an unduly large deflection of the middle armature leg  30 . The nubs  36  and  38  provide a nub air gap identified by “d” in FIG. 2 that is smaller than the second air gap  24 . 
     The nubs  36  and  38  provide shock resistance for the transducer  10  by inhibiting large deflections of the middle armature leg  30 . During a large shock, the middle armature leg  30  will deflect and potentially strike one of the nubs  36  or  38 . Without the nubs  36  and  38  during a shock, the middle armature leg  30  may deflect a large amount and possibly strike the magnetic element that may cause plastic deformation. The nubs  36  and  38  are positioned to limit the movement of the middle armature leg  30  to inhibit plastic deformation. 
     As depicted in FIG. 1, the nubs  36  and  38  are located on the magnetic elements  18  and  20  away from the free end of the middle armature leg  30  to allow freedom of movement of the middle armature leg  30  during normal operation of the transducer  10 . This positioning of the nubs  36  and  38  avoids the nubs  36  and  38  from rubbing the free end of the middle armature leg  30  during normal operation to ensure maximum output of the transducer  10 . Preferably, the nubs  36  and  38  are positioned at the coil end of the magnetic elements  18  and  20  to allow the free end of the middle armature leg  30  greater freedom of movement. This orientation of the nubs  36  and  38  also supports the middle armature leg  30  in the middle of its length during the shock. However, the nubs  36  and  38  may be positioned anywhere along the magnetic elements  18  and  20  such that the middle armature leg  30  has free movement during normal operation, but does not experience large deflections during shock. 
     As depicted in FIGS. 1 and 2, the nubs  36  and  38  are substantially symmetrically positioned around a longitudinal plane through the middle armature leg  30 . This longitudinal plane is perpendicular to the direction of the operational motion of the middle armature leg  30 . In other embodiments, the nubs may be asymmetrical to the longitudinal plane and have different orientations as long as the middle armature leg has freedom of movement in normal operation and large deflections of the middle armature leg are inhibited. In FIGS. 1 and 2, the nubs  36  and  38  have a rounded exterior (i.e. a drop shape). In other embodiments, the nubs may have a different shape. Although only one pair of nubs is illustrated, additional pairs of these nubs may be applied to the magnetic elements  18  and  20  to provide shock resistance. 
     In one embodiment, the nubs  36  and  38  comprise drops of UV-cured adhesive adhered to the magnetic elements  18  and  20 . In other embodiments, different materials secured to the magnetic elements may be used to meet the movement limiting function of the nubs  36  and  38 . Furthermore, the nubs  36  and  38  may be unitary with the magnetic elements  18  and  20  such as deformations on the surface of the magnetic elements  18  and  20 . 
     Not only do the nubs  36  and  38  limit large deflections of the middle armature leg  30 , but the nubs  36  and  38  may be configured to also cushion the middle armature leg  30  during shocks. In the cushioning embodiment, the nubs  36  and  38  comprise a softer material such as an elastoner, an epoxy, or a plastic. When the nubs are comprised of softer material, the nubs  36  and  38  may be considered a cushioning element. For cushioning, the approximate hardness for the material comprising the nubs  36  and  38  may be less than about Shore D 90. In some embodiments, the material comprising the nubs may be about Shore A 60. One example of a cushioning element is the Epoxy Technology UV-cured adhesive OG115 from Billerica, Mass. with a Shore D hardness of approximately 86 that tends to absorb shock. When the middle armature leg  30  deflects and strikes one of the cushioning elements or nubs  36 ,  38 , the cushioning element would absorb a portion of the impact of the middle armature leg  30 . The cushioning nature of the nubs  36  and  38  further inhibits plastic deformation and damage to the middle armature leg  30  providing greater shock resistance. 
     The nubs  36  and  38  of the present invention are easy to apply to the transducer  10 . In one embodiment, drops of adhesive are simply applied to the surface of the magnetic elements  18  and  20  prior to assembly of the transducer  10 . The present invention requires no additional parts, apart from these simple nubs. The transducer  10  may be easily assembled, and the armature may be adjusted with a rather high degree of accuracy. 
     The transducer  10  of the present invention also provides excellent shock resistance. Shock resistance tests were performed on several samples of the transducer  10  depicted in FIGS. 1-4 “Inventive” hereinafter). For the Inventive transducers, the middle armature leg  30  has a thickness of about 0.2 mm, and the second air gap  24  is approximately 0.35 mm. Drops of UV-cured adhesive from the Lord Corporation having a hardness of about Shore D 75 formed the nubs  36  and  38  on the magnetic elements  18  and  20 . The nubs  36  and  38  have a size that provides the nub air gap “d” between the tips of the nubs of approximately 0.26 to 0.27 mm. The nubs  36  and  38  have a diameter of approximately 0.5 mm. The nubs  36  and  38  are secured to the magnetic elements  18  and  20  with an edge of the nub&#39;s rounded exterior aligned with the end of the magnetic elements  18  and  20  adjacent to the coil. 
     To compare the shock resistance of the transducer  10  to conventional transducers, a transducer similar to the transducer  10  but without the nubs  36  and  38  “Nubless” hereinafter) was tested. Additionally, transducers produced by Knowles (Model ED1913) having deformations on the armature leg within the coil “Knowles” hereinafter) was tested. Furthermore, a Microtronic transducer (Model 2313) was tested which had a coil rotated at about 7°-8° to limit the deflection of the armature “Microtronic” hereinafter) was tested. 
     A free fall drop test was conducted to compare shock resistance of the Inventive, Nubless and Microtronic transducers. The test was conducted by dropping from varying heights (0 to 175 centimeters) the transducers upon a laboratory floor comprised of concrete covered by vinyl. The orientation of the transducers toward the floor was random. The distortion of the dropped transducers was measured after the free fall with a nominal input of 0.35 mVA at 1150 Hz. Table 1below illustrates the results of the free fall test with the data within the table representing percentage distortion at 1150 Hz. Table 1 also illustrates the distortion levels with symbols. No symbol represents a distortion level less than 5% distortion, an asterisk symbol (*) represents 5-10% distortion, an at symbol (@) represents 10-15% distortion and a number symbol (#) represents greater than 15% distortion. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Free Fall Test Results 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Trial 
                 Height 
                 Height 
                 Height 
                 Height 
                 Height 
                 Height 
                 Height 
               
