Patent Abstract:
A geometrically shaped acoustic polymer ribbon with shape memory, high conductivity, high toughness. A method of manufacturing the ribbon comprises: forming a sized, elongated, coated or coatable polymeric substrate film between a pair of opposed, geometrically shaped dies, pinching the dies about the polymeric substrate film to form an assembly, heating the dies and the pinched die and polymeric film assembly to a temperature of at about 300 degrees F. for a period of about 15 minutes to set the elongated film into a predetermined geometric pattern, cooling the assembly, removing the film from the dies; and if not already coated, coating the geometrically formed, set, elongated film with a conductive coating.

Full Description:
One embodiment of the present invention relates to microphone elements which are responsive to minute acoustic vibration of fluids such as air over the frequency and amplitude range of human hearing, yet which are tough and resistant to various damaging forces, and a method for manufacturing thin film acoustic ribbons particularly for ribbon microphones, and is a continuation-in-part of our U.S. patent application Ser. No. 11/242,611 filed Oct. 3, 2005 now U.S. Pat. No. 7,894,619 and Ser. No. 11/242,612 filed Oct. 3, 2005, now U.S. Pat. No. 7,900,337 which were based on Provisional patent application Ser. No. 60/620,934 filled 21 Oct. 2004, each of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     One aspect of the present invention to provides an arrangement for producing an improved material that may be used as acoustic ribbons that overcome the disadvantages of the prior art. 
     Another aspect of the present invention provides polymer acoustic ribbons that have minimum mass and high sensitivity and with high signal conductivity coatings thereon. 
     Another aspect of the present invention provides polymer ribbon articles that may retain a geometric shape after being distorted. 
     In another aspect of the invention the sound sensor is usable as a wide bandwidth microphone having a frequency response approximately corresponding to that of the entire range of the human ear. 
     BRIEF SUMMARY 
     One embodiment of the present invention comprises a low mass, high conductivity, shape memory acoustic film material and a process utilized for the forming of an arrangement of polymer film acoustic ribbons, and the like, which ribbons are to be used in acoustic transducers including for example, ribbon microphones as fluid responsive moving sound sensors. 
     Another embodiment of the present invention comprises the method of designing and using such an arrangement to produce a fluid responsive sound sensor having particular mechanical, electrical and shape-memory characteristics that result in a durable yet light and low mass, highly sensitive fluid responsive sound sensor. 
     DISCUSSION OF THE PROCESSES AND APPLICATIONS 
     The processes of designing and fabricating practical ribbon structures begin with selecting a candidate material for fabricating sound responsive structures which may be, for example, a thin-film polyethylene terephthalate, or PET. The PET material may be supplied in roll or sheet form. The size selected may be about 3 inches wide, and the thickness selected may be about 2.5 microns, which is very thin, and light. PET is a high strength polymeric material that, during or after the foil ribbon manufacturing process, may be prepared with a layer of aluminum, gold, or other conductive metallic material as will be further taught herein. 
     Before the forming process can begin the polymer material must be cut into the desired shape. For a ribbon microphone application, where typically an elongated sound sensor is suspended between the poles of parallel magnets, the polymer material may be cut into rectangular strips. The preferred cutting method includes the use of a precision blade shear or a laser cutter. During the blade shearing process, the thin film polymer material may stick to the shear blade due to a static charge therewith. The polymer film also has a tendency to push away during the shearing process leaving the cut strip slightly tapered. To solve these problems, the material may be held for example, between two layers of waxed paper and adhesively taped along its leading edge with masking tape. This arrangement provides a static barrier and adds a temporary mass and rigidity to the polymer film, virtually eliminating the tendency for it to be pushed away during the shearing process. 
     Once the material is cut into the desired shape, in this example, a rectangle, a forming process may then be performed. The material may be thermally formed, or set, into the desired shape by placing the material between a pair of shaped steel dies, or forming dies. An identical repeating zig-zag or sawtooth pattern, for example, on the forming dies allows the two sides of the dies to mesh together with the material to be formed in between. With the two halves of the forming dies securing the material therebetween, the arrangement is ready to be thermally set. 
