Abstract:
A noise suppressing microphone employing a thin film of piezoelectric material sandwiched between two conductive layers, one of which is laminated to a flat inflexible laminate. The sandwich is embedded in a mechanical sound filter except for one exposed face which is placed in contact with a user&#39;s body for sound pickup. The filter is composed of sound dissipating material, alone or in complementary configurations, and in sandwiches including a sound absorbing and sound reflecting metal layer, or a pillow of mastic-like sound-deadening material

Description:
INTRODUCTION 
     The present invention relates to communications and particularly voice communications. More particularly, the present invention relates to a noise suppressing microphone apparatus for high clarity two way radio communications in high noise, and environmentally extreme conditions. 
     BACKGROUND OF THE INVENTION 
     Two-way radio communications in high noise, environmentally extreme conditions is difficult, or often impossible. Such communications are often erratic, intermittent, and subject to various forms of environmental and operational interference and disruption. This is particularly true where high noise levels, moisture, and other challenging conditions of the particular applications environment are simultaneously effective. Such conditions are commonly found in military applications involving high-noise operations in the air such as helicopters and other types of aircraft, on the surface, such as tanks, air-cushion vehicles, and personnel carriers, and on or beneath the surface of a body of water, such as high speed boats, air-cushion watercraft, and submersibles. 
     Likewise, civilian applications include, but are not limited to, motorcycles, jet skis, skydiving, motor boating, firefighting, video games, voice input to computers, police work, voice recognition for computers, and hazardous materials applications. In most, if not all of such exemplary applications, hands-free operation is desirable, if not essential. 
     The environmental exposure to which such a microphone may be subjected includes high and low temperature, extraordinary shock and vibration effects during handling and use, high levels of audible sound interference, moisture, toxic, and chemically damaging agents. One of the best examples of an environment containing the broadest spectrum of challenging and extreme environmental factors is that of firefighting. The firefighting application demands the use of special apparel such as gloves, helmet liners, and face and eye protective elements that may contribute adversely to the use and operational effectiveness of prior art microphones. 
     Prior art conventional microphones are typically positioned on a boom in front of the user&#39;s mouth to pick up speech. In this location, the microphone will pick up ambient background noise including wind and breathing noise, and other external noise. Also, microphones mounted on or within a helmet or mask worn by a user are prone to pick up not only the aforementioned sounds but resonation&#39;s and reverberations of same. Such noise may completely drown out or obscure otherwise intelligible voice communications. Noise cancellation provided by special circuitry may be applied to the microphone output to achieve some degree of improvement. Such circuitry requires a power source and physical and functional support. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to provide a microphone having noise suppression capability for use in a high-noise operationally challenging environment. 
     It is yet another object of the present invention to provide a solid state microphone having integrated mechanical noise filtration. 
     It is still another object of the present invention to provide a piezoelectric microphone having integrated mechanical noise filtration. 
     These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     The present invention, the Noise Suppressing Microphone, is a piezoelectric sandwich transducer embedded in a mechanical noise filter, except for one transducer face employed for contact with the user&#39;s body for pickup of user-created sound. The sandwich is a thin layer of piezoelectric film between two conductive thin film metallic layers to which output leads are attached. The sandwich is securely mounted to a flat, substantially inflexible substrate. This assembly is covered by a hydrophobic coating and then embedded in a single or multiple layer mechanical sound suppressing filter. The filter absorbs and dissipates ambient sound impinging on the filter material surrounding those portions of the transducer not in direct contact with the user&#39;s body. 
     The microphone of the present invention is particularly effective when used in contact with the user&#39;s forehead. However, using the invention to detect maximum sound pickup at various points on the user&#39;s body will permit optimum location for a particular application. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
     FIG. 1 is side view of the microphone embedded in a single layer filter. 
     FIG. 2 is a side view of the microphone embedded in a multi-layer filter. 
     FIG. 3 is a side view of the microphone embedded in a multi-layer sandwich filter. 
     FIG. 4 is a side view of the microphone embedded in another version of a multi-layer sandwich filter. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the microphone assembly  10  of the present invention, the High-Noise Suppression Microphone is shown. 
     A thin piezoelectric-film  18 , made for example of polyvinylidene fluoride (PVDF), is sandwiched between two conductive layers  20  and  21 , which may be thin metallic films. This forms a piezoelectric sandwich element, or more specifically a PVDG sandwich element. The conductive film layers  20  and  21  coat the bottom and top surface of the piezoelectric film  18  and are constructed from conductive material such as aluminum or nickel. Wires  28  for connections from the sandwich are attached to the top  20  and bottom  21  conductive layers using silver epoxy. The sandwich element is then firmly mounted or laminated on a solid, flat, substantially inflexible, substrate  24 , which is preferably a piece of printed circuit board or equivalent material. 
     The wires  28  connected to conductive layers  20  and  21  are connected to the inputs of an impedance matching circuit  50  shown in FIG. 4 to address the high natural impedance of the piezoelectric sandwich. Details of the impedance matching and power connections are discussed in U.S. patent application Ser. No. 08/136,856, as they are not essential to the disclosure and description of the present invention. 
