Patent Publication Number: US-10791401-B2

Title: Compact electroacoustic transducer and loudspeaker system and method of use thereof

Description:
RELATED PATENT APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Ser. No. 62/697,055, filed on Jul. 12, 2018, to Joseph F. Pinkerton et al., entitled “Compact Electroacoustic Transducer And Loudspeaker System And Method Of Use Thereof.” 
     This application is related to U.S. Pat. No. 10,250,997, issued on Apr. 2, 2019, to Joseph F. Pinkerton et al., and is entitled “Compact Electroaccoustic Transducer and Loudspeaker System and Method Of Use Thereof” (the “Pinkerton &#39;997 Patent”), which issued from U.S. patent Ser. No. 15/333,488, filed on Oct. 25, 2016. 
     This application is also related to U.S. Pat. No. 9,167,353, issued on Oct. 20, 2015, to Joseph F. Pinkerton et al., and is entitled “Electrically Conductive Membrane Pump/Transducer And Methods To Make And Use Same” (the “Pinkerton &#39;353 Patent”), which issued from U.S. patent application Ser. No. 14/309,615, filed on Jun. 19, 2014, and which is a continuation-in-part to U.S. patent application Ser. No. 14/161,550, filed Jan. 22, 2014 (which issued as U.S. Pat. No. 9,264,795 on Feb. 16, 2016). 
     This application is also related to U.S. Pat. No. 9,143,868, issued Sep. 22, 2015, to Joseph F. Pinkerton et al., which issued from U.S. patent application Ser. No. 14/047,813, filed Oct. 7, 2013, and which is a continuation-in-part of International Patent Application No. PCT/2012/058247, filed Oct. 1, 2012, which designated the United States and claimed priority to provisional U.S. Patent Application Ser. No. 61/541,779, filed Sep. 30, 2011. Each of these patent applications is entitled “Electrically Conductive Membrane Pump/Transducer And Methods To Make And Use Same.” 
     This application is also related to U.S. Pat. No. 9,924,275, issued Mar. 20, 2018, to Joseph F. Pinkerton et al., and entitled “Loudspeaker Having Electrically Conductive Membrane Transducers,” which issued from U.S. patent application Ser. No. 15/017,452, filed Feb. 5, 2016, and which claimed priority to provisional U.S. Patent Application Ser. No. 62/113,235, entitled “Loudspeaker Having Electrically Conductive Membrane Transducers,” filed Feb. 6, 2015. 
     This application is also related to U.S. Pat. No. 9,826,313, issued Nov. 21, 2017, to Joseph F. Pinkerton et al., and entitled “Compact Electroacoustic Transducer And Loudspeaker System And Method Of Use Thereof,” (“the Pinkerton &#39;313 Patent,”) which issued from U.S. patent application Ser. No. 14/717,715, filed May 20, 2015. 
     U.S. patent application Ser. No. 15/647,073, filed Jul. 11, 2017, to Joseph F. Pinkerton et al., and entitled “Electrostatic Membrane Pump/Transducer System And Methods To Make And Use Same,” (the “Pinkerton &#39;073 Application”). 
     This application is also related to International Patent Application No. PCT/2019/30438, filed May 2, 2019, to Joseph F. Pinkerton et al., and entitled “Loudspeaker System and Method of Use Thereof,” (the “Pinkerton &#39;438 PCT Application”), which designated the United States and claimed priority to U.S. Patent Application Ser. No. 62/666,002, entitled “Audio Speakers,” filed May 2, 2018. 
     This application is also related to International Patent Application No. PCT/2019/33088, filed May 20, 2019, to David A. Badger et al., and entitled “Compact Electroacoustic Transducer And Loudspeaker System And Method Of Use Thereof,” (the “Badger &#39;088 PCT Application”), which designated the United States and claimed priority to U.S. Patent Application Ser. No. 62/673,620, filed May 18, 2018. 
     This application is also related to U.S. Patent Application Ser. No. 62/697,141, filed Jul. 12, 2018 to Joseph F. Pinkerton et al, and entitled “Cover-Baffle-Stand System For Loudspeaker System And Method Of Use Thereof.” 
     All of these above-identified patent applications are commonly assigned to the Assignee of the present invention and are hereby incorporated herein by reference in their entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     The present invention relates to loudspeakers, and in particular, to loudspeakers having an electrostatic transducer or an array of electrostatic transducers. The electrically conductive transducers generate the desired sound by the use of pressurized airflow. 
     BACKGROUND 
     Conventional audio speakers compress/heat and rarify/cool air (thus creating sound waves) using mechanical motion of a cone-shaped membrane at the same frequency as the audio frequency. Most cone speakers convert less than 10% of their electrical input energy into audio energy. These speakers are also bulky in part because large enclosures are used to muffle the sound radiating from the backside of the cone (which is out of phase with the front-facing audio waves). Cone speakers also depend on mechanical resonance; a large “woofer” speaker does not efficiently produce high frequency sounds, and a small “tweeter” speaker does not efficiently produce low frequency sounds. 