     A convenient method for heating the forming die assembly and polymer material is to place the articles, in a meshed state, into a preheated oven that is controlled by a thermostat. The forming die and polymer are then subjected to a suitable temperature over a period of time, and then removed from the oven and allowed to cool, when the polymer material will be found to have assumed the shape of the forming dies. 
     The material may be tested to determine its linear tensile strength and demonstrate that the PET material is about 8 times stronger than the thin aluminum material commonly used for ribbon microphone applications. 
     The material tensile, elongation and shape memory properties are important to the longevity and performance of acoustic ribbon elements. Because of the high strength, a polymeric ribbon element such as that made as taught herein is very desirable. This material when used as a ribbon element has the ability to resist wind blasts, high sound pressure levels, and electrical jolts such as those caused by the application of phantom power, without breaking or sagging. Phantom power is the 48 volt DC power supplied by mixing boards and microphone amplifiers used to power condenser type microphones. In some instances, the 48 volt DC power can be unintentionally routed to the ribbon microphone, which can then permanently distort and ruin the ribbon. 
     The excellent shape memory of preformed ribbons made from polyethylene terephthalate and other polymer and composite substrates allows them to retain their geometry and they can be extended to the point where the corrugations are flattened and readily returned, unstrained, to the original corrugated state. This material also has high strength which means the thickness of the material can be decreased to about 1.5 microns or less, thereby reducing the mass of the ribbon element which is also desirable because a lower mass ribbon is more responsive to incoming sound waves. Lower substrate and ribbon mass results in greater sensitivity and is desirable as it allows the faintest sounds to be converted into electrical energy. Having a low “substrate mass” may also be desirable in some instances where other high conductivity coatings with relatively greater mass than aluminum, such as gold and gold alloys for example, are used therewith. 
     Aluminization using direct evaporation of aluminum atoms upon the thin substrate material, applied evenly to one or both sides, has been found effective to produce a desired structure that is tough yet relatively low in mass, highly flexible and with good shape memory, and highly conductive, all of which are required for the successful sound-responsive ribbon-type element that may be used in a ribbon microphone or the like. 
     One embodiment of the invention comprises a ribbon microphone that accommodates the entire range of human hearing, both in frequency response and also in dynamic range including high sound pressure levels exceeding safe hearing limits but within recordable limits using the improved microphone and ribbon assembly. Such sound pressure levels start below about 10 dB, which is at the lowest threshold of natural hearing, and extend to above 150 dB, which is well above a safe sound level for humans, but within normal operating range of the ribbon microphone assembly as taught herein. 
     Another aspect of the invention includes a fluid coupled sound sensor having a corrugated ribbon-like form comprised of layers of conductive and nonconductive materials, the nonconductive materials having a thickness of about 3 microns or less and the conductive coating having a thickness of at least 100 nanometers, with a total weight of about 0.004 grams per square inch or less. The structure is produced whereby the conductive and nonconductive materials work in unison to produce a highly flexible, shape memory, highly sound responsive component, and whereby said sound responsive component comprises the ribbon in a ribbon microphone assembly having an acoustic responsivity of about 2 Hz to about 20 KHz. 
     Further, the ribbon structure is effective to return to a corrugated shape after extension to a flat shape, as encountered during wind blasts, for example, thereby overcoming limitations in the prior art, while maintaining sensitive sound responsivity over the acoustic range of human hearing. 
     One embodiment of the invention thus comprises a fluid coupled sound-sensor having a corrugated ribbon-like form comprised of layers of conductive and nonconductive materials, the nonconductive materials having a thickness of about 3 microns or less and the conductive layer having a thickness of at least 100 nanometers, with a total weight of about 0.004 grams per square inch or less, whereby the conductive and nonconductive materials work in unison to produce a highly flexible, shape memory, sound-responsive component, and whereby the sound responsive component comprises a ribbon in a ribbon microphone assembly having an acoustic responsivity of about 20 Hz to about 20 KHz. The conductive layer may be comprised of aluminum. The non-conductive layer may be comprised of a polymer. The corrugated ribbon-like form is cyclable from said corrugated form to a flat form and back to a corrugated form. 
