Patent Publication Number: US-2007121967-A1

Title: Hearing aid with large diaphragm microphone element including a printed circuit board

Description:
RELATED APPLICATIONS  
      This application is a continuation of U.S. application Ser. No. 11/359,990, filed on Feb. 21, 2006, which is a continuation of U.S. application Ser. No. 09/477,700, filed on Jan. 6, 2000, which claims the benefit of U.S. Provisional Application Ser. No. 60/115,011, filed on Jan. 7, 1999, U.S. Provisional Application Ser. No. 60/134,896, filed on May 19, 1999 and U.S. Provisional Application Ser. No. 60/157,872, filed on Oct. 6, 1999, and U.S. Patent Application entitled “Microphone Assembly for Hearing Aid With JFET Flip-Chip Buffer”, U.S. application Ser. No. 09/478,389, filed on Jan. 6, 2000, now U.S. Pat. No. 6,366,678, the contents of each of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The performance of a hearing aid depends, among other things, upon the design of the microphone pickup. The microphone is a substantial part of the hearing aid. Further, where a hearing aid uses a circuit board which requires electrical connections to be completed during the hearing aid assembly, the ease and simplicity with which the electrical connections can be made impacts the cost of manufacture. Hearing aids which can be manufactured at relatively lower cost are desirable, since they can be disposed of after use.  
      Examples of the use of hearing aid microphones or transducers are known in published literature.  
      U.S. Pat. No. 5,388,163 to Elko et al. teaches an electret foil transducer array comprising an electret foil having a layer of insulating material and a layer of metal in contact therewith. The transducer portion of the array comprises one or more discrete areas of foil with the surrounding areas removed. Alternatively, the discrete areas of foil could be formed by selective metal deposition. Electrical leads are coupled to the discrete areas of metal. By means of the electrical leads, electrical signals produced by each transducer in response to acoustic signals which become incident in use on the areas of foil are used for further processing. The electret foil is made up of the discrete areas of foil with a backing of polytetrafluoroethylene PTF or, alternatively, Mylar®. The electret foil is backed by a porous backplate (e.g., of sintered aluminum) with a rough surface to provide air channels. The porous backplate may be supported by a uniformly supporting metal screen to provide increased rigidity.  
      Nevertheless, despite such prior art, a need exists for a hearing aid with a relatively large diaphragm and improved low noise microphone characteristics performing with high efficiency, which is capable of being manufactured at low cost and economy, thereby facilitating the manufacture of hearing aids which are sufficiently inexpensive so that they can be disposed of after short periods of use. Additionally, there is a need for a hearing aid wherein electrical connections, which need to be made during manufacture, can be completed in a simple and economical manner and in a less labor intensive and effective process.  
     SUMMARY OF THE INVENTION  
      This invention is directed in particular, to disposable hearing aids, i.e., inexpensive hearing aids capable of lasting at least a limited period of time. Traditional hearing aids use microphones having relatively small size diaphragms, generally of the capacitive or electret type. Microphones for the hearing aid industry have continually become smaller in design, allowing hearing aids to also become smaller. However, as these microphones become smaller, they tend to become more expensive. This invention, inter alia, aims at reducing the cost of manufacturing the microphone assembly while maintaining high performance and at the same time allowing for automated assembly of the microphone into the hearing aid electronics. These goals will allow manufacturing cost of hearing aids to be lowered significantly, which is necessary to enable manufacture of disposable hearing aids.  
      The invention, in one embodiment, resides in a disposable hearing aid including an electret type microphone comprising a metallic diaphragm having a front face on which sound waves impinge in use. The diaphragm is glued to a grate-like support plate placed in apposition to and supporting the metallic diaphragm on its back face. The metallic diaphragm consists of a thin plastic film such as PTF coated with a metallic layer. The support plate functionally divides the diaphragm into a plurality of active diaphragm areas which produces a single transducer output whereby the sound waves are converted to electrical pulses. In this way, the advantages of low noise generation in a relatively large diaphragm owing to its larger area and higher capacitance are retained without sacrificing performance and economy.  
      Another embodiment of the invention uses an open-ended metal housing which is enclosed at the open end by a printed circuit board (PCB) carrying all the components needed for signal processing. An electrical connection is made between the printed circuit board and the microphone backplate for coupling the electrical pulses from the diaphragm areas to electrical components for signal processing. Different types of electrical connections which lend themselves effectively for mass production without sacrificing quality are described herein. In addition, the PCB has a ground plane connected to the metal housing to provide an EMI shielding.  
      In another embodiment of the invention, a large diameter capacitor microphone such as an electret microphones commonly used in hearing aids is provided. Traditional hearing aid microphones generally have a single circular or rectangular diaphragm of relatively small dimensions. A large diaphragm microphone herein is used in the disposable hearing aid of the invention to increase sensitivity and to reduce noise. Because the microphone does not have to share space on the hearing aid faceplate with an access door to the hearing aid battery a large diaphragm microphone can be employed which is disposed parallel and proximal to the hearing aid faceplate. The faceplate is provided with multiple inlet holes resulting in improved noise performance and unrestricted flow of sound to the microphone. However, a single large diaphragm has the problem of instability. As the charge on the capacitor is increased to increase sensitivity, the diaphragm is attracted towards the backplate with a higher force. As the distance between the diaphragm and the backplate decreases, the force increases. At some point, the diaphragm becomes unstable, and is attracted to and might stick to the backplate, rendering the hearing aid nonfunctional. The present invention minimizes the instability problem of large diaphragms and provides a hearing aid construction which is inexpensive, reliable, and economical. It also simplifies an electrical connection in the hearing aid which can be accomplished during the step of assembly of the hearing aid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
      A more detailed understanding of the invention may be had from the following description of preferred embodiments, given by way of example and to be understood in conjunction with the accompanying drawing, wherein:  
       FIG. 1  is a schematic cross-sectional view of a microphone assembly having a large diaphragm enclosed in a housing with the complete electronic components and a PCB included for a hearing aid.  
       FIG. 2  is a view similar to that in  FIG. 1 , but including a buffer/amplifier.  
       FIG. 3  is a view similar to that in  FIG. 1 , but including a spring contact type electrical connection between the backplate and PCB.  
       FIG. 4  is a partial cross section view of a disposable hearing aid in accordance with the invention with a microphone assembly and in an enclosure in which the present invention can be implemented.  
       FIG. 5A  is a plan view of a large area single circular diaphragm.  
       FIG. 5B  is a plan view of a diaphragm having a support structure which is used in the present invention.  
       FIGS. 6A and 6B  show plan views of a large diaphragm divided into four sectional diaphragms of equal size,  FIG. 6A , and four sections of equal size and one of dissimilar size,  FIG. 6B , respectively.  
       FIG. 7  is an electrical schematic of a noise model for the noise output from an electret microphone.  
       FIG. 8  is an enlarged diagrammatical cross section of a microphone assembly and electronics for a hearing aid according to one embodiment of the invention.  
       FIGS. 9A, 9B  and  9 C show steps in the process of forming a wire connection according to one embodiment  
       FIG. 10A  is top view of an alternate wire connection.  
       FIG. 10B  is a side view as in  FIG. 10A .  
       FIG. 11A  is a side view of a first step in forming another connection.  
       FIG. 11B  shows the completed connection.  
       FIG. 12  shows an alternate connection.  
       FIG. 13 a  plan view of an array of connections.  
       FIGS. 14A, 14B  and  14 C illustrate a process for making a plurality of alternate type electrical connections from the array of  FIG. 13 .  