     Thermoacoustic (TA) speakers use heating elements to periodically heat air to produce sound waves. TA speakers do not need large enclosures or depend on mechanical resonance like cone speakers. However, TA speakers are terribly inefficient, converting well under 1% of their electrical input into audio waves. 
     The present invention relates to an improved loudspeaker that includes an array of electrically conductive membrane transducers such as, for example, an array of polyester-metal membrane pumps. 
     Graphene membranes (also otherwise referred to as “graphene drums”) have been manufactured using a process such as disclosed in Lee et al. Science, 2008, 321, 385-388. PCT Patent Appl. No. PCT/US09/59266 (Pinkerton) (the “Pinkerton &#39;266 PCT Application”) described tunneling current switch assemblies having graphene drums (with graphene drums generally having a diameter between about 500 nm and about 1500 nm). PCT Patent Appl. No. PCT/US11/55167 (Pinkerton et al.) and PCT Patent Appl. No. PCT/US11/66497 (Everett et al.) further describe switch assemblies having graphene drums. PCT Patent Appl. No. PCT/US11/23618 (Pinkerton) (the “PCT US11/23618 Application”) described a graphene-drum pump and engine system. 
       FIGS. 1-5  are figures that have been reproduced from FIGS. 27-32 of the Pinkerton &#39;353 Patent. As set forth in the Pinkerton &#39;353 Patent: 
       FIGS. 1A-1E  depict an electrically conductive membrane pump/transducer  2700  that utilizes an array of electrically conductive membrane pumps that cause a membrane  2702  to move in phase.  FIGS. 1A-1B  are cross-sectional views of the pump/transducer that includes electrically conductive members  2701  (in the electrically conductive membrane pumps) and a speaker membrane  2702 . Speaker membrane  2702  can be made of a polymer, such as PDMS. Each of the electrically conductive membrane pumps has a membrane  2701  that can deflect toward downward and upwards. Traces  2605  are a metal (like copper, tungsten, or gold). The electrically conductive membrane pumps also have a structural material  2703  (which can be plastic, FR4 (circuit board material), or Kapton® polyimide film (DuPont USA)) and support material  2704  that is an electrical insulator (like oxide, FR4, or Kapton® polyimide film). Support material  2704  can be used to support the pump membrane, support the stator and also serve as the vent structure. Integrating these functions into one element makes device  2700  more compact than it would be with multiple elements performing these functions. All of the non-membrane elements shown in  FIG. 1A-1E  can be made from printed circuit boards or die stamped sheets, which enhances manufacturability. 
     Arrows  2706  and  2707  show the direction of fluid flow (i.e., air flow) in the pump/transducer  2700 . When the electrically conductive membranes  2701  are deflected downward (as shown in  FIG. 1A ), air will flow out of the pump/transducer device  2700  (from the electrically conductive membrane pumps) as shown by arrows  2706 . Air will also flow from the cavity  2708  into the electrically conductive membrane pumps as shown by arrows  2707  resulting in speaker membrane  2702  moving downward. When the electrically conductive membranes  2701  are deflected upwards (as shown in  FIG. 1B ), air will flow into the pump/transducer device  2700  (into the electrically conductive membrane pumps) as shown by arrows  2706 . Air will also flow into the cavity  2708  from the electrically conductive membrane pumps as shown by arrows  2707  resulting in speaker membrane  2702  moving upward. 
       FIG. 1C  is an overhead view of pump/transducer device  2700 . Line  2709  reflects the cross-section that is the viewpoint of cross-sectional views of  FIGS. 1A-1B .  FIGS. 1D-1E  shows the flow of air (arrows  2707  and  2706 , respectively) corresponding to the deflection downward of electrically conductive membranes  2701  and speaker membrane  2702  (which is shown in  FIG. 1A ). The direction of arrows  2707  and  2706  in  FIGS. 1D-1E , respectively, are reversed when the deflection is upward (which is shown in  FIG. 1B ). 
     The basic operation for pump/transducer  2700  is as follows. A time-varying stator voltage causes the pump membranes  2701  to move and create pressure changes within the speaker chamber  2708 . These pressure changes cause the speaker membrane  2702  to move in synch with the pump membranes  2701 . This speaker membrane motion produces audible sound. 
     The ability to stack pumps in a compact way greatly increases the total audio power. Such a pump/transducer stacked system  2800  is shown in  FIG. 2 . 
     For the embodiments of the present invention shown in  FIGS. 1A-1E and 2 , the individual pump membranes  2701  can be smaller or larger than the speaker membrane  2702  and still obtain good performance. 
     Pump/transducer system  2700  (as well as pump/transducer speaker stacked system  2800 ) can operate at higher audio frequencies due to axial symmetry (symmetrical with respect to the speaker membrane  2702  center). Each membrane pump is approximately the same distance from the speaker membrane  2702  which minimizes the time delay between pump membrane motion and speaker membrane motion (due to the speed of sound) which in turn allows the pumps to operate at higher pumping/audio frequencies. 
