Abstract:
A vibration module for applying vibrational tractions to a wearer&#39;s skin is presented. Use of the vibration module in headphones is illustrated for providing tactile sensations of low frequency for music, for massage, and for electrical recording and stimulation of the wearer. Damped, planar, electromagnetically-actuated vibration modules of the moving magnet type are presented in theory and reduced to practice, and shown to provide a substantially uniform frequency response over the range 40-200 Hz with a minimum of unwanted audio.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/054,712, entitled “DAMPED ELECTROMAGNETICALLY ACTUATED PLANAR MOTION FOR AUDIO-FREQUENCY VIBRATIONS,” filed Sep. 24, 2014, and U.S. Provisional Patent Application Ser. No. 62/101,985, entitled “SYSTEMS AND METHODS FOR PROVIDING DAMPED ELECTROMAGNETICALLY ACTUATED PLANAR MOTION FOR AUDIO-FREQUENCY VIBRATIONS,” filed Jan. 10, 2015, each of which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to tactile transducers that produce bass frequency vibrations for perception by touch. 
       BACKGROUND OF THE INVENTION 
       [0003]    Below about 200 Hz, the lower the frequency of sound, the more it is perceived not only by vibration of the ear drum but also by touch receptors in the skin. This sensation is familiar to anyone who has “felt the beat” of strong dance music in the chest, or through the seat of a chair, or has simply rested a hand on a piano. The natural stimulus is both auditory and tactile, and a true reproduction of it is possible only when mechanical vibration of the skin accompanies the acoustic waves transmitted through the air to the ear drum. 
         [0004]    The prior art in audio-frequency tactile transducers primarily employ axial shakers.  FIG. 1  shows an exploded view of a prior art headphone set  10  that includes axial shaker  100 , including moving mass  114  suspended on spiral-cut spring  112 , stator  116 , and voice coil  118 . The construction of such axial shakers mimics conventional audio drivers in which the light paper cone is replaced with a heavier mass, and a more robust suspension is provided, typically a spiral-cut metal spring. 
         [0005]    A drawback of this construction is the production of unwanted acoustic noise. This occurs because the axial shaker is mounted in the headphone ear cup with the motion axis pointed at the opening of the ear canal.  FIG. 2A  shows a perspective view of prior art headphone set  20  that includes an axial shaker that vibrates along the z-axis and stimulates the skin by plunging the ear cup against the side of the user&#39;s head. Axial movement of the mass causes a countermovement of the entire ear cup itself, which is typically sealed over the pinna. Thus, the same force that stimulates the skin under the ear cup cushions unfortunately also plunges air into the listener&#39;s ear canal, overwhelming the output of the audio driver and generating the excess unwanted acoustic noise. 
         [0006]      FIG. 2B  shows a graph illustrating the excess apparent bass audio generated by the prior art headphones of  FIG. 2A . In particular,  FIG. 2B  illustrates that the relatively flat acoustic frequency response of the headphones alone (traces labeled “off”) is degraded when the inertial shaker is turned on to progressively stronger levels (traces labeled “on”). In this example, significant audio is added to the acoustic frequency response, causing an undesirable bump of 10-20 dB in the 50-100 Hz range. The result is a bass-heavy sound in which upper frequencies are underrepresented, and the user&#39;s perception is one of muffled, muddy sound. 
         [0007]    The problem of uneven frequency response is typically made worse by a lack of mechanical damping. Leaving the system underdamped means that steady state signals near mechanical resonance achieve high amplitude, leading to a peaked response, and that the system rings after excitation is stopped, further degrading audio fidelity. Such a bump is evident in the frequency response of the prior art ( FIG. 2B ), where actuating the tactile transducer increases the acoustic output of the headphone 10 to 20 dB above the 90 dB Sound Pressure level that is indicated by the “0” reference line. 
         [0008]    Another approach in the prior art, also problematic, is the use of un-damped eccentric rotating motors (“ERMs”) and un-damped linear resonant actuators (“LRAs”). Small, un-damped ERMs are incompatible with high-fidelity audio for a few reasons. First, it generally takes about 20 milliseconds to “spin up” an ERM to a frequency that produces an acceleration large enough to be felt. By then an impulse signal (for example, the attack of a kick drum) will have passed. Second, in an ERM the acceleration, which can be likened to a “tactile volume,” and frequency, which can be likened to a “tactile pitch” are linked and cannot be varied independently. This linkage is fundamentally incompatible with acoustic fidelity. 
         [0009]    The main drawback of LRAs is the dependence on the “resonance,” that the name suggests. The devices are designed for tactile alerts, not fidelity, and so they resonate at a single frequency and produce perceptible vibration at only that frequency. For example a typical LRA might produce up to 1.5 g of acceleration at 175±10 Hz, but less than 0.05 g outside this 20 Hz range. Such a high Q-factor renders this sort of device useless for high fidelity reproduction of low frequency tactile effects in the 15-120 Hz range. Despite these problems, LRAs have been contemplated for vertical mounting in the top cushion of a headphone bow. 