       FIG. 15A  is a top plan view of the microphone assembly of  FIG. 15B .  
       FIG. 15B  is a side view of another embodiment of a microphone assembly.  
       FIG. 15C  is a bottom view of  FIG. 15B .  
       FIG. 16A  is an enlarged partial view of a portion of  FIG. 15A .  
       FIG. 16B  is an enlarged partial section of a portion of  FIG. 15B  showing the details of the diaphragm  103  and support frame  320 .  
       FIG. 16C  is a top view of the diaphragm  103  and support frame  320  of  FIG. 15B .  
       FIG. 17A  is a sectional view of the backplate  324  of  FIG. 16B .  
       FIG. 17B  is a top plan view of  FIG. 17A .  
       FIG. 18A  is a top plan view of the mounting ring  322 .  
       FIG. 18B  is a side view of the mounting ring  322 .  
       FIG. 18C  is a bottom plan view of the mounting ring  322 .  
       FIG. 19A  is a schematic side view of another embodiment of a microphone and electronic assembly housing of the invention which includes an intermediate PCB shield between the microphone and a JFET to form a separate compartment from the other electronics mounted on a second PCB.  FIG. 19B  is an enlarged view of a circled portion of  FIG. 19A .  
       FIG. 20  is a schematic side view of a microphone and electronic assembly housing of another embodiment of the invention which includes a single PCB shield between the microphone and a JFET mounted on the shield PCB wherein the remaining electronics are suspended from the shield PCB.  
       FIG. 21  is an assembly as in  FIG. 20  wherein the suspended electronics are enclosed in a second metallic housing connected to the microphone housing.  
       FIG. 22  is a schematic side view of a microphone assembly in which a JFET buffer is provided with source/drain flip-chip pads and a backside gate that is fastened to the microphone backplate.  
       FIG. 23A  is an exploded view of the assembly of  FIG. 22 .  
       FIG. 23B  is an enlarged schematic detail of the JFET buffer portion of  FIG. 22  prior to assembly.  
       FIG. 23C  is a detail as in  FIG. 23B  after assembly.  
       FIG. 24  is a cross-sectional view of an EMI shielded microphone assembly in which the JFET function is included in an IC on the PCB.  
       FIG. 25  is an equivalent circuit of a prior art microphone.  
       FIG. 26  is an equivalent circuit of one embodiment of an improved microphone of the invention having sensitivity control capability.  
       FIG. 27  is an equivalent circuit of an alternate embodiment of the improved microphone of the invention having sensitivity control capability.  
       FIG. 28  is a circuit schematic of an alternate embodiment of the invention in which the microphone amplifier is powered by electrochemical cells integrated into the microphone housing.  
       FIG. 29  is a mechanical schematic of the circuit of  FIG. 28 .  
       FIG. 30  is a circuit schematic of an alternate solar cell embodiment of the invention.  
       FIG. 31  is a mechanical schematic of the circuit of  FIG. 30 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       FIG. 1  shows a first embodiment of the invention illustrated pictorially in a cross sectional view of a hearing aid microphone assembly  100 . A metal housing  101  adapted to be disposed inside an enclosure such as the enclosure  408  shown in  FIG. 4 ; with sound inlets  102  contains, inter alia, front chamber  104 , a diaphragm  103 , a backplate  105 , a back chamber  108 , and electrical components  109 . In addition, a printed circuit board  106  on which the components are mounted, and an electrical connection  107  is included in the housing  101 , thereby providing all the electrical components (except the battery and a receiver) required for a hearing aid. The diaphragm  103  consists of a sheet of a thin flexible material (e.g., metallized mylar) that is stretched tight and glued to a support element  501 . As shown in  FIGS. 5 and 6 , the support element  501  may take many shapes. In the  FIGS. 5 and 6  embodiments, a separate spacer is inserted between the diaphragm (with its support element) and the backplate  105 . The separate spacer maintains an accurate distance between the diaphragm and the backplate. Also, in such embodiments, the backplate  105  is coated with a thin layer (typically about 1 mil) of Teflon®and charged.  
      The sound inlets  102  may be in the form of perforations in the metal housing, or a single opening about equal to or less than the diameter of the diaphragm to enable external sound to pass through the ports  409  in the faceplate of enclosure  400  and impinge on the front of the diaphragm so as to enable the hearing aid to perform its function. The perforations/openings  102  lead to the front chamber  104 , which is partly defined by the laterally extending diaphragm  103 . As shown, this embodiment of the invention comprises an electret microphone element mounted to cooperate with a printed circuit board  106  containing the hearing aid electronics  109 . The microphone housing  101  may be acoustically sealed to the printed circuit board (PCB), for example, by epoxy resin (not shown) applied at the periphery of the base of the housing as it interfaces the PCB  106 , thereby providing a sealed back chamber for the microphone assembly. Other methods of joining and sealing the microphone to the PCB are within the scope of this invention.  
      The backplate  105  is electrically connected to electronic components in one of several ways.  FIG. 1  shows a direct electrical connection to a conductive trace (not shown) on the PCB  106 . The backplate signal then proceeds along the conductive trace on the PCB to connect to other electronic components which may, for example, be a separate buffer amplifier or an integrated circuit containing a buffer amplifier as will be discussed later in connection with  FIGS. 2 and 20 - 24 . Using the connection method shown in  FIG. 1 , the PCB  106  must be of high enough impedance so as not to degrade performance of the microphone. This will restrict the materials that may be used for the PCB and, hence, may drive up the cost of the PCB. Metal housing  101  is one terminal of the microphone element and is electrically connected to circuit ground. With the physical configuration shown in  FIG. 1 , the metal housing  101  is either soldered to a metal trace on the PCB  106 , or connected with conductive epoxy to the conductive trace on the PCB.  
      A support element facilitates functionally-dividing the diaphragm  103  into a plurality of smaller sized active diaphragm areas, the output of which is spatially coupled from backplate  105  to connector  107  for processing by the electronic components  109  on the PCB  106 . Note: The term “spatially coupled” means that no output lead is attached to each active diaphragm area. Rather a single connection is made to a point on the backplate to obtain the voltage change output from the backplate representing the summation of all the voltage modulations induced in the microphone by the acoustic/sound wave input to the diaphragm.  
       FIGS. 8 and 16 - 18  show some of the details of the backplate  105 , which is electrically conductive and has spaced ridge formations or spacer bumps  326 , which are provided to contact the diaphragm at certain locations to facilitate dividing the large diaphragm  103  into smaller functional active diaphragms areas. The ridge formations can be of several desired configurations, such as, for example, triangular, semicircular, square, or trapezoidal cross section. Details of an alternate method of dividing the diaphragm will be provided in connection with  FIGS. 5 and 6 .  
      The backplate is electrically connected to the printed circuit wiring board  106  during assembly of the hearing aid. Details of the electrical connections are discussed in the description relating to  FIGS. 8-14 .  
      An alternative embodiment of the invention will now be described in connection with  FIG. 2 . In this embodiment, a separate buffer/amplifier  210  is connected between the microphone backplate  105  and the PCB  106 . The buffer/amplifier  210  has a very high input impedance suitable for use with an electret microphone element. Also, the buffer/amplifier  210  may be a unity gain buffer (e.g., a source follower), or a low-noise amplifier with gain. A typical gain might be 10 to 20 dB. This input to the buffer/amplifier  210  is electrically connected from the backplate. Suitable methods of making the connection to the backplate include, but are not limited to, welding the lead from the buffer/amplifier  210  to the backplate, or using conductive epoxy (not shown). The buffer/amplifier  210  may be attached to the side of the microphone housing  101  with epoxy (as shown in  FIG. 2 ) or with other suitable means. The power, ground, and output signal leads of the buffer/amplifier  210  are connected to the respective contacts (not shown) on the PCB. As shown in  FIG. 3 , the leads are preferably bent to lay flat on the PCB. Solder or conductive epoxy may be used to make the electrical connection to the PCB. If the leads are made of a resilient/springy material (i.e., beryllium copper), the leads may make a spring contact with the PCB, and solder or conductive epoxy would be unnecessary. In yet another embodiment, the separate buffer/amplifier  210  would not be attached to the side of the microphone housing, but rather would be suspended between the backplate and the PCB by its electrical connections.  