     It also means that pressure waves from each membrane pump  2701  arrive at the speaker membrane  2702  at about the same time. Otherwise, an audio system could produce pressure waves that are out of synch (due to the difference in distance between each pump and the speaker membrane) and thus these waves can partially cancel (lowering audio power) at certain pumping/audio frequencies. 
     Pump/transducer system  2700  (as well as pump/transducer speaker stacked system  2800 ) further exhibit increased audio power. Since all the air enters/exits from the sides of the membrane pump, these pumps can be easily stacked (such as shown in  FIG. 2 ) to significantly increase sound power. Increasing the number of pump stacks (also referred to “pump cards”) from one to four (as shown in  FIG. 2 ) increases audio power by approximately a factor of 16. As can be seen in  FIG. 2 , the gas within the chamber is sealed by the membrane pump membranes and the speaker membrane. The gas in the sealed chamber can be air or another gas such as sulfur hexafluoride that can withstand higher membrane pump voltages than air. 
     Audio output is approximately linear with electrical input (resulting in simpler/cheaper electronics/sensors). Another advantage of the design of pump/transducer  2700  is the way the pump membranes  2701  are charged relative to the gates/stators. These are referred to as “stators,” since the term “gate” implies electrical switching. Pump/transducers have a low resistance membrane and the force between the stator and membrane is always attractive. This force also varies as the inverse square of the distance between the pump membrane and stator (and this characteristic can cause the audio output to be nonlinear/distorted with respect to the electrical input). The membrane can also go into “runaway” mode and crash into the stator. Thus, in practice, the amplitude of the membrane in pump/transducer is limited to less than half of its maximum travel (which lowers pumping speed and audio power). 
     The issues resulting from non-linear operation are solved in the design of pump/transducer  2700  by using a high resistance membrane (preferably a polymer film like Mylar with a small amount of metal vapor deposited on its surface) that is charged by a DC voltage and applying AC voltages to both stators (one stator has an AC voltage that is 180 degrees out of phase with the other stator). A high value resistor (on the order of 10 8  ohms) may also be placed between the high resistance membrane (on the order of 10 6  to 10 12  ohms per square) and the source of DC voltage to make sure the charge on the membrane remains constant (with respect to audio frequencies). 
     Because the pump membrane  2701  has relatively high resistance (though low enough to allow it to be charged in several seconds) the electric field between one stator and the other can penetrate the charged membrane. The charges on the membrane interact with the electric field between stator traces to produce a force. Since the electric field from the stators does not vary as the membrane moves (for a given stator voltage) and the total charge on the membrane remains constant, the force on the membrane is constant (for a give stator voltage) at all membrane positions (thus eliminating the runaway condition and allowing the membrane to move within its full range of travel). The electrostatic force (which is approximately independent of pump membrane position) on the membrane increases linearly with the electric field of the stators (which in turn is proportional to the voltage applied to the stators) and as a result the pump membrane motion (and also the speaker membrane  2702  that is being driven by the pumping action of the pump membrane  2701 ) is linear with stator input voltage. This linear link between stator voltage and pump membrane motion (and thus speaker membrane motion) enables a music voltage signal to be routed directly into the stators to produce high quality (low distortion) music. 
       FIG. 3  depicts an electrically conductive membrane pump/transducer  3000  that is similar to the pump/transducers  2700  and  2900 , in that it utilizes an array of electrically conductive membrane pumps. Pump/transducer  3000  does not utilize a speaker membrane (such as in pump/transducer  2700 ) or a structure in place of the speaker membrane (such as in pump/transducer  2900 ). Pump/transducer  3000  produces substantial sound even without a speaker membrane. Applicant believes the reason that there is still good sound power is that the membrane pumps are compressing the air as it makes its way out of the inner vents (increasing the pressure of an time-varying air stream increases its audio power). Arrows  3001  show the flow of air through the inner vents. The pump/transducer  3000  has a chamber that receives airflow  3001  and this airflow exhausts out the chamber by passing through the open area (the chamber exhaust area) at the top of the chamber. In order to produce substantial sound the total area of the membrane pumps must be at least 10 times larger than the chamber exhaust area. 
       FIG. 3  also shows an alternate vent configuration that has holes  3003  in the stators that allow air to flow to separate vent layers. The cross-sectional airflow area of the vents (through which the air flow is shown by arrows  3001 ) is much smaller than the pump membrane area (so that the air is compressed).  FIG. 3  also shows how a simple housing  3004  can direct the desired sound  3005  toward the listener (up as shown in  FIG. 3 ) and the undesired out of phase sound away from the listener (down as shown in  FIG. 3 ). The desired sound  3005  is in the low sub-woofer range to mid-range (20 Hz to about 3000 Hz). 
