Abstract:
An acoustic transducer includes a housing, a plurality of diaphragms suspended from the housing and separated into one or more groups, and one or more motors combined with the housing that operate in response to an electrical signal. The diaphragms of each group are driven by a respective motor to which all the diaphragms in the group are coupled and at least one motor has an indirect coupling with no direct mechanical connection to the diaphragms driven thereby. One or more electromagnetic motors that drive one or more sets of multiple diaphragms to provide acoustically efficient loudspeaker systems having dimensions that allow use in applications that would be difficult or impossible with traditional transducers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is a divisional of U.S. patent application Ser. No. 11/628,394 filed Sep. 5, 2008, which is a National Phase entry of International Patent Application Number PCT/US05/019443 filed Jun. 3, 2005, which claims priority to U.S. Provisional Patent Application Ser. No. 60/576,990 filed Jun. 3, 2004, U.S. Provisional Patent Application Ser. No. 60/622,259 filed Oct. 25, 2004, U.S. Provisional Patent Application Ser. No. 60/641,620 filed Jan. 5, 2005, U.S. Provisional Patent Application Ser. No. 60/667,248 filed Apr. 1, 2005, and U.S. Provisional Patent Application Ser. No. 60/685,161 filed May 26, 2005, where the contents of all of said applications are herein incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention is related to the field of audio systems and acoustics, and pertains more specifically to providing an improved form factor for an acoustic transducer that converts electrical signals into acoustic radiation. 
     BACKGROUND ART 
     The general principles of moving coil electrodynamic loudspeakers are well understood. Central to the ability of a transducer to generate sound is the concept of volume displacement. The volume displacement of a transducer with a single diaphragm is equal to the effective surface area of the diaphragm multiplied by the excursion capability of that diaphragm. The greater the volume displacement of a transducer, the greater its potential for generating sound. The need for large volume displacement is especially pronounced at low frequencies. The traditional methods for achieving greater volume displacement in a transducer are to increase the surface area of the diaphragm, to increase the excursion capability of the diaphragm, or both. 
     Traditional transducers that are used to produce significant low frequency energy incorporate a single diaphragm with a large surface area and use motors and housings that provide for adequate excursion of the diaphragm. This leads to certain minimum dimension requirements for the diaphragm of a loudspeaker, which in turn imposes minimum dimension requirements on the loudspeaker enclosure. It is very difficult to use traditional transducers with good low-frequency response in applications such as flat-panel television and computer monitors. In these applications, the current solution is to use a separate subwoofer box to reproduce low frequency sound, resulting in added cost and inconvenience. The same holds true of automotive sound system applications, where designers struggle to find a place to hide the subwoofer in the car, which is usually in the trunk or under the seats. 
     DISCLOSURE OF INVENTION 
     It is an object of the present invention to provide for an acoustic transducer that can reproduce low-frequency sound with high fidelity at high sound pressure levels in applications that cannot be addressed satisfactorily by traditional transducers. 
     According to one aspect of the present invention, the sound-producing surface area of an acoustic transducer is distributed across multiple diaphragms in a form factor that is much more suitable for use in applications such as flat panel television and computer monitors as well as automotive sound systems. These multiple diaphragms can be separated into one or more groups, with the diaphragms of each group being driven synchronously by at least one motor to which all the diaphragms in the group are connected. Any motor capable of converting electrical audio signals into motion can be used to drive the diaphragms in a group. For example, motors consisting of a moving voice coil and a non-moving magnet can be used. 
     The specific implementations of an acoustic transducer that are described herein either use a single motor that drives all the diaphragms or the housing to which all the diaphragms are mounted, or use each of two motors to drive half of the diaphragms. In principle, the number of motors is largely independent of the number of diaphragms. For example, an acoustic transducer may have one group of four diaphragms that is driven by two motors and another group of three diaphragms that is driven by one motor. 
     Each driving motor may be connected directly or indirectly to all the diaphragms that it drives. An indirect connection may be achieved by directly connecting the motor to a housing that is in turn connected to the diaphragms by their surrounds or suspensions, or by using a gas or liquid fluid to couple the motor to the diaphragms. All the motors in a particular acoustic transducer may receive essentially the same audio signal and can be connected either in series or in parallel with one another. 
     The materials that are used in the construction of various implementations of the present invention may be materials that are used in the construction of typical acoustic transducers. The housing, connecting rods and motors may be made of materials whose modes of resonance, vibration, or flexure have characteristic frequencies that are outside the audio spectrum of interest. Since these components preferably are not part of the sound generation mechanism, the use of materials with modes in the audio spectrum of interest could result in unwanted audio artifacts. Preferably, moving elements such as the diaphragms and connecting rods are made of materials that are as light as possible to improve the efficiency of the device. For example, a glass-filled or mica-filled polypropylene-polyphenylene-oxide-styrene material or a carbon-fiber material may be used. 
     The implementations described herein utilize a tubular form factor with a cylindrical housing and round diaphragms; however, the cross-sections of the housing and the diaphragms do not have to be round. They could be oval, rectangular or essentially any other shape that may be desired. 