       FIG. 4  illustrates an alternate hearing aid microphone assembly  100  (described in more detail in connection with  FIGS. 22 and 23 ). The assembly  100  is disposed at a proximal end of an enclosure  408  for a disposable hearing aid  400 . The microphone including the housing  101 , diaphragm assembly  103 / 105 , and a back-end PCB  106  is shown to be about 2-3 mm in longitudinal length “L”. The shorter the microphone assembly  100  is, the better for purposes of wearing by a user. The microphone housing  101  occupies a substantial portion of the diameter adjacent the faceplate  406 . A flex circuit (not shown) may be used to couple the amplified output of the microphone from the PCB components  109  to a receiver  402  at the distal end of the hearing aid  400 . A stepped battery  404  is provided between the microphone and the receiver/speaker end  407 . Since the hearing aid  400  is disposable, the battery  404  may be permanently connected to the circuit elements and does not need to be accessed. The need to access the battery is a disadvantage. In prior art devices, an access door was required on the hearing aid faceplate  406  at the proximal end of the enclosure  408  of the hearing aid  400 . Traditionally, the access door would be located where the faceplate  406  of the molded shell-like enclosure  408  containing the hearing aid components is located. The battery access door is normally located on the faceplate since it is a surface not in contact with the ear canal, thereby minimizing ingress of contaminants and potential irritation. In the prior art non-disposable hearing aids, both components i.e., door and microphone, would have to share the same space on the faceplate. The diaphragm for the microphone would, therefore, be substantially smaller than the faceplate.  
      To the contrary, in the invention shown in  FIG. 4 , the microphone diaphragm occupies a substantial portion of the entire surface area adjacent the faceplate  406 . Moreover, because the microphone diaphragm  103  is located proximally adjacent to the faceplate unrestricted sound is allowed to flow through sound ports  409  provided in the faceplate  406  only a short distance from the diaphragm  103 . Thus, hearing aid  400 , not only provides a large area diaphragm, but the microphone assembly provides a high aspect ratio for a hearing aid, in the sense that, the width W versus length L of the microphone assembly versus assembly length is greater than 2:1 whereas in the past, many microphones were of necessity disposed perpendicular to the faceplate so that the aspect ratio was less than 1:1.  
       FIG. 8  shows an embodiment of the invention in which a spring contact element  301  is used to make the electrical connection between the backplate and the PCB. The spring contact element may be permanently connected to the backplate with the spring contact contacting at the PCB side. In another configuration, the spring contact is made at the backplate and the permanent connection made at the PCB side. In yet another configuration, spring contacts may be used at both the backplate and the PCB sides.  
      The PCB  106  may contain one or more copper layers L 1 , L 2  for making electrical connections to signal components, and to ground. The PCB may either be a rigid board (e.g., glass epoxy FR-4) or a flex-circuit (e.g., polymide). Other details of PCB construction are well known in the industry. Preferably, the PCB contains at least two layers L 1 , L 2  of which one layer is substantially a power or ground plane, and in conjunction with the metal housing provides electrical shielding of the integral electronics from interference, i.e., EMI. In one embodiment, the PCB extends beyond the metal housing (as shown) in  FIG. 8 . Electrical pads or terminals may be positioned on the PCB. In the embodiment shown in  FIG. 8 , these terminals may be located outside the metal housing to make electrical connections to other components such as a battery  404  or to a receiver (see  FIG. 4 ). This allows easy connections of a mechanical on/off switch spring element (not shown) and a wire harness (not shown) for electrical connections to the receiver and the negative terminal of the battery. The battery has a diameter of about the same dimensions as the metal housing  101  of the microphone. Therefore, there is not much room to make electrical connections to the PCB  106  within the diameter of the metal housing  101 . In the embodiments of the invention shown in  FIGS. 20 and 21 , the PCB  106  does not extend beyond the metal housing of the microphone. In these embodiments, the electrical connections to the PCB  106  must be made within the bounds (i.e., diameter) of the metal housing  101 .  
      It is envisioned that at least one of the electrical components  109  within the metal housing is an integrated circuit that provides certain hearing aid functions. Preferably, only one integrated circuit is needed. This single integrated circuit contains a high-impedance buffer to interface with the high-impedance electret microphone element, the signal processing circuitry of the hearing aid, and an output amplifier to drive a receiver. In an alternative embodiment, the high-impedance buffer /amplifier is external to the main integrated circuit of the components  109 . In addition to the components disclosed herein, only a battery and receiver are needed to functionally complete the electronics of a hearing aid.  
      As previously noted, the microphone element and, in particular, the diaphragm  103  of the microphone of this invention, is much larger than in traditional microphones. The microphone element disclosed herein is simple in construction and less costly to manufacture than traditional hearing aid microphones. The large diaphragm has a higher capacitance, and hence, lower impedance, than traditional hearing aid microphones. This results is lower noise than in traditional hearing aid microphones. Also, the large diaphragm microphone achieves higher sensitivity than traditional microphones. These features allow a lower cost, standard CMOS process to be used for the high-impedance buffer, and still provides low system noise. Traditional microphones require a more expensive JFET, BICMOS, or special low-noise CMOS process to implement the low-noise high-impedance buffer. Since this invention allows standard CMOS processes to be used, the complete hearing aid electronic system can be included in a single integrated circuit, thereby minimizing system costs.  
      One aspect of the inventive concept lies in the use of multiple diaphragm portions of different areas to improve the performance of the microphone. An additional advantage is that the microphone is mounted parallel to and adjacent to the faceplate which faces outwardly from the inner ear to provide an optimal acoustical path for sound to reach the microphone diaphragm. It is desirable to keep this acoustical path as short as possible, to obviate undesired resonance, which may otherwise be introduced into the frequency response of the hearing aid system. These undesired resonances will degrade the sound quality of the hearing aid. In the embodiment of  FIG. 8 , a microphone with a large diaphragm is divided by bumps  326  in another embodiment ( FIG. 6 ), a frame-like support structure permits the large diaphragm to be divided into multiple diaphragms having different areas acting together. In either case, the diaphragm is disposed substantially parallel to the faceplate and located just behind the faceplate, with a short acoustical path to external sound waves for improved performance and, in particular, improved sound quality with low noise.  
      The following data provides an insight into how the inventive hearing aid with a large area microphone with increased area and a high capacitance results in a relatively low noise device without sacrificing performance. A typical prior art type hearing aid microphone diaphragm may have a circular shape, measuring 2 mm in diameter with an area of 3.14 sq mm. A typical large area diaphragm microphone built using the concepts of this invention will have a diameter of 4 mm with an area of 12.6 sq mm. The improvement between the large area diaphragm and the prior art smaller diaphragm is shown in Table 1 below.  
                               TABLE 1                                       Conventional   Inventive               microphone   microphone                                                                area   3.14   mm 2     12.6   mm 2             active capacitance   0.557   pF   2.227   pF           estimated stray   1   pF   1   pF           (i.e., parasitic) capacitance           total capacitance (active   1.557   pF   3.227   pF           and stray capacitance)                      
 