       FIG. 4  depicts an electrically conductive membrane pump/transducer  3100  that is the pump/transducer  3000  that also includes an electrostatic speaker  3101  (which operates as a “tweeter”). An electrostatic speaker is a speaker design in which sound is generated by the force exerted on a membrane suspended in an electrostatic field. The desired sound  3102  from the electrostatic speakers  3101  is in a frequency in the range of around 2 to 20 KHz (generally considered to be the upper limit of human hearing). Accordingly, pump/transducer  3100  is a combination system that includes a low/mid-range speaker and a tweeter speaker. 
       FIG. 5  depicts an electrically conductive membrane pump/transducer  3200  that is the pump/transducer  3100  that further includes the speaker membrane  3202  (such as in pump/transducer  2700 ). 
       FIGS. 6A-6C and 7  are figures that have been reproduced from FIGS. 16A-16C and 17 of the Pinkerton &#39;313 Patent. As set forth in the Pinkerton &#39;313 Patent: 
       FIG. 6A  illustrates an electroacoustic transducer  1601  (“ET,” which can also be referred to as a “pump card”) and its solid stator  1602  (shown in more detail in  FIG. 6B ). Vent fingers  1603  are also shown in ET  1601 .  FIG. 6B  is a magnified view of ET  1601  and shows how there are membranes  1604  and  1605  on each side of shared stator  1602 . 
       FIG. 6C  shows the electroacoustic transducer  1601  having a single stator card before trimming off the temporary support  1606  that supports the vent fingers  1603  (as shown in  FIGS. 6A-6B ). This process enables a low cost die stamping construction. Parts can be stamped out (which is very low cost), then epoxied together, and then the part  1606  that temporarily holds all the vent fingers  1603  in place can be quickly stamped off or trimmed off. 
       FIG. 7  is an exploded view of ET  1601 . From top to bottom:  FIG. 7  shows an electrically conductive membrane  1604 , a first metal frame  1701 , first non-conductive vent member  1702  (with its 23 vent fingers  1703 ), solid metal stator  1602 , second non-conductive vent member  1704 , and second metal frame  1705 . (The second membrane is not shown). These parts can be joined together with epoxy, double-sided tape, sheet adhesive or any other suitable bonding process. After membrane  1604  is bonded to frame  1701  its entire outside edge (peripheral edge) is supported by frame  1701 . 
       FIGS. 8A-8B  are figures that have been reproduced from FIGS. 8A-8B of the Badger &#39;088 PCT Application. As set forth in the Badger &#39;088 PCT Application: 
       FIG. 8A  illustrates an exploded view of an electroacoustic transducer  801  that has two pump cards. This is similar to the electroacoustic transducer  1601  shown in  FIG. 7 . However, electroacoustic transducer  801  does not have metal frames  1701  and  1705 . I.e., the double stack cards of electroacoustic transducer  801  lack any frames. 
     From top to bottom:  FIGS. 8A-8B  shows a first non-conductive vent member  802  (with its 23 vent fingers), a first electrically conductive membrane  803 , a second non-conductive vent member  804 , a first solid metal stator  805 , a third non-conductive vent member  806 , a second electrically conductive membrane  807 , a fourth non-conductive vent member  808 , and a second solid metal stator  809 . As before, these parts can be joined together with epoxy, double-sided tape, sheet adhesive or any other suitable bonding process.  FIG. 8B  shows the electroacoustic transducer  801  after its parts (as shown in  FIG. 8A ) have been bonded together. 
     The membranes (membranes  803  and  807 ) are supported by the pair of non-conductive vent membranes above and below the membrane. For example, first non-conductive vent member  802  supports a portion of a first electrically conductive membrane  803  and second non-conductive vent member  804  supports the other portion of first electrically conductive membrane  803 . No non-conductive vent by itself can support the electrically conductive membrane. 
     This absence of the frames from electroacoustic transducer  801  was significant and provided advantageous and unexpected results. The frames in the earlier pump cards (such as the electroacoustic transducer  1601  shown in  FIG. 7 ) were expensive, difficult to make (the metal spans being both thin and narrow) and also had a tendency of causing electrical arcs to the stator. By removing the frames, the electrical arcing has been eliminated in electroacoustic transducer  801 . 
       FIGS. 9A-9B  are figures that has been reproduced from FIGS. 9A-9B of the Pinkerton &#39;073 Application. As set forth in the Pinkerton &#39;073 Application: 
       FIGS. 9A-9B  show a speaker  900  that utilizes EVMP card stacked arrays  901 - 903 . Each of the EVMP card stacked arrays has a face area, such as face area  909  of EVMP card stacked array  903 . Each of EVMP card stacked array  901 - 903  has two face areas, on one side of speaker  900  (such as face area  909  for EVMP card stacked array  903 ) and the other side of the speaker  900  (which is hidden in the view of  FIGS. 9A-9B ). Air enters and exits the EVMP card stacked arrays through each of the EVMP card stacked array face areas (In fact air enters and exits the EVMPs in the EVMP card stacked arrays through each of the face areas of the EVMP cards). 