     The increased complexity and additional parts needed to implement various aspects of the present invention may increase manufacturing costs and reduce reliability of the transducer. These problems can be mitigated or avoided by employing a modular design where, for example, one type of module, referred to herein as a motor module, contains a magnet assembly, a coil, and a diaphragm or cone, and another type of module, referred to herein as a diaphragm module, contains a section of the housing, a diaphragm, a suspension, and a set of rods that are coupled to the diaphragm. The motor module is designed to mate with a diaphragm module and may contain a set of rods that mechanically couple the motor in the motor module to the diaphragm in the adjacent diaphragm module. Alternatively, the motor module may contain a diaphragm that fluidically couples to the diaphragm in the adjacent diaphragm module. A diaphragm module is designed also to mate with another diaphragm module. Essentially any number of the diaphragm modules can be assembled into a linear array of modules. The rods in each diaphragm module pass through openings in the immediately adjacent diaphragm module and mechanically connect to the diaphragm in the next diaphragm module. The section of housing in each of the diaphragm modules is adapted to mate with the section of housing in adjacent diaphragm modules to form a chamber between modules. The air in a respective chamber is either acoustically isolated from the air outside the housing or it is acoustically coupled to the air outside the housing through a port, vent or other opening. 
     An acoustic transducer according to the present invention produces a front wave and a rear wave. It is anticipated that the transducer usually will be enclosed by a housing having openings appropriately oriented with respect to a listener through which the front wave may exit. There are many well-known methods for dealing with the rear wave in standard acoustic transducers and any of those methods can be used in the present invention. For example, the rear wave can be vented through a transmission line that introduces delay, it can be vented into a large enclosure that acts as a baffle, or it can be vented directly into the surrounding air. The latter method generally reduces the audio efficiency of the transducer in the low frequencies. 
     The overall size of an acoustic transducer according to the present invention is highly dependent on the desired level of audio efficiency at low frequencies. Higher audio efficiency can be achieved either by increasing the surface area of individual diaphragms, by increasing the excursion of individual diaphragms, by increasing the number of diaphragms, by optimizing the acoustic impedance matching between diaphragms and air, or by any combination of these factors. 
     According to one teaching of the present invention, the transducer includes a single motor actuating multiple diaphragms by using a single drive rod that is attached to each diaphragm. One side of each diaphragm faces an opening to the listening environment. The other side of each diaphragm is isolated from the listening environment by a baffle. The drive rod may pass through openings in the baffles and/or in the diaphragms. Seals may be used to prevent or substantially reduce unwanted air leakage in any openings through which the drive rod may pass. 
     According to another teaching of the present invention, the transducer includes two motors, each actuating multiple diaphragms. The diaphragms are arranged in two groups; diaphragms in one group are driven by one motor and diaphragms in the other group are driven by the other motor. Preferably, the groups of diaphragms are driven in opposition to one another. The diaphragms are actuated by the motors using drive rods. The drive rods may pass through openings in the baffles and/or in the diaphragms. Unfortunately, air can leak through these openings and cause large amounts of intermodulation and harmonic distortion. This leakage can also significantly reduce sound output levels. Seals may be used to prevent unwanted air leakage in any openings in the diaphragms including those through which the rods may pass. 
     These seals may be formed from one or more pieces of lightweight foam, each piece of which is compressible and expandable and affixed to a rod near an opening. A piece of foam is compressed when the rod pushes it toward the opening, and it expands when the rod pulls it away from the opening. These seals may also be made of a pleated fabric such as the fabric used in bellows, which can expand and contract as needed. Alternatively, the drive rods may be routed in such a way that they do not pass through any diaphragms or baffles, thereby eliminating the need for seals. 
     For those implementations having drive rods passing through diaphragms and/or baffles, it may be desirable to avoid the use of seals because the seals add cost and complexity to the implementation. This may be achieved by designing the size of the opening in the diaphragms and/or baffles through which drive rods pass to optimize overall performance. These openings are referred to herein as “pass-through openings.” Any air leakage through the pass-through openings in the diaphragms may generate undesirable artifacts in the form of audible distortion or noise and/or a reduction in the overall volume displacement of air. These air leakage artifacts can be reduced by increasing the resistance of the opening to air flow or by diffusing the air that passes through the openings so that it generates less audible noise. The resistance can be increased, for example by increasing the length of the path through which the air has to travel or by reducing the size of the opening. Several techniques for reducing the air leakage noise are described in the following paragraphs; these techniques may be used individually or in combination to achieve the desired outcome. 
     According to one technique, the resistance to air flow is increased by using thicker diaphragms to increase the length of the air travel path. This typically has the effect of increasing the mass of the diaphragms and reducing the maximum excursion for a given overall transducer volume. 
     According to another technique, the diaphragm thickness is increased by using a “sandwich” of two diaphragms with a layer of damping material such as a visco-elastic polymer between them. The resulting composite diaphragm is highly damped, which is often desirable in acoustic transducers because it can help reduce sonic artifacts. The presence of the damping material allows the diaphragms to be formed from a much lighter material, thereby mitigating an undesirable increase in the moving mass of the transducer. 
     According to another technique, the diaphragm thickness is increased by using a “sandwich” of a skin material that doesn&#39;t stretch, such as paper, and a lightweight spacing material such as polyurethane foam. The resulting composite diaphragm is typically lighter and stiffer than a monolithic diaphragm. 
     According to another technique, the resistance to air flow is increased by adding cylindrical “sleeves” to the diaphragms around the pass-through openings. The use of sleeves has the added effect of minimizing the increase in diaphragm mass. It may be preferable for the sleeves to be shaped differently on the two sides of the diaphragm. For example, on the outside face of the diaphragm, which transmits the front wave of the sound that is heard by the listener, the cylindrical sleeve may be shaped like a funnel to reduce the turbulence noise of the air that passes through the openings. 
     According to another technique, resistance to air flow is increased by adding sleeves made of an airflow resistant material around the pass-through openings. The inner diameter of these sleeves may be small enough that the sleeve fits somewhat tightly around the drive rod passing through the opening. The material used for these sleeves is preferably soft and slippery to reduce undesirable friction noise when the sleeve comes into contact with the drive rod, and possesses an airflow resistance sufficient to reduce the amount of air that passes through the opening. Examples of suitable materials include fabrics made of silk, polyester, soft wool, and other materials in combination with an elastic weave. These soft fabric sleeves are preferably mounted around shorter cylindrical sleeves made of a hard material such as plastic or metal. 