      The capacitance of the diaphragm is given by the following equation: 
 
 C=ε·A/d  
 
      wherein C is the active capacitance of the microphone (in farads), ε (epilson) is the permittivity of air and has a value of 8.859×10 −12  F/m, and d is the distance between the diaphragm and the backplate (in meters). For the example, d has a value of 50 μm.  
       FIG. 7  shows an exemplary noise model circuit wherein the total noise produced in a diaphragm is expressed as a function of the total capacitance C total , resistance value R which is shown as R in  in the diagram, the noise current i n , and the noise voltage e n , which are the parameters which influence the output. As explained supra, as the value of total capacitance C increases, the noise contribution due to i n , decreases. The total noise is, in effect, inversely proportional to C, and C in turn is directly proportional to the diaphragm area, whereby it is clear that diaphragms with a relatively large area contribute less to the noise generated, resulting in lower noise in the amplified sound for the user.  
      From the noise model illustrated in a diagrammatical form in  FIG. 7 ,  
           total   ⁢           ⁢   noise     =           (       i   n     ⁢     R     1   +   SRC         )     2     +     ⅇ   n   2           ,       
 
      where C =C total  and R=R in    
      As C increases, the noise contribution due to i n  decreases. Therefore, relatively larger area diaphragms which result in relatively large values of C improve the signal to noise ratio by decreasing the noise content.  
      As discussed later in connection with  FIGS. 6A and 6B , a support structure  501  may be provided to divide a large area diaphragm into multiple active areas. These areas can be tailored to provide smoother response characteristics.  
      Neglecting air loading on the diaphragm, the frequency of natural oscillation of the first radial mode of a thin circular membrane (diaphragm) of radius R is given by:  
               f   1     =       1.2     π   ⁢           ⁢   R       ⁢       v   p                 (   1   )             
 
 where v is the tension per unit area at the circumference and p is the mass per unit area. 
 
      The second, third, and fourth modes are related to the first mode by: 
 
 f   2 =2.3( f   1  ) 
 
 f   3 =3.6( f   1 ) 
 
 f   4 =4.9( f   1t ) 
 
      For a microphone in which the first mode is at 3.0 kHz, the second, third, and fourth modes are at 6.9 kHz, 10.8 kHz, and 13.8 kHz respectively. If multiple diaphragms of different diameters are used, the resonant frequencies will also be different and the overall frequency response of the microphones can be made smoother than single-size diaphragms. The diaphragms need not be circular. Calculations for the resonant frequencies of non-circular diaphragms, and in particular of odd-shaped diaphragms, are beyond the scope of this disclosure. Those skilled in the art will recognize that finite-element-analysis (FEA) software programs can be used to determine the resonant frequencies of odd-shaped diaphragms.  
      As will be described in further detail, the benefits/features of this invention disclosed herein include the following: 
          (1) A support structure that divides a large, unstable diaphragm into smaller, stable active diaphragm areas;     (2) A non-circular diaphragm support maximizes active diaphragm area and hence, maximizes microphone sensitivity; and     (3) Non-equal diaphragm supports distribute resonant frequencies and hence, provide smoother overall frequency response.        