     Another embodiment of the invention comprises a geometrically shaped acoustic ribbon comprised of layers with one layer being a highly elastic shape memory material and at least one additional layer of highly conductive material, wherein the combination of the layers produces high elongation and toughness characteristics while maintaining low mass and high conductivity of said acoustic ribbon. 
     Another embodiment of the invention comprises a method of manufacturing a coated, geometrically shaped, shape memory acoustic ribbon with an elastic polymeric substrate material with a second highly conductive layer and weighing no more than 0.004 grams per square inch and comprising: forming a sized, elongated, coated or coat-able polymeric substrate film between a pair of opposed, geometrically shaped dies; pinching the dies about the polymeric substrate film to form an assembly; heating said dies and said pinched die and polymeric film assembly to a temperature of about 300 degrees F. for a period of about 15 minutes to set said elongated film into a predetermined geometric pattern; cooling the assembly; removing the film from the dies; and if not pre-coated with a conductive coating, coating the geometrically formed, set, elongated film with a conductive coating. The polymeric film may be comprised of polyethylene terephthalate. The coating may be comprised of a metal selected from the group comprises of: aluminum, gold, silver, nitinol, copper-zinc-aluminum and copper-aluminum-nickel. The method may include perforating the polymeric film with a plurality of spaced apart holes to minimize the mass thereof. The method may include applying a coating of wetting material to the film prior to the pinching of the film between the dies. The wetting material may be comprised of isopropyl alcohol. 
     Another embodiment of the invention includes an acoustic ribbon for use in a flux frame of an acoustic ribbon microphone, comprising: an elongated polymeric substrate coated with a conductive coating; an arrangement of holes spaced along the substrate through the conductive coating and the substrate. The conductive coating may be comprised of nickel titanium. The conductive coating may be comprised of a compound selected from the group comprised of aluminum, copper, zinc or nickel. The elongated polymeric substrate may be comprised of a zig-zag geometric shape. 
     Another embodiment of the invention includes an elongated acoustic ribbon for microphones, the ribbon being comprised of a polymer substrate and a first conductive layer of metal coated on a first side of the substrate, the acoustic ribbon having a total weight no greater than 0.004 grams per square inch. The substrate may consist of Polyethylene Terephthalate. The substrate may be coated by a second conductive layer of metal on a second side thereof. The elongated acoustic ribbon for microphones may reside unstrained, in a zig-zag shape in cross section. The elongated acoustic ribbon may be comprised of a “shape-memory” microphone element. The second conductive layer of metal may be comprised of metal of a different thickness than the first conductive layer. The second conductive layer of metal may be comprised of a different conductive metal than the first conductive layer. 