     Another method for reducing air leakage noise is to seal the pass-through openings with a material that effectively stops air flow while minimizing friction and noise. Examples of such materials include bellows made of soft and flexible fabric, and semifluid lubricants such as thixotropic gels. A similar effect can be achieved by using a ferromagnetic liquid between the rod and the sleeve. The ferromagnetic liquid may be held in place by a thin ring magnet that is attached to the diaphragm. 
     Another method for reducing air leakage noise is to diffuse the air that passes through the opening. One technique for achieving this is to add soft foam at the exit point of the air travel path. In particular, a cylinder of soft foam may be added either directly around the pass-through opening or indirectly around a shorter cylindrical sleeve made of a hard material such as plastic or metal. The foam may be configured so that it extends above the hard sleeve and curves inward so that it covers the opening and nearly touches the drive rod. The foam may be polyurethane reticulated open cell foam, which has the desirable properties of diffusing the air while reducing unwanted friction noise when it comes into contact with the drive rod. In some applications it may be preferable to place foam only on the inside face of the diaphragm, which transmits the rear wave of the sound that is not heard by the listener. This makes it possible to use longer foam sleeves with a smaller inside diameter. These foam sleeves may touch the drive rods more tightly so that they increase resistance to air flow in addition to diffusing the air that passes through the opening. The tighter touching of the drive rods will increase friction noise but that noise is contained in the rear wave and is therefore less objectionable to the listener. 
     The air leakage noise may be reduced through a combination of the techniques mentioned above; namely, adding sleeves to the diaphragm and increasing the thickness of the diaphragm itself. 
     An example of such a combined technique increases the resistance to air flow by forming a composite diaphragm consisting of a sandwich of two diaphragms, each having cylindrical sleeves around the pass-through openings on its outside face only, with a layer of damping material between them. The reduction in air leakage noise, the amount of increase in the moving mass and the amount of diaphragm damping can be customized to fit almost any application by adjusting the thickness of the damping material layer, the thickness of the component diaphragms and the length of the sleeves. 
     Another example of a combined technique for reducing air leakage artifacts is adding both soft foam and soft fabric sleeve around the pass-through openings. In particular, the soft foam may be added around the hard sleeve and the soft fabric may be added around the foam, thereby combining the effects of increasing resistance to air flow and diffusing the air that passes through the opening. 
     Another example of a combined technique for reducing air leakage artifacts is to use a tight bushing around the rod. The bushing is preferably made of a very low friction material such as a self-lubricating polymer. The bushing is preferably attached to the diaphragm via a flexible airtight material to allow limited movement and isolate the diaphragm from vibration. 
     The techniques described above for reducing air leakage noise are applicable to any transducer that uses a diaphragm or cone with a hole in it. These techniques are not limited to array transducers that use multiple diaphragms. 
     According to yet another teaching of the present invention, the transducer includes a motor that directly actuates one or more structures each containing a number of diaphragms that are suspended by surrounds, spiders, or other forms of suspension. The back wave of each diaphragm is acoustically isolated from adjacent diaphragms by baffles. The front wave of each diaphragm is allowed to pass through an opening to the listening environment. No drive rods are used and instead the diaphragms are driven inertially. This teaching may be extended to use multiple motors. In addition, different structures may be moved in opposition to one another. 
     According to a further teaching of the present invention, each driving motor is connected mechanically to a single diaphragm. That diaphragm is coupled by a fluid to another diaphragm, which in turn may be coupled mechanically to other diaphragms. In this way, one or more conventional loudspeakers can be used to drive multiple diaphragms indirectly. If a pneumatic fluid coupling such as an air coupling is used between the directly driven diaphragm and the indirectly driven diaphragms, the indirectly driven diaphragms operate as if they are driven by a signal that is passed through a filter with a low pass characteristic, while the directly driven diaphragm operates as if it is driven with a signal having a full frequency range. In an embodiment such as this, the directly driven diaphragm generates most of the high frequency sounds and the indirectly driven diaphragms generate most of the low frequency sounds. 
     According to yet a further teaching of the present invention, a transducer with a housing comprises a plurality of diaphragm modules each having a section of the housing, a diaphragm suspended from the section of the housing, and a set of one or more rods coupled to the diaphragm. The section of housing for a respective diaphragm module has a first surface and an opposing second surface. The first surface of the section of housing in one diaphragm module is designed to mate with the second surface of the section of housing in another diaphragm module in such a way that a chamber is formed between respective diaphragms of adjacent modules. The section of housing for a module may have ports, vents or other types of openings that allow air inside the chamber to be acoustically coupled to air outside the chamber. The rods in each diaphragm module pass through openings in the immediately adjacent diaphragm module and mechanically connect to the diaphragm in the next diaphragm module. In one implementation, the set of rods in one module protrude from one surface of the diaphragm and the opposite surface of the diaphragm has fixtures that are adapted to receive and mate with the ends of the rods of the module next to the adjacent module. In another implementation, a first set of rods protrude from one surface of a respective diaphragm and a second set of rods protrude from the opposite surface of the diaphragm. The ends of the rods in the two sets are adapted to mate with one another. 
     According to yet another teaching of the present invention, the diaphragm modules mentioned above do not have rods coupled to the diaphragm. Each diaphragm module consists of a section of the housing and a diaphragm suspended from the section of the housing. After the middle section of a transducer is assembled from a plurality of these diaphragm modules, rods are inserted and attached to the appropriate diaphragms with a bonding process such as gluing or sonic welding and one or more motor modules are attached to the ends of the middle section of the transducer. 