     By way of example, the EVMP card stacked array  901  can be a stacked array of 30 cards. Each card in the EVMP card stacked array can be about 1 mm thick so the EVMP card stacked array  901  stack of cards is about 30 mm thick. The face area of one EVMP card (in the EVMP card stacked array) is 1 mm times the stack width (for example 300 mm), which calculates to be 300 mm 2  per card for each face of the EVMP card (which means the combined area of the faces of an EVMP card in the EVMP card stacked array is 600 mm 2  per EVMP card). Thus, for an EVMP card stacked array having 30 cards, this calculates to be 18,000 mm 2  for the total face area of the EVMP card stacked array. I.e., the area of face area  909  would be 9,000 mm 2 , as it is one of the two faces of EVMP card stacked array  903 . 
     The membrane area of that same EVMP card is the depth of the card (for example 20 mm) times the card width (which, again, for example, is 300 mm). This calculates to be 6,000 mm 2  per EVMP card, which is 10 times larger than the face area of the EVMP card. Again, for a 30 card stacked array in an EVMP card stacked array, this calculates to a total membrane area of 180,000 mm 2 . This means that total membrane area of the EVMP card stacked array (such as EVMP card stacked array  903 ) is around 10 times the total face area of the EVMP card stacked array. It is worthwhile to note that speaker  900  shows three EVMP card stacked arrays (namely EVMP card stacked arrays  901 - 903 ), which can be run at different electrical phases. 
     The speaker  900  also utilizes two (one for each of the two stereo channels) “conventional” electrostatic audio actuator card stacks  904 - 905  (conventional in that the membrane pumping frequency equals the produced audio frequency). I.e., conventional card stacks  904 - 905  are stacks of electrostatic tweeter cards. The speaker  900  also includes electronics and battery  906  with control buttons  907 . Speaker  900  has three EVMP card stacked arrays  901 - 903 , and although all of the cards within these EVMP card stack arrays are similar in structure, each EVMP card stack arrays can be driven at a different electrical phase. For instance, the EVMPs in each of EVMP card stacked arrays  901 - 903  can have an electrical drive voltage phase of 0°, 120°, and 240°, respectively. I.e., the EVMPs in EVMP card stacked array  901  can be operated at 0°, the EVMPs in EVMP card stacked array  902  can be operated at 120°, and the EVMPs in EVMP card stacked array  903  can be operated at 240°. 
       FIGS. 10 and 11A-11B  are figures that has been reproduced from FIGS. 4 and 5A-5B of the Pinkerton &#39;002 Application. As set forth in the Pinkerton &#39;002 Application: 
       FIG. 10  is an illustration of a dipole speaker  400  that has all electrostatic transducers. Sound comes out from side  401  and oppositely phased sound comes out the other side (not shown). It also has control buttons  407  and MEMs microphone ports  408  (with the MEMs microphones located behind microphone ports  408 ). The MEMs microphones are for example Knowles SPK0412HM4H-B-7 (Knowles Electronics, LLC, Itasca, Ill.) and are operably connected to a power source and a CPU on the speaker  400 . The power source is generally the same power source as used for the speaker and the CPU controls the electrostatic transducers. 
     The MEMs microphone ports  408  on the speaker  400  have been positioned along the null sound plane (NSP) of the speaker  400  (which null sound plane  503  shown in  FIG. 5B ). 
       FIG. 11A  is a top view of speaker  400 , showing only the top. Opposite sides  401  and  501  are shown. Sound emits from side  401  and oppositely phased sound out side  501  in speaker  400  (which makes it a dipole speaker). 
       FIG. 11B  is a magnified view of box  502  shown in  FIG. 5A . The null sound plane  503  for speaker  400  is shown. The MEMs microphone ports are positioned along this null sound plane  503 . 
     SUMMARY OF THE INVENTION 
     The present invention relates to an improved loudspeaker that has a plurality of stacks of cards having electrostatic transducers, in which one stack of cards has a different width as another stack of cards in the plurality. At frequencies above 200 Hz, and at the same drive voltage and current, the stack of lesser width produced significantly greater mic voltage (which is proportional to sound pressure level) as compared to the stack of greater width cards. By combining the plurality of stacks of cards with different widths, this provides for the elimination of conventional traditional cone drivers, and provides for improved sound and higher microphone voltages both above and below 200 Hz using only electrostatic transducers. It also assists in maintaining the null sound plane, whereas conventional cone drivers cause some interference. 
     In general, in one aspect, the invention features a loudspeaker that includes a first stack of cards including electrostatic transducers. The first stack has a plurality of first cards having a first width. The loudspeaker further includes a second stack of cards including electrostatic transducers. The second stack has a plurality of second cards having a second width. The second width is greater than the first width. The first cards have a mic voltage characteristic relative to the second cards. The mic voltage characteristic includes that, throughout a range of 300 Hz to 1000 Hz, the first stack has a first height and is operable to produce a greater mic voltage than a same height stack of a plurality of the second cards stacked to the first height when operated at a same drive voltage and current. 