     Another embodiment of the invention comprises an elongated shape memory acoustic microphone ribbon element assembly, consisting of: an elongated shape memory polymeric substrate; and a conductive coating arranged on the substrate, wherein the ribbon element assembly weighs no more than 0.004 grams per square inch. The conductive coating may comprise carbon nanotubes. The substrate may have a predetermined shaped formed thereon. The coating may be arranged on both a first and a second side of the substrate. The substrate may be comprised of Polyethylene Terephthalate. The coating may be comprised of a metal. The coating may be comprised of aluminum. The assembly preferably has dimensions of about: length 3.4″ and 0.145″ wide and 2.5 microns thick, and weighs about 0.002 grams. The substrate may be perforated. The coating may be perforated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and advantages of the present invention will become more apparent when viewed in conjunction with the following drawings in which: 
         FIG. 1  is a perspective graphic showing a set of blades of a shear used to cut very thin foils in the construction of the present invention; 
         FIG. 2  is a perspective view showing a foil and wax paper sandwich taped together in preparation for cutting with a shear; 
         FIG. 3  is a side view of a strip of foil being sheared; 
         FIG. 4  is a side view of a forming die; 
         FIG. 5  is a perspective graphic of the two halves of a forming die set, with a strip of foil arranged therebetween; 
         FIG. 6  is a perspective view of foil being wetted with a fluid such as alcohol and brushed into an array of groove portions of the forming die; 
         FIG. 7  is a perspective graphic of a clamped forming die assembled with a strip of foil in place, indicating its position in the oven; 
         FIG. 8  is a side view of a tensile set up using a hanging weight method with a specified gauge length; 
         FIG. 9A  is a side view of an acoustic ribbon shown formed in a corrugated shape; 
         FIG. 9B  is a side view of an acoustic ribbon deformed into an almost flat shape; 
         FIG. 9C  is a side view of an acoustic ribbon shown having returning to its original formed corrugated geometric shape; 
         FIG. 10A  is a perspective representation of an acoustic ribbon assembly comprising a conductive composite ribbon placed in a magnetic field; 
         FIG. 10B  is a perspective representation of the acoustic ribbon assembly shown in  FIG. 10A , being subjected to a signal causing it to extend; 
         FIG. 10C  is a perspective representation of the acoustic ribbon assembly shown in  FIG. 10B , with its ribbon returning to its original shape after a signal is removed; 
         FIG. 11  is a block diagram of the assembly of  FIG. 10 , wherein a ribbon is connected to a transformer and a further preamplifier; 
         FIG. 12  is a block diagram of the assembly of  FIG. 10 , wherein the ribbon is directly connected to a device on an amplifier without an intervening transformer; 
         FIG. 13  is an enlarged side view of a corrugated polymer ribbon that is comprised of a shape memory polymer substrate with a conductive layer or coating applied thereon; 
         FIG. 14  is an enlarged side view of a polymer substrate with one conductive layer applied to one side thereof; 
         FIG. 15  is an enlarged side view of a polymer substrate sandwiched between two conductive layers. 
         FIG. 16  is a table of ribbon characteristics; and 
         FIG. 17  is a graph of ribbon frequency ranges. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail, and particularly to  FIG. 1 , there is shown an initial step in the process of the present invention, for the forming of polymer foil acoustic ribbons, which ribbons are to be used in acoustic transducers including for example, ribbon microphones. 
     The process begins with the selection of a suitable polymeric material, for example, a thin film polyethylene terephthalate  10 . This material is supplied in roll and sheet form and the material size selected is 3 inches wide and the thickness is 2.5 microns. This polymeric material is, during or after the foil ribbon manufacturing process, coated with a layer of aluminum, gold, or other conductive material. 
     Before the forming process can begin the polymer material  10  must be cut into the desired shape as represented in  FIG. 1 . For a ribbon microphone application, the polymer material is cut into rectangular strips that are for example, about 3 inches long by about 0.145 inches wide. The preferred cutting method includes the use of a precision shear  12 . During the shearing process the thin film polymer material may stick to the shear  12  because of a static charge therewith. The polymer film  10  also has a tendency to push away during the shearing process leaving the cut strip slightly tapered. 
     Referring to  FIG. 2  the polymer film  10  is sandwiched between two layers of waxed paper  14  and taping along its leading edge with masking tape  16 . This provides a static barrier and adds a temporary mass and rigidity to the polymer film  10  and eliminates the shearing problems of static and push away as described above. 
     Referring to  FIG. 3  the foil  10  is being sliced into a rectangular strip such as those used in microphone applications by the shear  12  which is also shown in  FIG. 1 . 
     Referring to  FIG. 4  the sliced rectangular polyethylene terephthalate film  10  is heat set into the desired shape by a sandwiching arrangement between a pair of steel dies  18  and  20 . An identical zig-zag shaped repeating pattern  19  allows the two sides of the dies  18  and  20  to mesh together. 