     In any of the implementations described above, sleeves may be added around pass-through holes or the diaphragm may be a composite diaphragm composed of two diaphragms with a layer of damping material sandwiched between them. The sandwich diaphragm may also incorporate cylindrical sleeves on one or both of its faces to reduce undesirable air leakage noise. 
     In any of the implementations described above, the diaphragm suspensions need not all have identical properties or orientations. For example, in implementations that drive diaphragms directly, it may be desirable to use stiffer suspensions near the motors to minimize movement in directions other than along the direction of the actuated drive rods. Furthermore, by orienting the suspensions of diaphragms that are actuated by a single motor so that some of the suspensions face in an opposite direction with respect to other suspensions, asymmetrical characteristics of the suspensions may be cancelled or reduced so that distortion characteristics of the transducer may be reduced. 
     The various features of the present invention and its preferred embodiments may be better understood by referring to the following discussion and the accompanying drawings. The contents of the following discussion and the drawings are set forth as examples only and should not be understood to represent limitations upon the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration of an implementation of the present invention using baffles, a single internal drive rod and a single motor. 
         FIG. 2  is a schematic illustration of an implementation of the present invention using no baffles, multiple internal drive rods and two motors. 
         FIG. 3  is a schematic illustration of an implementation of the present invention using baffles, multiple external drive rods and a single motor. 
         FIG. 4  is a schematic illustration of an implementation of the present invention using no baffles, multiple external drive rods and two motors. 
         FIG. 5  is a schematic illustration of an implementation of the present invention using baffles, no drive rods and a single motor. 
         FIGS. 6A-6C  are schematic illustrations of a diaphragm module that may be used to manufacture an acoustic transducer according to the present invention. 
         FIG. 7  is a schematic perspective illustration of an implementation of an acoustic transducer according to the present invention with a mechanically coupled drive using diaphragm modules like those illustrated in  FIGS. 6A-6C . 
         FIG. 8  is a schematic cross-sectional illustration of the transducer shown in  FIG. 7 . 
         FIG. 9  is a schematic perspective illustration of an implementation of an acoustic transducer according to the present invention with a fluidically coupled drive using modules like those illustrated in  FIGS. 6A-6C . 
         FIG. 10  is a schematic cross-sectional illustration of the transducer shown in  FIG. 9 . 
         FIGS. 11A-11C  are schematic illustrations of a composite diaphragm that is composed of two diaphragms with a layer of damping material sandwiched between them. 
         FIGS. 12A-12C  are schematic illustrations of a diaphragm module with cylindrical sleeves around the pass-through openings. 
         FIGS. 13A-13C  are schematic illustrations of a composite diaphragm that is composed of two diaphragms, each with cylindrical sleeves around the pass-through openings on its outside face only, with a layer of damping material sandwiched between them. 
         FIGS. 14A-14B  are schematic illustrations of a diaphragm with cylindrical sleeves around the pass-through openings and soft fabric sleeves around the cylindrical sleeves. 
         FIGS. 15A-15B  are schematic illustrations of a diaphragm with cylindrical sleeves around the pass-through openings and soft foam sleeves around the cylindrical sleeves. 
         FIGS. 16A-16B  are schematic illustrations of a diaphragm with cylindrical sleeves around the pass-through openings, soft foam sleeves around the cylindrical sleeves, and soft fabric sleeves around the foam sleeves. 
         FIGS. 17A-17B  are schematic illustrations of a diaphragm with funnel-shaped cylindrical sleeves around the pass-through openings on the outside face of the diaphragm and, on the inside face, cylindrical sleeves around the pass-through openings with soft foam sleeves around the cylindrical sleeves. 
         FIGS. 18A-18B  are schematic illustrations of a diaphragm with soft bellows around the pass-through openings on its inside face only. 
         FIGS. 19A-19B  are schematic illustrations of a diaphragm with cylindrical sleeves around the pass-through openings, ring magnets around the sleeves on its inside face only, and ferromagnetic liquid between the sleeves and the drive rods. 
         FIGS. 20A-20B  are schematic illustrations of a diaphragm with cylindrical sleeves around the pass-through openings on its outside face only, ring magnets around the pass-through openings on its inside face only, and ferromagnetic liquid between the magnets and the drive rods. 
         FIGS. 21A-21B  are schematic illustrations of a diaphragm with a semifluid lubricant covering the pass-through openings. 
         FIG. 22A  is a schematic illustration of a diaphragm module housing section with ribs. 
         FIG. 22B  is perspective schematic illustrations of an acoustic transducer comprising modular housing sections with ribs. 
         FIGS. 23A-23C  are schematic illustrations of a dome-shaped diaphragm with integrated rods and sleeves. 
         FIG. 24A  is a perspective schematic illustration of a modularly constructed transducer comprising modular housing sections with ribs and dome-shaped diaphragms with soft foam sleeves. 
         FIG. 24B  is a schematic cross-sectional illustration of a modularly constructed transducer with dome-shaped diaphragms and soft foam sleeves. 
     
    
    
     DETAILED DESCRIPTION 
     A. Direct Drive 
       FIG. 1  shows one implementation of the invention in which an electromagnetic motor comprises a magnet  1010  and a voice coil  1020  to which is mounted a mechanical coupling  1030  that is coupled to a drive rod  1040 . The drive rod is attached to the diaphragms  1050 , each of which are in turn attached to the housing  1060  by a respective suspension  1070 . When an audio signal is applied to the voice coil, the sound waves from one side of the diaphragms are allowed to radiate to the listening environment through the openings  1080 . The sound waves from the other side of the diaphragms are allowed to radiate from another set of openings  1085 . Unwanted air leakage is prevented or reduced substantially by the baffles  1090  and the seals  1100 . If desired, one or more bushings may be used in the motor to prevent undesirable voice coil motion. Alternatively, the drive rod  1040  can pass through some or all of the diaphragms  1050  without using seals. The size of the space between the diaphragms and the rods can be optimized to minimize air leakage while minimizing friction between the rods and the diaphragms. 