      The present invention provides a hearing aid overcoming the disadvantages of prior art by selectively combining (i) the functional advantages of a large diaphragm, (ii) the advantages offered by a plurality of smaller diaphragms, which may or may not be of the same size, (iii) a simple construction to effect electrical connection between a printed circuit board and a backplate of the diaphragm, during assembly, (iv) the advantage of the ability to use a single integrated circuit, and (v) the advantage of the microphone being mounted in parallel with and up against a faceplate, to provide an optimal acoustical path for sounds to reach the microphone diaphragm so that an inexpensive standard low cost CMOS process can be used to complete the hearing aid electronic circuit. The above features enable lower cost hearing aids to be manufactured, thus enabling the hearing aids to be made disposable, without sacrificing superior performance.  
      In  FIGS. 5A and 5B , a single large diaphragm  502 A configuration is compared with a multiple diaphragm configuration of the invention ( FIG. 5B ) of the same overall size. In this configuration, seven individual circular diaphragms  502 B are shown, although more or fewer diaphragms may be used. The larger circular support structure  501 A is about 9.5 mm in diameter. The support structure  501 B divides the diaphragm into seven active microphone diaphragm areas  502 B of about 2.5 mm diameter each. The active area of the single diaphragm of  FIG. 5A  is about 57 mm 2 , while the active area of the multiple smaller diaphragms shown in  FIG. 5B  is about 34 mm 2 . The support structure  501  B represents the inactive area that contributes to parasitic capacitance and may slightly reduce the sensitivity of the microphone.  
       FIGS. 6A and 6B  show two embodiments that provide multiple diaphragms with increased active area compared with the embodiment of  FIG. 5B . In  FIG. 6A , four active diaphragms of areas  502  of equal size are shown. The diaphragms are not circular, but rather they are pie-shaped quadrants, to maximize the active diaphragm area. The overall circular diaphragms may be divided into more or fewer than four sections as shown. By minimizing the area of the support structure  501 , and hence, maximizing the active diaphragm area, the active capacitance is increased and the parasitic capacitance is decreased. The active area of the configuration of  FIG. 6A  is about 48 mm 2 . The active area of the configuration of  FIG. 6B  is about 49 mm 2 .  FIG. 6A  has four equal-size active diaphragm areas  502 , hence the resonant frequencies of each diaphragm will be the same.  FIG. 6B  has two different sized active diaphragm areas and hence will have two different sets of resonant frequencies. The active diaphragm area arrangements may comprise a plurality of areas, all of similar or different sizes and shapes. The sizes, and hence, the resonant frequencies, may be chosen to optimize the frequency response. In general, the optimization will provide a smoother response than normally obtained with a single-size diaphragm.  
      In summary,  FIGS. 5A, 6A , and  6 B show a large diaphragm which can be used with a support structure, wherein the active area of the diaphragm is divided so as to create several smaller active diaphragm regions  502  each acting as individual diaphragms. Each arrangement shown in  FIGS. 5B, 6A , and  6 B has its own suitable support structure  501 . The arrangements shown in  FIGS. 5A, 6A , and  6 B offer the advantages of large capacitance and hence, an improved signal to noise ratio.  
       FIG. 8 , as previously discussed, illustrates an exemplary cross section of a large diaphragm microphone assembly, wherein the electrical connection between the backplate  105  and the PCB  106  is established by a spring contact  301 . The cross section shown in  FIG. 8  includes a housing  101 , sound inlets  102 , a charged diaphragm  103 , a backplate  105  functioning as a support plate, a retainer ring  807 , electronic circuit components  109 , and a PCB  106 . A spring contact  301  which is electrically attached to the PCB  106 , by virtue of its configuration and resilience, makes electrical contact after assembly with conductive backplate  105 . In general, only one electrical contact is needed. The electret microphone is a capacitor with a permanent charge. Since q= c·v , where q equals the charge, c equals the capacitance and v equals the voltage across the capacitance, if q is fixed (as it is in the electret microphone) as sound impinges upon the diaphragm (one plate of the capacitor), the diaphragm vibrates which in turn modulates the capacitance. As the capacitance modulates (changes), and with charge fixed, the voltage across the capacitor also modulates (changes). This changing voltage represents the sound pressure waveform (i.e., sound) impinging upon the diaphragm. The diaphragm is held at ground potential, therefore, this changing voltage appears at the backplate  105 . To couple this signal into the electronics, the backplate is coupled to the PCB, which in turn connects the signal through a conductive trace (not shown) to the signal processing electronics  109 . The diaphragm  103  and metal housing  101  are both connected to ground in this embodiment and act as an electromagnetic shield. Different configurations for the spring contact  301  are conceivable, and are within the scope of this invention. Spacer bumps  326  on the backplate  105  facilitate functionally dividing the area of the charged diaphragm  103  into smaller sized active diaphragm areas, without losing the advantages of the larger capacitance and consequent lower noise contributed by a large diaphragm. Other alternative provisions, e.g., a ridge or the like, may be used to, to facilitate dividing the diaphragm area into smaller active portions.  
      The cost of a hearing aid depends largely on the degree of automation and the number of parts and processes needed in large-scale manufacturing. The following description addresses some possible variations in the design of the electrical contact between the backplate and the PCB, which is a difficult, expensive, and a critical aspect of the manufacture.  
      The electrical connection between the backplate of the microphone and PCB is difficult and critical because it is completed by an act of assembly of the housing with the printed circuit board during manufacture. The connection needs to have minimum capacitance to the sidewalls; therefore, the connecting body must be very thin and, therefore, fragile. The connector is required to be just the correct length to bridge the gap between the backplate and the PCB.  
      A first approach to making the connection is shown in  FIGS. 9A-9C . A thin metal conductor  89  is formed generally in the shape shown in  FIG. 9A  with a long center tab  90  and two shorter side tabs  92  and  94 . For example, the conductor  89  may be formed of 0.001″ thick copper. When the center tab  90  is bent up 90° as shown in  FIGS. 9B and 9C , the base  96  which remains can be placed on solder dots  805  of a pad on a PCB and soldered along with the rest of the circuit components on the PCB. Four small solder dots (shown in phantom) are better for stability than one large dot. If the length of the center lead  90  is formed to be less than the assembled distance between the PCB and the backplate, an electrically conductive epoxy dot can be placed on the backplate to line up with this lead at assembly. When the assembly is made, the lead penetrates the epoxy dot to make the connection. The epoxy dot is sufficiently large to compensate for any tolerance build up in the assembled parts.  
      If the center lead is formed to be greater than the distance between the assembled backplate  105  and the PCB  106 , the lead  90  will buckle as it interfaces with the surface of the backplate during assembly as shown in  FIG. 8 . If the parts are gold-plated, this pressure contact may be sufficient to complete the assembly. An electrically conductive epoxy dot  805  on the backplate could also be part of this contact version if needed. To aid in controlling the position of the long contact lead as it stabs the backplate during assembly, a depression  806  can be formed in the backplate to corral the lead as shown in  FIG. 8 .  
      In each of the above versions, a small, pre-bent portion in the center lead will act as a strain relief during the life of the product as shown in  FIG. 8 . It is obvious that many other shapes and bends can be used that are similar to those in the description above.  
       FIGS. 10A and 10B  show another approach to making this connection by using a conductive wire  88 . A length of wire with a ring  98 , formed at one end and bent approximately at 90° to the wire can be soldered to a pad on the PCB. The other end  99  can mate with the backplate similar to the contact in  FIG. 9 .  
       FIGS. 11A and 11B  show a method of making electrical contact without any extra parts. A very thick electrically conductive epoxy dot  802  can be placed on both the PCB  106  and the backplate  105 . Both dots should be higher than half the distance between the two plates and should be aligned with each other during assembly. As the parts are assembled, the two epoxy dots join together and amalgamate to form the electrical connection ( FIG. 11 B ).  
       FIG. 12  shows another method of making electrical contact. The backplate  105  is lanced to provide a lead  823  to reach the PCB  106 . An electrically conductive epoxy dot  824  completes the contact. This lead is relatively stiff and should be shorter than the distance between the two parts.  
       FIG. 13  show a plurality of contacts in an array  854  that resembles a small surface mount plastic package. Small plastic cubes  852  are preferably injection molded onto a sheet metal frame array  854 , including suitable electrically conductive leads  856 . When each section  858  (shown in dotted lines) is separated, there are four leads  856  protruding from each of four sides of the cube  852 . As shown in  FIGS. 14A and 14B , three of these leads  856 B, C and D are bent around one face of the cube  852  to form three solder pads. The fourth lead  856 A is bent at an angle, as shown in  FIG. 14C  to become the spring contact connection to the backplate. This embodiment allows the lead connection to be placed and soldered on the PCB  106  with standard assembly equipment and processes. The lead  856 A that contacts the backplate  105  provide a pressure contact or a conductive epoxy contact can be applied to retain the lead in place. The material for the plastic and the electrically conductive leads are well known and may be chosen suitably by one skilled in the art.  
      Further details of the hearing aid microphone assembly described in  FIG. 8  will now be provided in connection with  FIGS. 15-18 . The basic parts of the assembly are the diaphragm  103 , the backplate  105  and the housing  101 . In addition, spacers as will be described, are provided to maintain the proper relationship of the parts. All of these parts are fastened to a circuit board  106  that contains all of the necessary electronics for the hearing aid. The cross-section of  FIG. 15B  shows the relationship of all the parts.  FIGS. 15A and 15C  are top and bottom views respectively.  FIG. 15A  shows a series of holes  102  to allow sound to reach the diaphragm. The bottom view,  FIG. 15C , shows tabs  304 , which are part of the housing, wrapped around the PCB  106  to clamp the housing  101  tightly to the circuit board. These tabs make electrical connection to the PCB ground plane  306  that covers the entire bottom of the PCB  106 . The tabs must be wrapped tight enough to insure that there is a good acoustic seal between the housing  101  and the top of the PCB. A soft coating (not shown) may be sprayed onto the top surface of the circuit board before installing the housing to insure a good seal.  FIG. 16  shows a partial enlargement of one end of the cross section of  FIG. 15B  to show more detail of the relationship of the internal parts.  
      The diaphragm  103  shown in detail in  FIGS. 16 and 17  is constructed of an extremely thin stretched metal coated dielectric film  342 , for example, 0.001″ thick Teflon® covered with a metal coating  344  on one side  103 A. The film is stretched and adhered to an annular conductive support frame  320  using a conductive adhesive  340  (see  FIG. 16B ). The conductive side of the film  103 A should make good electrical contact with the frame  320 . The diaphragm and frame assembly is placed into the housing so that the frame  320  contacts the housing at the raised ring spacer  111  which is coined into the planar top portion of the housing to establish the desired spacing between the diaphragm and the housing. Before assembly, a static charge is placed on the diaphragm film  103 . The charge can be placed onto the diaphragm  103  (or onto a Teflon®coating on the backplate  105 ) by one of several methods such as corona discharge or ion-beam deposition. It is also possible that the frame  320  can be adhered to the opposite side of the film  342  so the conductive side of the film contacts the housing directly. Then, the adhesive does not have to be conductive.  
      The backplate  105  shown in  FIG. 16A  must be located extremely close to the diaphragm  103 . Note: Unlike previous embodiments, no separate spacer  501  is used between the diaphragm and the backplate. Instead, a small ridge  324  is coined on the edge of the backplate. When the backplate is placed into the housing, the ridge presses against the frame  320  of the diaphragm to establish a space  104  that, for example, may be 50 microns. This diaphragm is much larger in diameter than is used in present day production. Therefore, the diaphragm  103  can be unstable when the bias voltage is applied. To break up the large unstable area, small projections  326  are coined into the backplate to support the center of the diaphragm the proper distance from the backplate. A bias voltage is provided to keep the diaphragm tight against the projections  326 .  
      An insulated mounting ring  322  shown in detail in  FIGS. 18A, 18B  and  18 C is provided to support the backplate  105  and clamp the backplate diaphragm frame  320 , and housing  101  together. An outer peripheral edge of the mounting ring  322  is shown with a plurality of small weak projections  323  that will easily collapse when all of the parts are clamped onto the circuit board. An alternative method of clamping the parts together is to press fit the ring into the housing to hold the parts together. Then, four or more indentations are punched into the sides of the ring for a more permanent anchor. Tight tolerances for the press fit parts can be relieved by molding ribs (not shown) into the side of the ring. The ribs will easily collapse during the press fit operation.  
      The housing and its assembled parts are fastened to the circuit board by 4 or more tabs  304  that penetrate slots in the circuit board ( FIG. 16A ). While the sandwich of parts is clamped tightly, the tabs are bent onto the copper layer  306  on the back of the circuit board. The copper layer and the metal housing make a shield for the circuit inside. This embodiment requires no solder adhesives, or welding for the final assembly.  
      As noted, the microphone assembly and electronics described above is intended to be part of a disposable, i.e.,“throw a way” hearing aid. It does not have to survive inventory plus 8 or more years of life. It is adapted to last 2 years in an inert atmosphere package plus 40 days in use.  
      Although the drawings show a circular microphone, any reasonable shape can be used. For example, there can be flats on the sides of the housing so that the housing is more form fitting to the internal circuit consisting of rectangular components. The advantage of this design is that volume allocated to exterior contacts and a switch is almost doubled. These flats will also serve as orientation and gripping surfaces for automation equipment. Because of the rectangular shape of the circuit components, four flats can be formed on the sides of the housing, if needed, for automation purposes.  
      The advantages of this embodiment are: 
          1. All of the metal parts can be manufactured similar to picture tube gun parts that are very low cost and with high tolerances.     2. Almost the entire diaphragm is active.     3. Coined features insure very accurate spacing and location of all the parts.     4. No solder, welding, or gluing is needed at final assembly. The diaphragm and frame are delivered to the line as a subassembly.     5. True layered assembly.     6. The flat sides of the housing allow room for test points, connection pads, and a switch.        