     Referring to  FIG. 5  the sliced rectangular polyethylene terephthalate film  10  is shown between the forming dies  18  and  20  before the dies are meshed together. There are two methods used for placing the film material  10  between the dies  18  and  20 . One method is to lay the material flat between the one face of the die  20  and place the other half of the die  18  on top of the film material  10 , also represented in  FIG. 5 , in a way that allows the two dies to mesh with one another. A light pressure is applied to the two dies  18  and  20  until they are firmly meshed together with the film material  10  in between. 
     The second method for placement of the film material  10  between the dies  18  and  20  is represented in  FIG. 6 , wherein the film material  10  is lain onto the first die  20  and using for example, a wetting agent such as isopropyl alcohol to wet the film material  10  with a small paint brush  24  or spraying or the like, and push the film material  10  into the die  20  so the capillary action between the ribbon film material  10  and the die  20  helps the film material  10  to follow the shaped geometry of the die  20 . When the film material  10  is in place, the second die  18  is placed thereon and pressure is applied thereto. 
     Referring to  FIG. 7  the dies  18  and  20  may be held in place sandwichingly around the film material  10  by use of a clip or wire means  26  bindingly wrapped theraround. With the two halves of the forming dies  18  and  20  securing the film material  10  therebetween, the film  10  is ready to be heat set. The preferred method for heating the forming die assembly ( 10 ,  18 ,  20  and  26 ) is for heat setting in an oven, (not shown for clarity of views). The temperature of the oven is set and may be monitored using a thermocouple. The oven may be preheated for 30 minutes and the temperature may be set for example, at about 300° F. The fluctuation in the oven set at 300° F. is recorded at about 295° F.-305° F. A flat tray is preferably placed in the oven when it is turned on and allowed to preheat with the oven. When the oven preheat is completed the forming die assembly ( 10 ,  18 ,  20  and  26 ) is placed in the oven on the tray in an upright position. The amount of time required for the film material  10  to permanently take the geometric shape of the dies  18  and  20  is for example, about 15 minutes. After opening the oven door the temperature drops and the oven must come back up to temperature before starting the timer. When the heat cycle is complete the dies  18  and  20  are removed from the oven and allowed to cool for about 10 minutes before the dies  18  and  20  are opened and the film material  10  removed therefrom. 
     Referring to  FIG. 8  the film material  10  may be tested to determine its linear tensile strength. There are a number of ways this can be achieved including the use of a commercial tensile tester, for example an Instron™ tensile device, or by use of a hanging weight method. Samples of polyethylene terephthalate film  30  were tested using the hanging weight method. The dimensions were about 3 inches long 0.157 inches wide and 2.5 microns thick. The polyethylene terephthalate film sample  30  was suspended from a beam  32  using adhesive tape to secure it to that beam  32 . The modified clamp  34  with a weight hanger  36  was attached to the opposite end of the polyethylene terephthalate  30 . The space between the beam  32  and the distal end of the jaws of the clamp  34  (gauge length) which in this example, was 1.5 inches. Weights  37  were gradually added to the weight hanger  36  until failure of the polyethylene terephthalate  30 . The polyethylene terephthalate  30  failed after a total of about 139 grams of weight were added to the hanger  36 . The polyethylene terephthalate  30  elongated about 0.5 inches total, most of this occurring after the last 20 grams of weight were added. As a comparison a sample of aluminum was tested using the same method. The aluminum was the same size as the polyethylene terephthalate film and the same gauge length was used. The aluminum failed after the addition of about 16.7 grams of weight with no noticeable elongation. Therefore the polyethylene terephthalate film is about 8 times stronger than the aluminum material commonly used for ribbon microphone applications. 
     Referring to  FIG. 9A  the excellent shape memory of preformed ribbons  31  made for example, from polyethylene terephthalate  30  and other polymer substrates, allows them to retain their geometry and can be extended to the point where the corrugations are flattened, as represented in  FIG. 9B , and then readily return, unstrained, to a preset corrugated state as represented in  FIG. 9C . Having high strength also means the thickness of the material can be decreased to about 1.5 microns or less, thereby reducing the mass of the ribbon element which is also desirable because a lower mass ribbon is more responsive to incoming sound waves. This will result in greater sensitivity and permit in some applications the use of high conductivity coatings with greater mass, such as for example gold, which has poor tensile strength when used alone. 