       FIG. 2  shows one implementation of the invention in which an electromagnetic motor comprises a magnet  2010  and a voice coil  2020  to which is mounted a mechanical coupling  2030  that is coupled to a drive rod  2040 . The drive rod  2040  is attached to the diaphragms  2050 , each of which are in turn attached to the housing  2060  by a respective suspension  2070 . The suspensions  2070  need not all have identical properties. It may be desirable, for example, to use stiffer suspensions near the voice coil to minimize movement of the voice coil in directions other than along the direction of the actuated drive rod. The stiffness of the suspensions  2070  may be controlled by manipulating suspension geometry or material. Furthermore, by orienting the suspensions of the diaphragms that are actuated by a single motor so that they face opposite directions, distortion characteristics of the transducer may be reduced. In this particular implementation, the drive rod  2040  passes through all but one of the diaphragms  2150  via openings that are sealed by the seals  2180 . A different motor comprises a magnet  2110  and a voice coil  2120  having a mechanical coupling  2130  that is coupled to a drive rod  2140 . The drive rod  2140  is attached to the diaphragms  2150 , each of which are in turn attached to the housing  2060  by a respective suspension  2170 . In this particular implementation, the drive rod  2140  passes through all but one of the diaphragms  2050  via openings that are sealed by the seals  2180 . The voice coils  2020  and  2120  are connected so that each diaphragm works in opposition to the diaphragms next to it. When an audio signal is applied to the transducer, the sound waves from the front of the diaphragms are allowed to radiate to the listening environment through the openings  2090 . Leakage between the front wave and rear wave is prevented or reduced substantially by the seals in the diaphragms. The rear wave is allowed to radiate through openings  2190 . Alternatively, the drive rods  2040  and  2140  can pass through some or all of the diaphragms  2050  and  2150  without using seals. The space between the diaphragms and the rods can be optimized to minimize air leakage while minimizing friction between the rods and the diaphragms. The net change of momentum of the mechanical parts in this implementation of the invention is zero or substantially zero after taking into account variations in the parts due to manufacturing tolerances; therefore, the transducer housing  2060  will be essentially free of vibrations. 
       FIG. 3  shows one implementation of the invention in which an electromagnetic motor comprises a magnet  3010  and a voice coil  3020  that is mounted to a mechanical coupling  3030  to which are coupled two drive rods  3040 . The drive rods  3040  are attached to the diaphragms  3050 , which in turn are attached to the housing  3060  by the suspensions  3070 . When an audio signal is applied to the transducer, the sound waves from one side of the diaphragms are allowed to radiate to the listening environment through the openings  3080 . The sound waves from the other side of the diaphragms are allowed to radiate through the openings  3180 . Unwanted air leakage between the individual chambers is prevented or reduced substantially by the baffles  3090 . 
       FIG. 4  shows one implementation of the invention in which an electromagnetic motor comprises a magnet  4010  and a voice coil  4020  that is mounted to a mechanical coupling  4030 , which is coupled to a drive rod  4040 . The drive rod  4040  is attached to the diaphragms  4050 , which are in turn attached to the housing  4060  by the suspensions  4070 . A different motor comprises a magnet  4080  and a voice coil  4090  having a mechanical coupling  4100  that is coupled to a drive rod  4110 . The drive rod  4110  is attached to the diaphragms  4120 , which are in turn attached to the housing  4060  by the suspensions  4130 . The voice coils are connected so that each diaphragm works in opposition to the diaphragms adjacent to it. When an audio signal is applied to the transducer, the sound waves from the front of the diaphragms are allowed to radiate to the listening environment through the openings  4160 . The sound waves from the rear of the diaphragms are allowed to radiate through the openings  4180 . The net change of momentum of the mechanical parts in this implementation of the invention is zero or substantially zero after taking into account variations in the parts due to manufacturing tolerances; therefore, the transducer housing will be essentially free of vibrations. 
     The main difference between the implementations illustrated by  FIG. 2  and  FIG. 4  is the configuration of each rod that drives half of the diaphragms in the transducer. Another implementation of the present invention uses two groups of rods, with each group comprising multiple rods. Each group of rods is connected to half the diaphragms and passes through the other half of the diaphragms. For example, the implementations illustrated in  FIGS. 7-10  use six rods that are symmetrically distributed in a circular pattern around the center of the diaphragms and adjacent rods are displaced from one another by an angle of 60 degrees. The six rods are divided into two groups of three rods, and the rods in these two groups are interlaced with respect to each other. This means that the three rods in each group are symmetrically distributed in a circular pattern at equal distance from the center of the diaphragms and adjacent rods in the group are displaced from one another by an angle of 120 degrees. Each group of three rods is attached to half the diaphragms and passes through the other half of the diaphragms via sealed or unsealed openings in a fashion similar to that described above for the rods  2040  and  2140  and illustrated in  FIG. 2 . In this arrangement, each diaphragm is actuated in a symmetric fashion by three rods whose three points of attachment to the diaphragm are symmetrically distributed and define a unique two-dimensional plane in three-dimensional space. If the rods and diaphragms are properly aligned so that all the rods are parallel to each other, all the diaphragms are parallel to each other, and all the rods are perpendicular to the surface of all the diaphragms, then the diaphragms will be subjected to a symmetrically distributed normal force that will tend to move them in the desirable longitudinal direction without exciting any undesirable vibrational modes that may result in undesirable sonic artifacts. 