      Another important feature of the invention shown in  FIGS. 15A, 15B  and  15 C involves the sound openings. Most persons with hearing loss have greater high frequency hearing loss than low or mid-frequency hearing loss. This causes such persons to miss or confuse softly spoken, low energy consonants such as t, b, v, k, p, s. Thus, one function of an appropriate hearing aid is to amplify high frequency energy sufficiently to make these low level sounds audible and at a comfortable listening level. The sound inlet for a hearing aid microphone typically is very narrow. When high frequency sounds from outside the hearing aid pass through this narrow opening, they are attenuated by inertance and acoustic resistance, resulting in a lower high frequency input to the hearing aid than desired, and possibly reduced audibility to important high frequency speech sounds. Additionally, too small an inlet may produce an acoustical resonance in the microphone system frequency response (as used in the hearing aid). Wind turbulence passing across and down the small cylindrical-shaped microphone inlet vibrates the microphone diaphragm, which results in a noise that interferes with the desired hearing aid operation.  
      The hearing aid microphone assembly  100  shown in  FIGS. 15A, 15B  ad  15 C has a very large microphone diaphragm  103  interfacing with multiple inlet holes  102  through the a housing  101 . Alternatively, the housing  101  may be further contained in an enclosure  408  (as shown in  FIG. 4 ) which also has multiple inlet holes  409 , in faceplate  406  in which case the diaphragm  103  may be fully exposed to the exterior faceplate with a single large aperture  102 B provided at the end face of the housing  101 . In the latter case, using more than one sound inlet hole in the enclosure effectively minimizes inertance and acoustic resistance and ensures that the aggregate sound inlet has a minimal effect on the acoustic response of the microphone system. If the combined area of the holes is large enough, the acoustic impedance will be very low. The holes in the faceplate  406  should be made as large as possible without allowing a wearer to insert pins through them. A 0.040″ diameter hole or smaller is desirable. The narrower and longer the holes, the more are needed. Flaring the outside and/or inside surfaces of the microphone sound inlet holes (see  102 A  FIG. 16A  or openings  409  of  FIG. 4 ) helps to reduce the turbulence produced by wind, and hence, wind-induced noises.  
      In another embodiment of the invention, a vibration isolation material, such as a thin piece of acoustically transparent felt  163  is placed between the metal housing  101  of the microphone assembly  100  and the enclosure  408  (see  FIG. 4 ). The felt  163  will damp mechanical vibrations produced by the hearing aid receiver conducted through the shell and transduced by the microphone. In addition, the felt will protect the microphone diaphragm from foreign objects.  
       FIG. 19A  illustrates in schematic form another embodiment of the invention. In the previous embodiments, the printed circuit board  106  provided an acoustical seal for the rear volume of the microphone, i.e., diaphragm  103 /backplate  105 . The electronic circuitry of the hearing aid was mounted on the printed circuit board  106 . In that embodiment, it is possible that signals from the electronics may be coupled to the backplate electrode of the microphone through parasitic capacitance. The invention disclosed in this embodiment provides an electrostatic shield  602  to prevent electromagnetic interference (EMI) between the electronics  109  and the back-plate electrode  105  as well as providing a shielded compartment for a high input impedance amplifier  604  used in conjunction with the electret microphone element.  
      In  FIG. 19A , an electret microphone is disposed in housing  101  having sound openings  102  located opposite diaphragm  103 , and backplate electrode  105 . Also shown is a substrate/shield  602  extending across the inner sides of housing  101 , an amplifier  604 , mounted to the substrate  602 , and an electrical connection  609  between the substrate/shield and the main PCB, wherein the PCB  106  contains the main electronic components of the hearing aid electronics.  
      Hearing aid electronics  109  may include class-D switching amplifiers, switched-capacitor filters, or digital electronics, such as one commonly found in digital signal processing circuits. Each of these type of circuits contain signals switching at high frequencies which may be coupled to the microphone diaphragm or backplate through parasitic capacitances. These high frequencies would, thereby, introduce noise into the microphone signal and possibly effect the operation of the circuit. The substrate/shield  602  contains at least two layers of metallization  602 A and  602 B, wherein one layer is primarily a ground plane and functions to shield the microphone elements from the high frequency signals in the hearing aid electronics.  
      Some of the benefits of this embodiment are as follows: 
          1. Inherent electrical shielding is provided by the combination of the metal housing  101  and the power and/or ground plane(s)  602 A/B on the substrate/shield  602 .     2. Allows the use of various types of JFET, BICMOS, or low-noise CMOS amplifiers  604  mounted on said substrate.     3. The substrate/shield  602  provides shielding between the amplifier  604  mounted thereon and the hearing aid electronics  109  mounted on the printed circuit board  106 .        