     Referring now to  FIG. 10A  a ribbon element  30  is shown at rest, suspended in a flux frame  48  between magnets  46  and fixed at both ends. As a way to reduce its mass the ribbon element  30  may have perforations  40 . The perforating may be done using a laser, a drill, high pressure water machining or a punch, (not shown for clarity of the figures). Once the perforated holes  40  are in place the ribbon element  30  may be coated on both sides with a metallic layer which allows current to flow through the ribbon element by way of the holes  40 . 
     Referring to  FIG. 10B  shows the ribbon element  30  being elongatively distorted or extended and subsequently returning to its formed geometric shape upon release of any strain thereto, as represented in  FIG. 10C . 
       FIG. 11  is a block diagram representation of the assembly represented in  FIG. 10A  connected by a proper circuit  50 , to a transformer  42  and also to a further preamplifier  44 . 
     Referring to  FIG. 12  is a block diagram representation of the assembly represented in  FIG. 10A , connected directly to an amplifier  56  without an intervening transformer. 
     Referring now to  FIG. 13  is an enlarged side view of a corrugated ribbon element  30  with two layers. One layer is a polymer substrate  64  and the second layer is a conductive coating  62 . The conductive coating may be comprised of gold, gold alloys and aluminum as taught herein, or may be comprised of shape memory alloys such as copper/zinc/aluminum, copper/aluminum/nickel, and nickel titanium. 
     Referring to  FIG. 14  is an enlarged side view of a thin film polymer substrate  64  with a conductive coating  62  before corrugation thereof. 
     Referring now to  FIG. 15  is an enlarged side view of a thin film polymer substrate  64  with a conductive coating  62  on one side and a conductive coating  66  applied to the opposite side to create a more symmetrical structure with respect to the substrate  64 , to improve responsiveness, evenness, and acoustic efficiency of the ribbon assembly  60 . 
     The combination of the polymer substrate  64  and the conductive coating(s)  62  and/or  66  for an acoustic ribbon  60  of the present invention should be limited in weight and/or mass per unit area to that of a single component acoustic aluminum ribbon of the prior art. Such a prior art acoustic aluminum ribbon have dimensions for example, of 3.4″×0.145″×2.5 microns and may weigh 0.002 grams. A strip of thin film polyethylene terephthalate that measures 3.4″×0.145″×2.5 microns weighs about 0.001 grams. This permits 0.001 grams of aluminum metal to be added as a coating to the polymer substrate while maintaining a similar mass compared to prior art ribbons. Thus, a conductive coating (of for example, aluminum of up to 0.001 grams per 3″ length) may be added to the polymer substrate as inclusive of the present invention. Such conductive coating (of aluminum) may be added to the substrate  64  in single layers, multiple layers or combinations of thicknesses to one or both sides of that substrate  64 . 
     Referring back to  FIG. 13  another method used to produce polymer ribbons is with a polymer substrate that has been pre-coated with a conductive material as taught herein. The conductive coating  62  may be applied using various methods including vacuum vapor deposition. Polyethylene terephthalate  10  is one suitable substrate material but other materials could also be used. Substrate materials may include nylons, polyesters, polyketones and acrylics such as polyaramid, polyurethane, polyimide, polypropylene, PVC, polyethylene, polyester, acetate, polyetheretherketone and other thermoplastic and thermoset polymers. Substrate materials may be configured as flat sheets or as a combination of fibers and or nano-fibers in woven or non-woven states. Materials such as aromatic polyimide, fiber glass, polyester, cotton, expanded PTFE, carbon nanotubes in both sheet and linear form may be used to produce fibers. The fibers may be pre-coated, or coated post processing with a conductive coating or adhered to a sheet of conductive material such as aluminum. Carbon nanotubes may also be attached to a polymer substrate  64  to enhance the electrical conductivity and strength of the applied coating or as the coating itself. 