     Implementations of the invention can include one or more of the following features: 
     The mic characteristic can further include that, throughout the range of 300 Hz to 1000 Hz, the first stack of the first cards is operable to produce a mic voltage that is at least 2.5 times greater than the same height stack of the second cards when operated at the same drive voltage and current. 
     The mic characteristic can further include that, throughout the range of 300 Hz to 1000 Hz, the first stack of the first cards is operable to produce a mic voltage that is between 2.5 and 5 times greater than the same height stack of the second cards when operated at the same drive voltage and current. 
     The mic characteristic can further include that, at 300 Hz, the first stack of the first cards is operable to produce a mic voltage that is at least 6 times greater than the same height stack of the second cards when operated at the same drive voltage and current. 
     The first cards can have an audio power characteristic relative to the second cards. The audio power characteristic can include that, at 300 Hz, the first stack of the first cards is operable to produce an audio power that is greater than the same height stack of the second cards when operated at the same drive voltage and current. 
     The audio power characteristic can further include that, at 300 Hz, the first stack of the first cards is operable to produce an audio power that is at least 30 times greater than the same height stack of the second cards when operated at the same drive voltage and current. 
     The mic voltage characteristic can further include that, at below 165 Hz, the same height stack of the second cards is operable to produce a greater mic voltage than the first stack of the first cards when operated at the same drive voltage and current. 
     The mic voltage characteristic can further include that, throughout the range of between 20 Hz and 200 Hz, the same height stack of the second cards is operable to produce a greater mic voltage than the first stack of the first cards when operated at the same drive voltage and current. 
     The loudspeaker can be not utilizing cone drivers. 
     The first stack of the first cards can be operable to be used to produce audible sound in place of cone drivers. 
     The first width can be approximately 12 mm and the second width can be approximately 21 mm. 
     The second width can be at least 1.7 times greater than the first width. 
     The second stack of cards can have a second height and the second height can be greater than the first height. 
     The second height can be at least 8 times greater than the first height. 
     The second cards can have a capacitance that is at least 1.8 times more than the first cards. 
     A first card in the plurality of first cards can have a greater microphone voltage that a second card in the plurality of the second cards. 
     The first cards in the plurality of first cards can have the same length as the second cards in the plurality of second cards. 
     The electrostatic transducers in the first stack of cards can have a vent structure that is the same as the electrostatic transducers in the second stack of cards. 
     The loudspeaker can have a null sound plane. 
     The null sound plane can be located along a center line of the speaker. 
     A microphone can be positioned in the null sound plane. 
     The microphone can be a MEMS microphone array. 
     The loudspeaker can further include a third stack of cards comprising electrostatic transducers. The third stack can have a plurality of third cards having a third width. The third width can be greater than the first width. The second width can be greater than the third width. 
     The third width can be at least 30% wider than the first width. The second width can be at least 30% wider than the third width. 
     In general, in another aspect, the invention features a method of using a loudspeaker. The method includes selecting a loudspeaker. The loudspeaker includes a first stack of cards including electrostatic transducers. The first stack has a plurality of first cards having a first width. The loudspeaker further includes a second stack of cards including electrostatic transducers. The second stack has a plurality of second cards having a second width. The second width is greater than the first width. The method further includes utilizing the first stack of cards to produce audible sound in a range of 100 Hz to 20 KHz. The method further includes utilizing the second stack of cards to produce audible sound in a range of 20 Hz to 400 Hz. 
     Implementations of the invention can include one or more of the following features: 
     The first stack of cards can be utilized to produce audible sound in a range of 200 Hz to 20 KHz. The second stack of cards can be utilized to produce audible sound in a range of 20 Hz to 300 Hz. 
     The loudspeaker can produce sound in a range of 20 Hz to 20 KHz without cone drivers. 
     The loudspeaker can be controlled using voice recognition. 
     The step of selecting a loudspeaker can include selecting any of the above-described loudspeakers. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIGS. 1A-1E  (which are reproduced from the Pinkerton &#39;353 Patent) depict an electrically conductive membrane pump/transducer that utilizes an array of electrically conductive membrane pumps that cause a membrane to move in phase.  FIGS. 1A-1B  depict cross-section views of the pump/transducer.  FIGS. 1C-1E  depict overhead views of the pump/transducer. 
         FIG. 2  (which is reproduced from the Pinkerton &#39;353 Patent) depicts an electrically conductive membrane pump/transducer that has a stacked array of electrically conductive membrane pumps. 
         FIG. 3  (which is reproduced from the Pinkerton &#39;353 Patent) depicts an electrically conductive membrane pump/transducer that utilizes an array of electrically conductive membrane pumps that operates without a membrane or piston. 
         FIG. 4  (which is reproduced from the Pinkerton &#39;353 Patent) depicts an electrically conductive membrane pump/transducer  3100  that utilizes an array of electrically conductive membrane pumps and that also includes an electrostatic speaker. 