      In the invention described in connection with  FIG. 19 , the amplifier  604  is mounted on one PCB  602  and the hearing aid electronics are mounted on a second PCB  106 .  FIG. 20  shows an alternate embodiment in which all components (amplifier and hearing aid electronics) are mounted on one PCB  602 .  
       FIG. 21  shows an optional shielding cover for the  FIG. 20  embodiment that provides EMI shielding for the electronics.  
      Note that  FIGS. 19-21  show an amplifier, preferably a JFET amplifier, that has been mounted to the printed circuit board using flip-chip technology. Conductive epoxy  610  connects the gate of the JFET  604  to the backplate  105  of the electret microphone shown generally at  606 .  
      As noted, in the embodiment of  FIG. 19A , one PCB is required for the JFET that serves as a buffer amplifier  604  for the electret microphone element and one PCB  106  for the hearing aid amplifier in the electronics  109 . The result is a relatively large and expensive microphone/amplifier assembly. One reason for separating the microphone from the IC amplifier in the electronics  109  is that microphone output signals from buffer amplifier  604  are low level, whereas IC amplifier output signals are  40 - 50  dB higher in level. If the amplifier output signal gets back into the microphone output signal, the audio signal processing performance may significantly degrade. Additionally, the microphone/amplifier assembly  606  must have shielding from external EMI signals such as digital wireless telephone interference sufficient for a hearing aid wearer to use a digital cellular telephone. This has been accomplished as disclosed previously by enclosure of the entire microphone/amplifier assembly in a metal can or housing  101  which is grounded to the ground plane of PCB  106 .  
      By making the PC board  602  such that components are mounted on two sides (as in  FIG. 20 ) rather than one side, the JFET buffer amplifier  604  can be placed on one side (the same side as the microphone element) and the amplifier IC and external components  109  can be placed on the other side of the same PCB  602  ( FIG. 20 ). The pre-amp (not shown) in the amplifier IC connects to the JFET through a via connection  612  in the PCB  602 . Metallization  611  on the JFET connects with conductive epoxy  610  to the backplate  105  of the microphone  606 . This results in a smaller and less expensive microphone/amplifier assembly, while isolating the high level output of the IC amplifier from the low level microphone output via the ground plane shield layer  602 B incorporated in the PCB  602 . EMI shielding can be retained by placing a second-metal can  616  over the amplifier IC and external components  109  on the bottom of the PCB  602 .  FIG. 21  shows such an overlapping configuration of top  614  and bottom  616  metal shield cans with respect to the printed circuit board. Other configurations are possible as well such as butting the two cans together and joining them with conductive epoxy.  
      As previously noted, an electret microphone for hearing aids typically uses a JFET buffer to convert the signal from the backplate a high impedance source (the microphone) to a low impedance source. This impedance conversion results in a higher level loaded output signal level to the hearing aid amplifier than would be produced from the condenser microphone element itself without a buffer. A JFET gate contact to the backplate of the microphone&#39;s condenser must somehow be made. A direct connection from a 4 mil square pad on the JFET to the microphone backplate is difficult to do and the use of an intermediate wire bond pad requires that the pad be mounted on ceramic, which complicates assembly. If the JFET gate connection is on the substrate, the substrate must have high resistivity to not compromise the input impedance of the amplifier. A ceramic (alumina) substrate has such properties. Traditionally, the electrical connections for the JFET have been wire bonded to the microphone element onto a ceramic substrate. Wire bonds are normally formed with a loop from pads on the die to extra bonding pads on the ceramic substrate, a practice that requires extra space vertically and horizontally and produces stray capacitance to ground and other circuit nodes which reduce sensitivity and introduce noise. Other disadvantages of a ceramic substrate itself are that it is relatively costly for use in a disposable hearing aid application and that it has a high dielectric constant which makes stray capacitance even higher.  
      In accordance with the embodiment shown in  FIGS. 22 and 23 A, B and C, flip chip technology is used to minimize the physical size and lead lengths required to connect die bond pads of the JFET  604  to reduce the lead length between the electret microphone backplate  105  and the JFET. The result is a lower noise and higher sensitivity connection than could be made by longer paths formed by conventional wiring. By keeping the JFET backside gate connection  762  of the FET off the PCB  602  substrate  764 , a lower cost substrate such as a glass-epoxy printed circuit board (e.g., FR 4 ) may be used. Since the JFET gate does not contact the substrate and then connect to the microphone backplate (rather the JFET is connected to the backplate directly), the stray capacitance should be lower and, hence, sensitivity should be higher.  
       FIGS. 23B and 23C  show details of the flip-chip JFET connections including the gate to backplate connection  762  using conductive epoxy  756 .  FIG. 23B  is an exploded view before assembly, while  FIG. 23C  shows the JFET after assembly with the PCB  602  and the backplate  105 . The metallization  754  on the top of the JFET die  604  is the gate connection, which is a very high impedance point. The solder bumps  752  on the bottom are the low impedance connections such as the drain and source connections. In this embodiment of the invention, four solder bumps: Drain, Source, Bias, and one dummy solder bump that is a No-Connect (NC) are provided. NC is not connected to any part of the JFET circuit. The underfill material  760  provides mechanical support.  
      This embodiment of the invention produces the following advantages: 
          a. A flip-chip JFET  604  with no gate contact made to the PCB, allows use of low cost FR 4  or other such materials instead of ceramic for the PCB substrate.     b. By controlling the depth of the front chamber  104  in the microphone assembly so that the spacing from the backplate to the PCB substrate is small enough, a single blob of conductive cement  756  is sufficient to bridge the gap, eliminating the need for wire bonds.     c. Stray capacitance from the gate to PCB substrate is reduced because of this gate isolation, resulting in decreased signal loss and decreased noise pickup.     d. The use of four dummy solder balls on JFET to provide better mechanical support and alignment during assembly. (Solder bumps on Drain, Source, Diode, and NC solder bumps  752 ).        

       FIG. 24  illustrates yet another embodiment of the invention comprising a reduced component count EMI shielded microphone/amplifier assembly for use in disposable hearing aid in which the JFET buffer function is incorporated in a hearing aid amplifier integrated circuit disposed on the bottom of PCB.  
      Previous embodiments required one printed circuit board for the JFET that serves as a buffer for the electret microphone element and one PC board for the hearing aid amplifier (e.g.,  FIG. 19 ). Without the JFET function, the microphone element output is a high impedance and low signal level. The JFET produces a low impedance/higher signal level microphone output. The result is a relatively large and expensive microphone/amplifier assembly. Another reason for separating the microphone from the amplifier and buffering its output with a JFET is that microphone output signals are low level loaded whereas amplifier output signals are 40-50 dB higher in level. If the amplifier output signal gets back into the microphone output signal, the audio signal processing performance may significantly degrade. Additionally, the microphone JFET amplifier assembly in the previous embodiments must have shielding from external EMI signals such as digital wireless telephone interference sufficient for a hearing aid wearer to use a digital cellular telephone. This has been accomplished and disclosed previously by an encapsulation of the entire microphone/amplifier assembly in a metal can.  
      In accordance with the embodiment of  FIG. 24 , the external JFET is eliminated by providing its impedance transforming functions within an amplifier integrated circuit  670  mounted on the bottom side of PCB  602 . Then, the two-sided PCB  602  is provided with a metal bump  672  (in place of the JFET) of previous embodiments, on one side (i.e., the same side as the microphone element) and the amplifier IC  670  and external components are placed on the other side of the PCB. A pre-amp in the amplifier IC  670  connects to the metal bump through a via connection  674  in the PCB. The metal bump connects with conductive epoxy  676 A to the backplate of the microphone. This results in a smaller and less expensive microphone assembly. A ground plane shield layer  678  is incorporated in the PC board. EMI shielding is retained by placing a second metal can  679  over the amplifier IC and external components on the bottom of the PCB  602  and joining can  679  with upper can  677  using conductive epoxy  676 B at the joints. Alternately, the two cans may be soldered, welded, or press fit together to make the electrical connection.  
      Further details of the invention will now be described in connection with  FIGS. 25-27  which relate to improvements in sensitivity of capacitor microphones such as electret microphones commonly used in hearing aids. Traditional hearing aids use small microphones, generally of the electret type. These traditional microphones have sensitivities of about −35 dB (re: 1V/Pa). At a sound pressure level of 94 dBSPL (re: 20 μPa), the output voltage of such microphones is about 17.8 mVrms (50 mVpp). Larger diaphragm microphones may achieve a sensitivity as high as about −15 dB (re: 1 V/Pa), or 178 mVrms (503 mVPP) at 94 dBSPL. Those skilled in the art of hearing aid design must make a tradeoff between system noise performance and signal overload. Those using a high sensitivity microphone or an expensive low-noise amplifier to increase the microphone signal above the noise floor of the remaining circuitry must risk signal overload for loud sounds, or accept poorer noise performance but have large headroom to prevent overload from loud sounds. To obtain the best of both worlds, some hearing aids include an input amplifier with input compression limiting. This amplifier has a gain of about 20 dB for low-level signals. However, for signals greater than about 90 dBSPL, the gain of the amplifier is reduced to prevent signal overload and distortion. The amplifier must be built from a low-noise semiconductor process so the amplifier itself does not introduce excessive noise into the system. In accordance with this embodiment of the invention, a microphone with higher inherent sensitivity is provided along with means to reduce the sensitivity for loud sounds. The higher sensitivity eliminates the need for an expensive low-noise amplifier, and hence total system costs will be reduced. The invention disclosed herein may be applied to capacitor microphones used in other than hearing aid applications. For example, electret microphones are commonly used in telephones, answering machines, portable tape recorders, and cellular telephones. Each of these applications generally uses some form of automatic gain control or compression limiting to prevent overload and distortion from large signals.  
      Most hearing aid microphones are small and hence use small diaphragms. In a previous embodiment, a large diaphragm microphone is disclosed. The large diaphragm microphone provides both a lower noise and higher sensitivity compared to traditional microphones. However, the higher sensitivity means that the hearing aid will overload and distort at lower sound pressure levels than traditional microphones.  
       FIG. 25  shows an equivalent circuit of a traditional microphone  900 . The voltage source V 1  produces a voltage proportional to sound pressure level. Capacitors C 1  and C 2  are the active capacitance and parasitic capacitance-of the microphone, respectively. Capacitor C 3  and resistor R 1  represent the input impedance of the electronics circuitry  902  that the microphone element drives. One skilled in the art can easily see that the components C 1 -C 3  and R 1  form a voltage divider that effects the effective sensitivity of the microphone.  
       FIG. 26  shows an equivalent circuit of a large diaphragm microphone of the invention driving electronics that include a variable capacitance diode (D 1 ). Components C 1 -C 3 , R 1  and the capacitance of D 1  form a voltage divider that effects the effective sensitivity of the microphone. With the connection of D 1  between the signal output and a control voltage  908 , a negative control voltage may be applied to the anode of D 1  to vary its capacitance. By varying the control voltage, the voltage divider is controlled and hence the effective sensitivity of the microphone  904  is controlled. The capacitance of variable capacitance diodes, such as Philips Semiconductor part BB  130 , can be varied from about 16 pF at a reverse voltage of  28 V, up to about 500 pF at a reverse voltage of 1 V. With the values of C 1 -C 3  shown in Table II below, the sensitivity of the microphone can be varied over a 23 dB range. However, the reverse voltage of up to 28 V is much higher than is practical for hearing aid circuits which are intended for operation from a 1.3 V battery source.  
                           TABLE II                                   COMPONENT   CAPACITANCE                          C1   10 pf           C2   10 pf           C3   1.0 pf                         
      Another embodiment of the invention is shown in  FIG. 27 . In  FIG. 27 , the variable capacitance diode of  FIG. 26  has been replaced with a series of capacitors (C 4 -Cn) and transistors (Q 4 -Qn) shown here as MOSFET type transistors forming a variable sensitivity circuit  906 . The transistors act as switches. Any number of capacitor/transistor pairs may be used. With all transistors turned off, the microphone sensitivity is at its maximum. As capacitor/transistor pairs are turned on, the voltage divider is changed and the effective sensitivity of the microphone is reduced. With reference to  FIG. 27 , the values of C 4 -Cn may be selected to provide attenuation steps of any value desired. Typical step values may be from about 1 dB to about 6 dB and preferably from about 1 to 3 dB. Other series/parallel combinations of switched capacitors can be used to implement digitally controlled sensitivity adjustment of the microphone.  
      Some of the benefits/features of the invention disclosed herein are: 
          1. Large output signal from microphone results in lower system noise.     2. Electronic control of microphone sensitivity prevents overload and distortion at high sound pressure levels.     3. A low noise gain controlled amplifier is not needed.     4. The use of a standard CMOS process is allowed, rather than more expensive JFET, BICMOS, or low noise CMOS processes for the input amplifier of the electronics, resulting in lower system costs.        