     Referring again to  FIG. 14  practical ribbon assemblies may be produced using various substrate and coating thickness combinations, including, for example, a 2 micron substrate  64  of polyethylene terephthalate with a 500 nanometer coating of aluminum  62 . In sheet form this material will curl tightly when not secured by its edges. The curling may be caused by the differences in mass and shrink rate between the substrate  64  and the coating  62 , or by thermal effects which relieve stresses present in the substrate polymer. In order to cut the material into the desired shape for further processing it is desirable for the material to lay reasonably flat. The material may be flattened by placing it between flat 0.015″ aluminum plates and heating in an oven. In one practical process embodiment, the oven is preheated to 320° F. and the material placed in the oven for 12 minutes, removed and allowed to cool for 10 minutes. 
     Referring again to  FIG. 15  another method to reduce curling of the material is to apply the conductive coating  62  and  66  to both sides of the substrate  64 . The advantage being that a bilateral structure and associated symmetry will exhibit self compensation of lateral forces and therefore may have less curl. Because the substrate material  64  may shrink to a greater degree when a first side is coated, a thicker coating may be applied to the first or second side to balance the curling forces. An alternative to this compensation process may be to deposit conductive layers on all sides of the substrate simultaneously, thereby balancing the resultant stress forces that may distort the underlying substrate and subsequently, the composite structure. A further benefit of dual layer deposition is the potential to use each side as a separate circuit or as separate series or parallel circuit elements that may have the advantage of producing a longer effective path through a magnet gap, and therefore higher sound to current conversion efficiency. 
     Returning back to  FIG. 2  the flattened pre-coated material is sandwiched between two layers of waxed paper  14 , and as shown in  FIG. 3  cut using a precision shear as taught here within. Once it has been cut the material can be formed between two heated dies as shown in  FIG. 7  and also taught here within. The coated substrate material is heated to 300° F. for 12 minutes and allowed to cool for 15 minutes. Other time and temperature combinations may be used to flatten and form the material as long as the material does not become brittle or lose its shape memory. 
     An advantage of the polyethylene terephthalate film is that it will not become brittle with age under normal conditions because it contains no plasticizers. Another advantage is that it offers good shape retention. Once cooled after the formation of the corrugation of the film, the structure retains its geometry as seen in  FIG. 9A , and even when it is deformed for a substantial period of time, represented by  FIG. 9B , it will naturally and spontaneously return to the formed geometry when released as seen in  FIG. 9C . This ability to remember the formed geometry is called shape memory, and is a desirable property for use in a delicate sound sensitive device such as a microphone, which must remain highly responsive and flexible, yet be rugged and tough enough to withstand high external air pressure forces and internal magnetic and electrical forces. 
     Referring back to  FIG. 10A , the polymer composite ribbon  30 , when used in a ribbon microphone, is placed between two strong permanent magnets such as neodymium iron boron  46  with a strong magnetic field and fastened at two ends. The ribbon is free to move between the magnets within a working gap. The polymer composite ribbon  30  is moved by incoming sound pressure as the ribbon  30  moves in the magnetic field and an electronic signal is produced and sent to an amplifier. The polymer ribbon  30  has the ability to pick up sound pressure levels (SPL) in a range as low as 10 db SPL and very loud sound pressure levels as high as 150 db SPL without sound distortion or damage to the ribbon  30  because of the low mass and high strength of the polymer composite and its ability to retain its geometry. 
     A ribbon microphone built with an aluminum ribbon as described in the prior art can be used in a bidirectional configuration allowing it to receive and process sound levels from multiple directions. It also can be used in a broad frequency range from 30 Hz to 20 KHz. The polymer composite ribbon of the present invention exhibits broad response from 20 Hz to 20 KHz, unlike ribbon tweeters or loudspeakers made from aluminum and or polymers, contrasted graphically in  FIGS. 16 and 17 , mainly because of the relatively high mass of a ribbon element used in a tweeter or other radiating structure sensitive and small enough to be used as a practical microphone. 