               
                 No. 
                 0 cm 
                 50 cm 
                 75 cm 
                 100 cm 
                 125 cm 
                 150 cm 
                 175 cm 
               
               
                   
               
            
           
           
               
            
               
                 Nubless Transducer Percentage Distortion from Free Fall Test 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 2.1 
                 1.7 
                 3.9 
                 2.9 
                 27.1# 
                 # 
                 # 
               
               
                 2 
                 3.6 
                 3.7 
                 2.3 
                 30.1# 
                 34# 
                 # 
                 # 
               
               
                 3 
                 2.7 
                 1.4 
                 14.3@ 
                 17.4# 
                 15.2# 
                 # 
                 # 
               
               
                 4 
                 2.4 
                 3.1 
                 2.1 
                 35# 
                 40.8# 
                 # 
                 # 
               
               
                 5 
                 1.5 
                 1.6 
                 14.7@ 
                 8.1* 
                 51.3# 
                 # 
                 # 
               
               
                 6 
                 2.3 
                 2.3 
                 4.3 
                 34.6# 
                 47.1# 
                 # 
                 # 
               
               
                 7 
                 1.6 
                 1.6 
                 1.2 
                 1.6 
                 33# 
                 # 
                 # 
               
               
                 8 
                 3.7 
                 26.9# 
                 9.3* 
                 56.1# 
                 80# 
                 # 
                 # 
               
               
                 9 
                 4.2 
                 4.1 
                 7.3* 
                 1.3 
                 50# 
                 # 
                 # 
               
               
                 10 
                 2.9 
                 4.6 
                 4.3 
                 13.6* 
                 54.8# 
                 # 
                 # 
               
            
           
           
               
            
               
                 Microtronic Transducer Percentage Distortion from Free Fall Test 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 2.9 
                 1.7 
                 2.6 
                 2.6 
                 1.9 
                 8.7* 
                 10.3@ 
               
               
                 2 
                 0.9 
                 1.3 
                 5.8* 
                 2.6 
                 5.3* 
                 11.6@ 
                 16.9# 
               
               
                 3 
                 1 
                 5 
                 3.1 
                 1.4 
                 1.8 
                 2.8 
                 1.9 
               
               
                 4 
                 0.9 
                 1.2 
                 1.3 
                 1.7 
                 1.3 
                 1.2 
                 6 
               
               
                 5 
                 1.7 
                 1.7 
                 1.7 
                 1.6 
                 13.1@ 
                 3.8 
                 13.1@ 
               
               
                 6 
                 0.9 
                 1.3 
                 5.4* 
                 10@ 
                 8.6@ 
                 16# 
                 20.3# 
               
               
                 7 
                 1.3 
                 1.7 
                 2.1 
                 1.9 
                 16.4# 
                 35.7# 
                 37.4# 
               
               
                 8 
                 2.6 
                 2.6 
                 3.1 
                 2.2 
                 2.4 
                 3.5 
                 16.9# 
               