     Another implementation of the present invention uses one rod and one tube that are concentric. The outer diameter of the rod is smaller than the inner diameter of the tube so that, when they are mounted in a concentric fashion, the rod does not touch the tube. The rod is attached to a first set of diaphragms consisting of half of all the diaphragms in the transducer and passes through one or more diaphragms in a second set of diaphragms consisting of the other half of the diaphragms. The tube is attached to the diaphragms in the second set of diaphragms and passes through one or more diaphragms in the first set of diaphragms. The rod passes through diaphragms in the second set of diaphragms by virtue of the fact that it is wholly contained inside the tube. The tube is composed of multiple sections that are connected to one another one or more connecting rods that pass through openings in the diaphragms of the first set. Preferably, three connecting rods are symmetrically distributed across the circumference of the tube sections. 
     For any of the direct-drive implementations described herein, the diaphragm suspensions need not all have identical properties or orientations. For example, it may be desirable to use stiffer suspensions near the motors to minimize movement in directions other than along the direction of the actuated drive rods. The stiffness of the suspensions may be controlled by manipulating suspension geometry or material. Furthermore, by orienting the suspensions of diaphragms that are actuated by a single motor so that some of the suspensions face in an opposite direction with respect to other suspensions, asymmetrical characteristics of the suspensions may be cancelled or reduced. In typical implementations, suspensions have an asymmetrical response to the forces generated by the driving motor. An asymmetrical response typically introduces distortion into the resulting sound wave generated by the moving diaphragms. By reversing the orientation of some of suspensions, the asymmetry of the overall suspension response may be reduced, thereby reducing distortion in the resulting sound wave. 
     B. Indirect Drive 
       FIG. 5  shows one implementation of the invention in which an electromagnetic motor comprises a magnet  5010  and a voice coil  5020  to which is mounted a mechanical coupling  5030  that is coupled to a housing  5040 . The housing is connected to the diaphragms  5050  by the suspensions  5060 . Individual chambers are created by the baffles  5070 . The sound waves from the front of the diaphragms are allowed to radiate to the listening environment through the openings  5080 . The sound waves from the rear of the diaphragms are allowed to radiate through the openings  5180 . Cancellation between the front and rear of the diaphragms is prevented or reduced substantially by the baffles  5070 . At frequencies well below the resonance of the diaphragm/suspension assembly, the diaphragms move largely in phase with the housing and substantially no sound will be created. At frequencies well above the resonance of the diaphragm/suspension assembly, the diaphragms are almost motionless and the relative motion between the housing and the diaphragms creates sound. As a result, the resonant frequency of the diaphragm/suspension assembly can be chosen to achieve the desired frequency response of the transducer. 
     The suspensions need not all have identical properties or orientations. By varying the orientation of the suspensions as discussed above, asymmetrical characteristics of the suspensions may be cancelled or reduced so that distortion characteristics of the transducer may be reduced. 
     C. Modular Construction 
       FIGS. 6A-6C, 7, and 8  illustrate another implementation of the present invention that allows the acoustic transducer to be assembled in modules. Such a modular implementation may allow for greater manufacturability, flexibility, and performance as compared with a non-modular implementation. 
       FIGS. 6A-6C  illustrate one implementation of a diaphragm module.  FIG. 6A  shows a front view of the diaphragm module,  FIG. 6B  shows a rear view of the same diaphragm module, and  FIG. 6C  shows a cross-sectional view of the same diaphragm module. The diaphragm module includes a diaphragm  6050  that is attached via a suspension  6070  to the housing section  6060 . The housing section  6060  incorporates an opening  6190  on the front side and another opening  6290  on the rear side. The housing section  6060  has protrusions  6162  on the front side and  6262  on the rear side, as well as corresponding slots  6164  on the front side and  6264  on the rear side, respectively. The diaphragm module also includes a section of three rods  6040 , each of which has a protrusion  6041  on the front side and a matching opening  6042  on the rear side. The rods  6040  may be integrated with the diaphragm  6050  for improved structural integrity. Such a diaphragm/rod component could be manufactured, for example, using a material such as glass-filled or mica-filled polypropelene-polyphenylene-oxide-styrene in a molding process. The diaphragm  6050  has three openings  6080  to allow the rods of an adjacent diaphragm module to pass through the diaphragm  6050 . If desired, diaphragm modules may have suspensions with different properties or different orientations as discussed above. 
     When two adjacent diaphragm modules are assembled together to form one implementation of a transducer, the front side of the first diaphragm is attached to the front side of the second diaphragm. The rods  6040  of the first diaphragm pass through the holes  6080  of the second diaphragm. The protrusion  6162  of each of the two diaphragm modules slide into the slot  6164  of the other module and may be bonded via an operation such as gluing or sonic welding. The front openings  6190  of the first and second diaphragms combine to create an opening for the front sound wave to be transmitted to the surrounding air. An assembly comprising two diaphragm modules that are assembled in this manner may be assembled with a third diaphragm module whose rear side is attached to the rear side of the second diaphragm module. The protrusion  6262  of each of the second and third diaphragm modules slide into the slot  6264  of the other module and may be bonded via an operation such as gluing or sonic welding. The rod protrusions  6041  of the first diaphragm slide into the rod openings  6042  of the third diaphragm and may be bonded via an operation such as gluing or sonic welding. The rear openings  6290  of the second and third diaphragms combine to create an opening for the rear sound wave to be vented to the surrounding air. 