     Referring to the side views shown in FIGS. 1,  2 , and  3 , the piezoelectric sandwich comprising piezoelectric film  18  and conductive films  20  and  21  is shown affixed to the circuit board which forms the inflexible substrate  24 . The substrate  24  is laminated to the piezoelectric sandwich along substantially the entire surface of one of the two conductive layers  21 . This sandwich has a square form of 0.75 inch by 0.75 in one preferred embodiment. 
     A ground shield  25  is preferably placed about the piezoelectric sandwich except for the exposed face  27  and another ground shield  52 , as shown in FIG. 4, over the impedance matching circuit if located separately, to allow use in an environment of high electromagnetic interference. 
     The surface of the film and circuit board is then covered with a hydrophobic epoxy layer  26  to provide environmental protection against water intrusion that would short out the piezoelectric film destroying its ability to function. The necessity in harsh environmental conditions of providing such a water-resistant layer is a primary reason why diaphragm-based piezoelectric microphones will not work under the conditions for which the present invention is needed. The above-described piezoelectric sandwich is embedded in a mechanical noise filter  42 . All faces of the sandwich are embedded in the filter except for the face of the piezoelectric sandwich opposite the face to which the substrate  24  is laminated. The noise filter  42  provides ambient noise suppression by means of absorption, dissipation, reflection, and other means, alone, and in combination. The filter  42  must be a single layer  32  of sound suppression or sound dissipating material as shown in FIG. 1, multiple layers  29  and  30  of complementary sound suppression material, as depicted in FIG. 2, a sandwich of complementary sound suppression material including layers  29  and  30  and a reflective and re-directive metal barrier layer  31  in-between, as shown in FIG. 3, and a sandwich of multi-layer composite layers  43  and  44  of complementary sound suppression materials including a reflective and re-directive metal barrier layer  31  in-between as shown in FIG.  4 . 
     In the embodiment of the microphone assembly  10  in FIG. 1, the noise filter  42  has a single layer  32  of sound suppression or sound dissipating material which absorbs impinging ambient sound energy. Material used for a single layer noise filter  32  includes sound absorbing film having an approximate weight of 0.2 lbs./sq. ft. 
     In the embodiment of the microphone assembly  10  in FIG. 2 employing a two layer  29 ,  30  noise filter  42 , complementary sound suppression or dissipating materials are selected to maximize the amount of sound energy absorption and thus virtually eliminate ambient sound or noise reaching the piezoelectric film  18 . The complementary sound suppression layers  29  and  30  may be made from various density rubber, rubber and foam composites, and polymeric materials having desirable sound energy absorbing characteristics. 
     In the embodiment of the microphone assembly  10  in FIG. 3, the noise filter  42  employs a metal layer  31  sandwiched between a first layer  29  of noise suppression material and a second layer  30  of noise suppression material. The sound absorbing materials for layers  29  and  30  are as described above for the filter  42  without the metal layer  31 . The sandwiched metal layer  31  acts as a reflector and absorber of sound energy in this arrangement. Any sound energy not absorbed and dissipated by entry layer  30  is in part reflected back into layer  30  and in part dissipated and communicated into inner layer  29 . The overall suppression of ambient sound reaching the piezoelectric film  18  with this filter  42  configuration is measurably better than the single layer  32  filter and the double layer  29  and  30  filter (FIG. 2) without the reflective dissipating metal layer  31 . The metal lead has been found particularly effective for use as the metal layer  31 , however, other metals may be used. In lieu of the metal layer, a pillow-like layer of mastic-like sound deadening material may be substituted. A mastic like floor tile adhesive offers excellent sound dissipating qualities. 
     In FIG. 4 the filter  42  is a sandwich of two multi-layer laminates of sound suppression and dissipation materials with a metal layer  31  in-between. Each multi-layer laminate consists of a layer  43  of high density material bonded or otherwise secured to a layer  44  of low-density material. Exemplary of such material is a {fraction (5/16)} inch thick composite material known as Quiet-Mat DSB-1 by Noise Reduction Enterprises of Essex, Mass. This material is a laminate of 1 lb./sq.ft. high-density sound barrier bonded to a ¼ inch foam decoupler. In this configuration, as above, a pillow-like layer of mastic-like material may be substituted for the metal layer for some applications. 
     FIG. 4 shows the arrangement of the epoxy sealed microphone element  41  installed in the metallic lead receptacle  49 , which rests in the cavity  46  in the filter  42 . More particularly the cavity  46  is shown located in the layer  44  of low-density material. The leads  28  for connecting the microphone element  12  to the impedance matching device  50  for ultimate connection to a radio are shown in FIG.  4 . 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. Thus, for example, other combinations of materials of high and low-density sound absorbing capacity may be used for particular applications. The use of confined or pillow-like mastic or gel-like layers between the layers of high and low density laminates and between these laminates and each side of a metal sheet  31  provides another means for dissipating ambient sounds constituting noise to clear communication. Thus it is, therefore, to be understood that, within the scope of the appended claims the invention may be practiced other than as specifically described.