         FIG. 5  (which is reproduced from the Pinkerton &#39;353 Patent) depicts an electrically conductive membrane pump/transducer  3200  that utilizes an array of electrically conductive membrane pumps that cause a membrane to move in phase and that also includes an electrostatic speaker. 
         FIG. 6A  (which is reproduced from the Pinkerton &#39;313 Patent) illustrates an electroacoustic transducer (“ET,” which is also referred to as a “pump card”) and its solid stator. 
         FIG. 6B  (which is reproduced from the Pinkerton &#39;313 Patent) is a magnified view of the electroacoustic transducer of  FIG. 6A . 
         FIG. 6C  (which is reproduced from the Pinkerton &#39;313 Patent) illustrates the electroacoustic transducer of  FIG. 6A  having a single stator card before trimming off the vent fingers. 
         FIG. 7  (which is reproduced from the Pinkerton &#39;313 Patent) is exploded view of the electroacoustic transducer of  FIG. 6A . 
         FIG. 8A  (which is reproduced from the Badger &#39;088 PCT Application) illustrates an exploded view of an electroacoustic transducer. 
         FIG. 8B  (which is reproduced from the Badger &#39;088 PCT Application) illustrates the electroacoustic transducer shown in  FIG. 8A  in fabricated form. 
         FIGS. 9A-9B  (which are reproduced from the Pinkerton &#39;073 Application) illustrate a loudspeaker with stacked arrays of electrostatic venturi membrane-based pump/transducer (EVMP) cards. 
         FIG. 10  (which is reproduced from the Pinkerton &#39;438 PCT Application) illustrates a dipole loudspeaker having electrostatic transducers. 
         FIGS. 11A-11B  (which are reproduced from the Pinkerton &#39;438 PCT Application) illustrate the null sound plane (NSP) of the speaker of  FIG. 10 . 
         FIG. 12  is an illustration of two different sized cards used in card stacks of the present invention. 
         FIG. 13A  is a graph showing the of mic voltage of two stacks of different width cards over the frequency range from 300 Hz to 1000 Hz. 
         FIG. 13B  is a graph showing the ratio of the mic voltages of the two stacks of different card widths over the frequency range from 300 Hz to 1000 Hz. 
         FIG. 14  is an illustration of a speaker with a narrow card stack and wider card stack (with the face plate of the speaker removed so that the card stacks can be viewed). 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a loudspeaker having improved pump cards that each include an array of electrically conductive membrane transducers (such as polyester-metal membrane pumps). The array of electrically conductive membrane transducers combine to generate the desired sound by the use of pressurized airflow. In the present invention, a plurality of stacks of cards having electrostatic transducers, in which one stack of cards has a different width as another stack of cards in the plurality, is employed. Surprisingly, at frequencies above 200 Hz, and at the same drive voltage and current, the stack of lesser width produced significantly greater mic voltage (which is proportional to sound pressure level) as compared to the stack of greater width cards. By combining the plurality of stacks of cards with different widths, this provides for the elimination of conventional cone drivers, and provides for improved sound and higher microphone voltages both above and below 200 Hz using only electrostatic transducers. It also assists in maintaining the null sound plane, whereas conventional cone drivers cause some interference. 
     It has been discovered that changing the card geometries between stacks in the plurality of stacks yields unexpected properties that can then be used advantageously for speakers. A more narrow card (such as one having a 12 mm span) was found to have a much larger microphone voltage than a wider card (such as one having a 21 mm span).  FIG. 12  is an illustration of two different sized cards used in card stacks of the present invention, namely a narrow card  1201  (having a 12 mm span in the width direction) and a wider card  1202  (having a 21 mm span in the width direction). Narrow card  1201  and wider card  1202  have the same length and, other than the width and thickness, the same vent structure, which is similar as described above for  FIGS. 8A-8B . 
     For frequencies above approximately 200 Hz: a stack of the narrow cards  1201  were found to had a much larger microphone voltage than a stack of the wider cards  1202 . A stack of the narrow cards  1201  (20 cards with a 12 mm membrane span) was compared to a stack of the wider cards  1202  (20 cards with a 21 mm membrane span) with height of stacks adjusted to be equal over the frequency range from 300 to 1000 Hz. 
       FIG. 13A  is a graph showing the of mic voltage of each of the stack the narrow cards  1201  and the stack of the wider card  1202  over the frequency range from 300 Hz to 1000 Hz (plots  1301 - 1302 , respectively). 
       FIG. 13B  is a graph showing the ratio of the mic voltages of the two stacks of different card widths over the frequency range from 300 Hz to 1000 Hz. Plot  1303  reflects the ratio of (a) the mic voltage of the narrow cards  1201  to (b) the mic voltage of the stack of the wider card  1202  over the frequency range from 300 Hz to 1000 Hz. As shown in  FIG. 13B , this ratio was between 2.5 to 5 throughout the range of 300 to 1000 Hz. This was both surprising and remarkable, particularly, as it was believed that the narrow card would have acted to the contrary (i.e., it was expected that the mic voltage of the narrow card stack would be less, and much less, than the mic voltage of the wider card stack). Indeed, below 200 Hz, the wider card did have a greater mic voltage than the narrow card stack (as anticipated). 