     In preferred implementations, the housing section  6060  of a diaphragm module is made of a material that has sufficient strength and rigidity to provide a stable supporting structure for the diaphragms so that the transducer does not generate objectionable artifacts. If the housing section is made of a rigid plastic material such glass-filled or mica-filled polypropelene-polyphenylene-oxide-styrene, however, the rigidity of the resulting transducer may not be sufficient. In that case, the rigidity of the modular assembly may be improved by adding ribs to the outer wall of the housing section.  FIG. 22A  illustrates a housing section  22060  with integrated flanges  22160  and ribs  22260  on its outer surface. Adjacent housing sections may be attached to one another with glue and screws through the openings  22460  for additional rigidity. The resulting modular transducer assembly  22000  is shown in  FIG. 22B . 
     The assembly procedure outlined above may be continued to add additional diaphragm modules to form a linear array of diaphragm modules of essentially any desired length. A second type of module, referred to herein as a motor module, includes a mechanical coupling that is designed to attach to the rear side of a diaphragm module. 
     A linear array of diaphragm modules may be assembled with one or more motor modules to create a complete transducer. For example,  FIGS. 7 and 8  illustrate one implementation of a transducer according to the present invention that is composed of two motor modules  7100  and twelve diaphragm modules. Each motor module  7100  comprises a magnet assembly  7110 , a coil  7120  and a mechanical coupling  7130  that connects the motor to a first diaphragm and from there to the other diaphragms through the rods  6040 . The number of diaphragm modules that can be connected together in this fashion can be chosen to create a transducer of arbitrary length and arbitrary volume displacement, provided the motors have enough power to actuate the load presented by the selected number of diaphragm modules. 
     D. Fluidic Drive 
       FIG. 9  and  FIG. 10  illustrate another implementation of the present invention in which the motor module  9100  is similar to a motor used in traditional transducers, and comprises a magnet assembly  9110 , a coil  9120 , and a cone  9130 . The cone  9130  is fluidically coupled to the first diaphragm  9140  through the fluid contained in the sealed chamber  9150 . The diaphragm  9140  is mechanically coupled to the remaining diaphragms  6050  through the rods  6040 . The rear wave from the directly driven cones  9130  may contribute to the front waves of the diaphragms  6050 . If the fluid used in the sealed chambers  9150  between the directly driven cones  9130  and indirectly driven diaphragms  6050  is a gas such as air, the fluidic drive includes a low pass filter. In this case, the directly driven cones  9130  may be driven to generate significant acoustic energy throughout their full frequency range while the indirectly driven diaphragms  6050  generate significant acoustic energy only at the lower frequencies. 
     E. Reduced Air Leakage Noise 
       FIGS. 11A-11C, 12A-12C, and 13A-13C  illustrate three different techniques that may be used in various combinations to reduce undesirable air leakage noise through the pass-through openings of the diaphragms. 
       FIGS. 11A-11C  illustrate one technique using a composite diaphragm  11050 .  FIG. 11A  shows an exploded view of the composite diaphragm  11050  with two component diaphragms  11150  and  11250  and a layer of damping material  11350  between them. The layer of damping material  11350  may be attached to the component diaphragms  11150  and  11250  using a process such as gluing or molding.  FIG. 11B  shows a rear view and  FIG. 11C  shows a cross-sectional view of the composite diaphragm  11050 . 
       FIGS. 12A-12C  illustrate another technique using a diaphragm  12050  with sleeves around its pass-through openings.  FIG. 12A  shows a rear view,  FIG. 12B  shows a front view and  FIG. 12C  shows a cross-sectional view of the diaphragm  12050  with the sleeves  12450  around its pass-through openings. 
       FIGS. 13A-13C  illustrate yet another technique using a composite diaphragm  13050  with sleeves around its pass-through openings.  FIG. 13A  shows an exploded view of the composite diaphragm  13050  with two component diaphragms  13150  and  13250  and a layer of damping material  13350  between them. The layer of damping material  13350  may be attached to the component diaphragms  13150  and  13250  using a process such as gluing or molding. The two component diaphragms  13150  and  13250  each have sleeves  13450  around their corresponding pass-through openings. The sleeves are formed on the outside face of each component diaphragm, which is the side that faces away from the damping material  13350 .  FIG. 13B  shows a rear view and  FIG. 13C  shows a cross-sectional view of the composite diaphragm  13050 . 
       FIGS. 14A-14B  illustrate another technique using a diaphragm  14050  with hard sleeves and soft fabric sleeves around its pass-through openings.  FIG. 14A  shows a side view and  FIG. 14B  shows a cross-sectional view of the resulting subassembly, which includes the diaphragm  14050  with hard cylindrical sleeves  14450  around each of its pass-through openings on both sides of the diaphragm  14050 . The soft fabric sleeves  14550  are attached to the outside of the hard sleeves  14450  and extend past them, almost touching the rods  14040  that slide through the pass-through openings of the diaphragm  14050 . 
       FIGS. 15A-15B  illustrate another technique using a diaphragm  15050  with hard sleeves and soft foam sleeves around its pass-through openings.  FIG. 15A  shows a side view and  FIG. 15B  shows a cross-sectional view of the resulting subassembly, which includes the diaphragm  15050  with hard cylindrical sleeves  15450  around each of its pass-through openings on both sides of the diaphragm  15050 . The soft foam sleeves  15650  are attached to the outside of the hard sleeves  15450  and preferably extend past them, curving in and almost touching the rods  15040  that slide through the pass-through openings of the diaphragm  15050 . 