      Hearing aid microphones of the electret type typically produce an output signal which is amplified by a junction field-effect transistor (JFET) amplifier. Such hearing aids are powered by a single zinc-air cell that produces about 1.3 volts. Electrical noise on the 1.3 volt power is reduced by a resistor-capacitor filter, or by an active voltage regulator. In either case, the final dc voltage available for the JFET amplifier circuit is about 0.90 to 0.95 volts. This low voltage imposes tight tolerances on the JFET device parameters, in particular on the pinch-off voltage parameter. Therefore, the yield of the JFET devices is low and the costs are relatively high. In previous embodiments of the invention, the microphone element is generally of the electret type and the amplifier is of the JFET type and is located within the cover of the microphone. The main electronics are mounted on a PCB in the microphone housing and the remainder of the electronics in the hearing aid enclosure. The remaining electronics include a separate battery and a receiver which may be either a passive receiver or one containing an integral class-D amplifier. Microphones and receivers of these types are commercially available from several source including Knowles Electronics, Inc. (Itasca, Ill.), Microtronic A/S (Roskilde, Denmark), and Industries (Camden, Me.). In general, the commercially available microphones are intended to operate on a voltage of about 0.9 volts to 1.5 volts, and generally are operated at about 0.9 volts to 0.95 volts.  
      The embodiments shown in  FIGS. 28-31  provides a supplemental power source for the microphone JFET amplifier per se which is integral to the microphone housing and free of noise from the main power source in the hearing aid. It provides higher operating voltages for the JFET amplifier so that the tight tolerances of the JFET parameters are no longer necessary, and the cost of the JFET may be reduced.  
      As shown in  FIGS. 28 and 29 , microphone amplifier J 1  is powered by one or more electrochemical cells B 1 , B 2  connected in series. As shown in the figure, two lithium cells B 1 , B 2  provide a total of 6 volts to a JFET amplifier J 1 . The microphone  103  has three electrical connections (terminals) labeled “GND”, “OUT”, and “BAT”. To turn on the microphone, terminal “BAT” is connected to terminal “GND” by a suitable switch (not shown). The output signal appears between “OUT” and “GND”. The electrochemical cells may be of any type such as zinc-air, carbon-zinc, alkaline, silver-oxide, or lithium (shown in the figure). A preferred embodiment uses two lithium cells connected in series and physically located in the back chamber  108  of the housing cover  101  along with the amplifier J 1 . Electrical connections (not shown) are made between the cells B 1 , B 2  and the JFET by conductive traces on the substrate of the PCB. Alternatively as shown in  FIGS. 30 and 31 , the amplifier J 1  may be powered by a solar cell array D 1 . The solar cell array may contain any number of parallel-series combinations of individual solar cell elements as long as it is sufficient to provide the desired voltage and current. An optional filter capacitor C 6  and an optional voltage regulator VR 1  may be included individually or combined in the array. The filter capacitor and voltage regulator will both reduce noise picked up from modulation of the illumination on the solar cell, for example, 60 Hz modulation from indoor lighting. In a preferred embodiment, both a filter capacitor and a simple voltage regulator diode are included.  
      In  FIG. 31 , the basic physical construction of the microphone assembly is shown. The solar cell array D 1  may be mounted on the face of the microphone exposed to the source of illumination. Although the solar cell array D 1  is mounted to partially block the sound inlet to the diaphragm of the microphone, sufficient area is left open so as not to degrade the acoustical performance of the microphone  103 . Electrical connections (not shown) provide the electrical connection between the solar cell array and the JFET amplifier. An alternate location for the solar cell array is shown at D 1 ′.  
      Equivalents  
      The electret type diaphragm, its preferred dimensions, the different alternative configurations of the spring contact, and the methods of obtaining the electrical contact by electrically conductive epoxy resin are all exemplary in the context of the embodiments described hereinabove. Likewise, the division of the large diaphragm to obtain smaller sized active diaphragms are for illustration only and can be replaced with other substantially similar alternatives. For example, the single large diaphragm may be subdivided into two or three portions as long as the advantages of the relatively large capacitance of the single large diaphragm can still be used to derive the benefit of low noise. The sound inlets  102  in  FIGS. 1 and 8  or  409  in  FIG. 4  may be of any convenient shape and number without limitation. The electrical connection  107  shown in  FIG. 1  or  301  shown in  FIG. 8  may be formed suitably in a manner different from what is illustrated.  
      Also note certain phrases in the claims should be given the broadest possible meaning, for example, in the claims, the phrase, “electrical connection” is used to describe the connection between the backplate and a component on the PCB. This phrase also encompasses an intermediate connection between a trace or conductive element on the PCB substrate and from these to the component.  
      While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form, modification, variation and details may be made therein without departing from the scope of the invention as defined by the appended claims.