     Furthermore, as the capacitance of the more narrow cards  1201  was about 1.8× less than the wider cards  1202 , this meant that the stack of more narrow cards  1202  requires 1.8× less current/power. For a given drive voltage and current, the stack of more narrow cards  1201  produces 6× the mic voltage (which is proportional to sound pressure level) and 36× the audio power as the stack of the wider cards. Stated another way, the stack of more narrow cards  1201  is 36× more efficient as the wider cards at 300 Hz (which is very important for a battery-powered device). Furthermore, the stack of more narrow cards  1201  are also approximately 6× lighter and 6× less expensive than the stack of wider cards  102  for a given audio power output above 300 Hz. 
     Due to the surprising advantages of the narrow cards, these can be used to replace traditional cone drivers with the narrow card stack.  FIG. 14  shows, for instance, a 12 mm card stack (approximately 22 cards) above the 21 mm card stack (approximately 110 cards) and is clearly much shorter (1.6 cm vs. 13 cm) than the 21 mm card stack (even though it produces more audio output power per electrical input watt above 300 Hz). However, the 21 mm cards produce more audio power than the 12 mm cards below approximately 165 Hz (this is why both types of cards are used to cover the full audible frequency range of 20 Hz to 20 kHz). Replacing traditional sealed cones with a 12 mm card stack has the added benefit of creating a null sound plane along the centerline of the speaker and this enables very high resolution voice recognition when MEMS microphones are located in this plane (as described in the Pinkerton &#39;438 PCT Application). 
     Due to the surprising advantages of the narrow (12 mm) electrostatic cards it is possible to eliminate traditional cones, such as speaker  1400  shown in the illustration in  FIG. 14 , which has a narrow card stack  1401  (approximately 22 cards of 12 mm width) above a wider card stack  1402  (approximately 110 cards of 21 mm width). The height of the narrow card stack  1401  is clearly much shorter (1.6 cm vs. 13 cm) than the wider card stack  1402  (even though the narrow card stack  1401  produces more audio output power per electrical input watt above 300 Hz). The wider card stack  1402  produces more audio power than the narrow card stack  1401  below approximately 165 Hz. Thus, both types of cards (narrow and wider) are used to cover the full audible frequency range of 20 Hz to 20 kHz. 
     Replacing traditional sealed cones with narrow car stack  1401  was found to add an additional benefit by creating the null sound plane along the centerline of speaker  1400  and this enables very high resolution voice recognition when MEMS microphones are located in this plane (as described in the Pinkerton &#39;438 PCT Application). 
     Generally, for cost purposes, there are only two different widths of card stacks utilized (i.e., one or more stacks are stacks of narrow cards and one or more stacks are stacks of wider cards). However, embodiments of the present invention can include three (or more) different widths of card stacks (i.e., one or more stacks are stacks of narrow cards, one or more stacks are stacks of wider cards, and one or more stacks of stacks of even wider cards). For example, an embodiment of the present invention can have a stack of narrow (12 mm) electrostatic cards, a stack of wider (16 mm) electrostatic cards, and a stack of even wider (21 mm) electrostatic cards. 
     While embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described and the examples provided herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. The scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. 
     The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated herein by reference in their entirety, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein. 
     Amounts and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of approximately 1 to approximately 4.5 should be interpreted to include not only the explicitly recited limits of 1 to approximately 4.5, but also to include individual numerals such as 2, 3, 4, and sub-ranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only one numerical value, such as “less than approximately 4.5,” which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the presently disclosed subject matter belongs. Although any methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of the presently disclosed subject matter, representative methods, devices, and materials are now described. 
     Following long-standing patent law convention, the terms “a” and “an” mean “one or more” when used in this application, including the claims. 
     Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently disclosed subject matter. 
     As used herein, the term “about” and “substantially” when referring to a value or to an amount of mass, weight, time, volume, concentration or percentage is meant to encompass variations of in some embodiments ±20%, in some embodiments ±10%, in some embodiments ±5%, in some embodiments ±1%, in some embodiments ±0.5%, and in some embodiments ±0.1% from the specified amount, as such variations are appropriate to perform the disclosed method. 
     As used herein, the term “substantially perpendicular” and “substantially parallel” is meant to encompass variations of in some embodiments within ±10° of the perpendicular and parallel directions, respectively, in some embodiments within ±5° of the perpendicular and parallel directions, respectively, in some embodiments within ±1° of the perpendicular and parallel directions, respectively, and in some embodiments within ±0.5° of the perpendicular and parallel directions, respectively. 
     As used herein, the term “and/or” when used in the context of a listing of entities, refers to the entities being present singly or in combination. Thus, for example, the phrase “A, B, C, and/or D” includes A, B, C, and D individually, but also includes any and all combinations and subcombinations of A, B, C, and D.