       FIGS. 16A-16B  illustrate another technique using a diaphragm  16050  with hard sleeves, soft foam sleeves, and soft fabric sleeves around its pass-through openings.  FIG. 16A  shows a side view and  FIG. 16B  shows a cross-sectional view of the resulting subassembly, which includes the diaphragm  16050  with hard cylindrical sleeves  16450  around each of its pass-through openings on both sides of the diaphragm  16050 . The soft foam sleeves  16650  are attached to the outside of the hard sleeves  16450 . The soft fabric sleeves  16550  are attached to the outside of the soft foam sleeves  16650  and extend past them, almost touching the rods  16040  that slide through the pass-through openings of the diaphragm  16050 . 
       FIGS. 17A-17B  illustrate yet another technique using a diaphragm  17050  with hard sleeves and soft foam sleeves around its pass-through openings.  FIG. 17A  shows a side view and  FIG. 17B  shows a cross-sectional view of the resulting subassembly, which includes the diaphragm  17050  with hard cylindrical sleeves  17450  around each of its pass-through openings on both sides of the diaphragm  17050 . The soft foam sleeves  17650  are attached to the outside of the hard sleeves  17450  only on the inside face of the diaphragm  17050 , and they tightly touch the rods  17040  to further reduce resistance to air flow. The sleeves  17450  have a funnel shape on the outside face of the diaphragm  17050  to provide a greater reduction in air leakage noise. 
       FIGS. 18A-18B  illustrate a technique for preventing air leakage using a diaphragm  18050  with soft bellows around its pass-through openings.  FIG. 18A  shows a side view and  FIG. 18B  shows a cross-sectional view of the resulting subassembly, which includes the diaphragm  18050  with soft bellows  18750  on its inside face. One side of the bellows  18750  is connected to the diaphragm  18050  around each of its pass-through openings. The other side of the bellows  18750  is connected to the rod  18040 . The soft bellows  18750  stretch and contract as the diaphragm  18050  and the rods  18040  move relative to each other. 
       FIGS. 19A-19B  illustrate another technique for preventing air leakage using a diaphragm  19050  with hard sleeves, ring magnets, and ferromagnetic liquid.  FIG. 19A  shows a side view and  FIG. 19B  shows a cross-sectional view of the resulting subassembly, which includes the diaphragm  19050  with hard cylindrical sleeves  19450  around each of its pass-through openings on both sides of the diaphragm  19050 . The ring magnets  19950  are attached to the outside of the hard sleeves  19450  on the inside face of the diaphragm  19050 , and they are preferably polarized in the vertical direction for improved efficiency. The ferromagnetic liquid  19960  is placed between the sleeves  19450  and the rods  19040 , and is held in place by the magnetic force of the ring magnets  19950  as the rods  19040  move relative to the diaphragm  19050 . 
       FIGS. 20A-20B  illustrate another technique for preventing air leakage using a diaphragm  20050  with hard sleeves, ring magnets, and ferromagnetic liquid.  FIG. 20A  shows a side view and  FIG. 20B  shows a cross-sectional view of the resulting subassembly, which includes the diaphragm  20050  with hard cylindrical sleeves  20450  around its pass-through openings on the outside face of the diaphragm. The ring magnets  20950  are attached around the diaphragm  20050  on the inside face of the diaphragm  20050 , and they are preferably polarized in the vertical direction for improved efficiency. The ferromagnetic liquid  20960  is placed between the ring magnets  20950  and the rods  20040 , and is held in place by the magnetic force of the ring magnets  20950  as the rods  20040  move relative to the diaphragm  20050 . 
       FIGS. 21A-21B  illustrate another technique for preventing air leakage using a diaphragm  21050  with a semifluid lubricant, such as a thixotropic gel.  FIG. 21A  shows a side view and  FIG. 21B  shows a cross-sectional view of the resulting subassembly, which includes the diaphragm  21050  with the semifluid lubricant  21980  covering its pass-through openings on both sides of the diaphragm. The lubricant  21980  allows the rods  21040  to slide through the openings but otherwise seals the openings to essentially eliminate air flow through the openings. 
     The thickness of the diaphragm and the length of the sleeves may be adjusted so that the total length of the air path through the pass-through openings is as short as 2 mm or as long as 25 mm or more. The air path length may be set according to the needs of the application and the desired level of audio quality. A path length of about 15 mm is preferred for many applications. 
     The drawings illustrate implementations of acoustic transducers that have flat or planar diaphragms. The shape of the diaphragms is not critical in principle. Other shapes such as cones or domes may be used. 
       FIGS. 23A-23C  illustrate a dome-shaped diaphragm  23050  with integrated rods  23040  and sleeves  23450 .  FIG. 23A  shows a front side view,  FIG. 23B  shows a rear side view, and  FIG. 23C  shows a cross-sectional view of the diaphragm  23050 . Because of the dome shape of the diaphragm, flat landings are added to accommodate air leakage reduction components and improve rigidity. The flat landings  23455  surrounding the sleeves  23450  are used to attach components for reducing air leakage noise such as, for example the soft foam sleeves  17650  shown in  FIG. 17  or the ring magnets  19950  shown in  FIG. 19 . The flat landings  23045  surrounding the rods  23040  are added to make the diaphragm  23050  more amenable to volume manufacturing methods such as injection molding. The gussets  23047  are also added for structural support of the joint between the rods  23040  and the landing  23045 . The flat landings  23045  and  23455  are pushed towards the front side of the diaphragm  23050  to increase the clearance between neighboring diaphragms, which increases the maximum allowed excursion of the overall transducer. 
       FIG. 24A  shows a perspective view and  FIG. 24B  shows a cross-sectional view of a modularly assembled transducer  24000  with integrated flanges  24160  and ribs  24260  on its outer surface, and dome-shaped diaphragms  24050  with integrated rods  24040  and sleeves  24450  that are surrounded on their rear side by soft foam sleeves  24650 .