               
                 9 
                 2.3 
                 3.2 
                 1.8 
                 15.6# 
                 1.8 
                 1.6 
                 2.2 
               
               
                 10 
                 1.7 
                 4.4 
                 2.5 
                 1.5 
                 1 
                 1.1 
                 17# 
               
            
           
           
               
            
               
                 Inventive Transducer Percentage Distortion from Free Fall Test 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 1.5 
                 1.1 
                 1.2 
                 1.4 
                 2.2 
                 0.9 
                 1.9 
               
               
                 2 
                 1.1 
                 1.3 
                 1.3 
                 1.5 
                 1.2 
                 1.2 
                 8.8* 
               
               
                 3 
                 1.6 
                 1.9 
                 2 
                 2.5 
                 4.6 
                 2.2 
                 16.9# 
               
               
                 4 
                 0.9 
                 1.4 
                 1.3 
                 0.7 
                 0.9 
                 13.1@ 
                 16.4# 
               
               
                   
               
            
           
         
       
     
     Table 1 illustrates that the Inventive transducers suffered the least distortion as of the free fall and impacts. The Microtronic transducer performed better than the Nubless transducer. Thus, the Inventive transducer provides superior shock resistance. 
     Additionally, to compare the shock resistance of the Inventive transducer to the conventional transducers, a mechanical shock test was performed. The mechanical shock test is illustrated in FIG.  5 . The test apparatus  50  comprises a steel ball  52  having a weight of approximately one kilogram connected to a steel bar  54  having a length of approximately one meter by a string  56 . A steel block  58  weighing approximately 100 kg reinforces the base of the bar  54 . The shock test was conducted by fixing the transducers on the flat side of the ball  52  using double-sided tape. Although the tape most likely added mechanical damping, all transducers were tested using the same tape. Also adhered to the ball  52  is an accelerometer (B&amp;K 8300 accelerometer)  60  to measure peak acceleration of the ball  52 . The shock test comprises releasing the ball  52  at a certain distance such that the ball  52  will strike the block  58  reinforced bar  54  with the desired acceleration. 
     The Inventive transducer was tested with five samples being mounted to the ball  52  “cover up” and five samples mounted “cover down.” When the transducers are mounted “cover down,” the cover side of the transducer is affixed with the double tape to the flat side of the ball  52 . When the transducers are “mounted cover up,” the cover side of the transducer is opposite the flat side of the ball  52 . The reason for these separate measurements is that if the armature is asymmetrically mounted, the armature can move more freely in one direction and much less in the other direction, thus the shock resistance is also asymmetrical. Ten samples of each the Nubless, Microtronic and Knowles transducers were also tested. 
     FIG. 6 illustrates a graph of the shock resistance test with acceleration on the x-axis and percentage distortion at 1150 Hz on the y-axis. The distortion of the tested transducers was measured after the shock with a nominal input of 0.35 mVA at 1150 Hz. Referring to FIG. 6, the cluster of the lines I represents the test results for five Inventive transducers tested “cover down.” The cluster of the Lines  2  represents the test results for five Inventive transducers tested “cover up.” If the results of the Inventive transducers were graphed as an average, it would be nearly a horizontal line across the y-axis at less than 2% distortion. If the results of the Inventive transducers were graphed as an average, it would be nearly a horizontal line across the y-axis at less than 2% distortion. Line  3  illustrates the average test results for the Microtronic transducer. Line  4  illustrates the average test results for the Knowles transducer. Line  5  illustrates the average test results for the Nubless transducer. 
     FIG. 6 clearly illustrates the improved shock resistance of the Inventive transducer over the conventional transducers. The Nubless transducers have a level of 10% distortion at approximately 6000 g. The Knowles transducers have a level of 10% distortion at approximately 10500 g. The Microtronic transducers have a level of 10% distortion at approximately 11500 g. None of the Inventive transducers have a distortion of greater than a 5% distortion over the entire test range of 16000 g. Shock resistibility is generally defined as the level for which the distortion exceeds 10%. Thus, the Inventive transducers provide significantly more shock resistance then the other transducers. 
     It will be appreciated that the present invention has been generally described with reference to a particular embodiment illustrated in the figures, but the present embodiment is not limited to the particular embodiments described herein. For example, the present invention may include a U-shaped armature or other suitable form instead of the illustrated E-shaped armature. For the U-shaped armature embodiment, one of the nubs  36  may be mounted on the left on the upper magnetic element  18  and the other nub  38  on the right on the lower magnetic element  20 . Additionally, it is also possible that the first air gap and/or the second air gap has a non-rectangular cross section. Similarly, the nubs may have varying positions, shapes and compositions. 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations will be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.