     Referring back to  FIGS. 10A and 10B  a polymer composite ribbon  30  may be less susceptible to environmental conditions weakening it such as wind blasts, humidity in the air and moisture. This is because the polymer composite ribbon  30  is less likely to corrode when exposed to moisture, and the process of vapor deposition affords ample opportunity to apply very thin sealing layers, oxide layers etc., which improve environmental durability. The polymer composite ribbon  30  exhibits high strength and has shape memory property enabling it to return to its original geometry when stretched or extended as seen in  FIGS. 9A ,  9 B and  9 C. This can be appreciated by comparing a 2 micron polymer ribbon with a 500 nanometer conductive coating of aluminum as seen in  FIG. 13  and prepared according to the teachings of this invention, to a conventional aluminum ribbon as used in the prior art, and noting the differences. The process for comparison is as follows: Each ribbon made for the comparison is cut into strips 3.4 inches long by 0.145 inches wide and corrugated. The polymer composite ribbon  30  is corrugated by heat setting as demonstrated in  FIGS. 5 ,  6  and  7  and taught herein, and the conventional aluminum ribbon is corrugated using mechanical distortion means. Mechanical distortion, or bending, can be produced by passing the thin conventional ribbon material through a set of enmeshed gears as is commonly encountered in the prior art. The corrugated ribbons thus produced have a reduced overall corrugated lengths of 2.480 inches for the conventional aluminum ribbon used in the prior art, and 2.906 inches for the polymer ribbon  30 . 
     Once prepared, each ribbon is extended under moderate tension in the axial direction as demonstrated in  FIG. 9B  until the corrugations may be observed to be flat or as nearly so as practical. Once extended and flattened, each ribbon is then released and allowed to relax, and the result observed and measured. It can be observed that the conventional aluminum ribbon, when extended so that the corrugations became flat or as nearly so as practical, and it is allowed to relax was 0.73 inches longer than the original corrugated length. Therefore the total resting elongation of the aluminum ribbon after extension and relaxation is 23%. Such an elongated ribbon is not suitable for use in a ribbon microphone. 
     By contrast, the polymer composite ribbon  30  when also extended so that the corrugations become flat or nearly so can be observed to return to its original length when released, as demonstrated in  FIGS. 9A ,  9 B and  9 C. The relaxed length after extension is 2.906 inches or 0.00 inches longer than the original corrugated length. Therefore the total resting elongation of the polymer ribbon after extension and relaxation is negligible. This ability of a thin, light, low mass, highly conductive, air responsive ribbon like structure to return to shape after extension is highly desirable and is a significant improvement over the prior art. This toughness is the result of a system that has adequate elasticity and strength to maintain shape memory under similar stretching and/or extension as commonly encountered in sound sensing applications yet retains high conductivity needed for effective transducer efficiency. 
     The process of forming a conductive, shape memory ribbon  30  may also be performed in reverse, by providing a conductive substrate first such as an aluminum ribbon used in ribbon microphones, and then depositing a settable polymer onto the conductive substrate through vapor deposition. Such polymeric vapor deposition may be performed in a controlled chamber with heat, gases, ultraviolet curing lamps and polymer vaporization capabilities which may include plastic films such as thermoplastics like PET, PEEK, Kapton or Parylene, or carbon deposition of nanotubes and films. The polymeric vapor may be effective to conform to a preformed ribbon, further aiding the shape retention qualities, and may be enhanced by the application of fibrous substances, particles, and at various thicknesses at different locations. Alternatives also include lamination processes or any process of providing combined physical properties, with the object to provide the elongation and toughness characteristics, while maintaining low mass and high conductivity, all required to produce a successful sound sensor ribbon microphone arrangement which is one object of the present invention.

Technology Classification (CPC): 7