         [0010]    In addition to the limited frequency range of LRAs there is a another problem with using LRAs as audio-frequency tactile transducers is that a transducer mounted vertically between the headphone bow and the top of the skull flexes the bow. At a fine scale, this flexion makes the bow flap like the wings of a bird, where an ear cup is situated at each wing tip. The inward-outward component of the flapping plunges the ear cups against the sides of the wearer&#39;s head, again producing undesirable audio that competes with and distorts the acoustic response of the audio drivers in the ear cups. 
         [0011]    To avoid such unwanted audio, one approach is to construct a low-profile, vibrating module which moves a mass in-plane (i.e. in the x-y plane of  FIG. 2A ). This approach minimizes the surface area that is oriented to cause the problematic axially directed acoustic radiation. When mounted in an ear cup, such an in-plane vibrating module produces motion parallel with the surface of the side of the head. This movement effectively shears the skin, creating tactile sensation with little effect on the volume of air trapped between the ear cup and the ear drum. Acoustic noise is therefore minimized. Consider the difference between sliding a glass over a table top (planar motion of the present invention) and plunging a toilet (axial motion, as used in prior art). Although this in-plane approach has been contemplated, the dielectric elastomer actuators proposed for this purpose are expensive and complex devices that require high voltage electronics. Another drawback of this approach was that no provision was made for critically damping those transducers. Accordingly the tactile acceleration frequency response was underdamped, with a claimed Q-factor of 1.5 to 3. 
         [0012]    In terms of electromagnetic actuation, a relatively thin, flat arrangement of a coil and two magnets that produces planar motion has been disclosed. In particular, the vibration module includes a single-phased electromagnetic actuator with a movable member comprised of two parallel thin magnets magnetized transversely in opposite directions and connected by a magnet bracket, and a means for guiding the magnet bracket. 
         [0013]    Although this general approach to providing electromagnetic actuation has not been applied in headphones, it has been applied to the problem of providing haptic feedback in computer input devices like joysticks. One such device includes an actuator comprising a core member having a central projection, a coil wrapped around the central projection, a magnet positioned to provide a gap between the core member and the magnet, and a flexible member attached to the core member and the magnet. In this design, the motion is guided by a parallel pair of flexures. 
         [0014]    A drawback of this guiding approach is the vulnerability of flexures to buckling when loaded by longitudinal compression. Compressive longitudinal loads on the flexures arise naturally from the attraction of the magnet pair riding the flexures to iron flux guides on the coil side, such as the E-core that provides the central projection supporting the coil. Accordingly, the flexures must be thick enough to resist this load without Euler buckling. This thickness comes at the expense of increased stiffness in the motion direction, which may undesirably impede movement. 
         [0015]    Despite this drawback, the general approach has been applied elsewhere. For example, a flexure-guided surface carrying the magnets has been contemplated for use as the face of a massaging element. One approach to mitigating the buckling problem is to bear the compressive load on an elastic element such as foam. Supporting the load with an elastic element has some undesirable drawbacks, however. The foam adds stiffness in the direction of travel, and may significantly increase the thickness of the assembly, since the foam layer must be thick enough that the maximum shear strain (typically &lt;100%) allows adequate travel. 
         [0016]    An alternative approach to suspending a moving element arranges the long axis of the flexures in the plane of a substantially flat transducer. Because slender flexures resist transverse shear loads more effectively than longitudinal compressive loads, thinner flexures may be used, providing less impediment to motion. 
         [0017]    Therefore, there exists a need for novel audio-frequency tactile transducers and devices. 
       SUMMARY OF THE DISCLOSURE 
       [0018]    In some embodiments, proposed herein is a thin, flat vibration module with a movable member that is electromagnetically actuated to produce motion in-plane. Motion of the movable member can be damped so that the steady-state sinusoidal voltages applied to the module at different frequencies produce an acceleration response of the movable member that is substantially uniform over the range of 40-200 Hz. The module can be mounted in a headphone so that the motion axis lies substantially parallel to the sagittal plane of the wearer&#39;s head, so that the motion does not plunge the ear cup toward the wearer&#39;s ear canal, which produces unwanted audio and/or distortions. 
         [0019]    In some embodiments, the module may consist of a mass and thin magnets, polarized through their thickness, where the mass and magnets are movably suspended inside a housing. The suspension may include flexures, bushings, ball bearings, or a ferrofluid layer, for example. The housing may include one or more conductive coils that carry electrical current used to vibrate the movable portion. To facilitate mounting of the module in the ear cup of a headphone, the geometry of the mass, coil, and housing may be substantially planar, (e.g. with a thickness less than one-third the length or width). The vibration of the moving portion may be damped using a suitable approach, such as the shearing of a layer of ferrofluid, oil, grease, gel, or foam, or the passage of air through an orifice, for example. 
         [0020]    In some embodiments, flexures suspending the mass and magnets can be molded into the housing. In yet another embodiment, flexures may have tabs that engage receiving holes in the housing. 
         [0021]    In some embodiments, the mass may have a central pocket that provides space for the magnets and coil. In other embodiments, the mass may lie adjacent to the magnets. In still other embodiments, the mass may be a battery for powering the module. 
         [0022]    In some embodiments, the flexures can extend radially from a central hub to guide torsional rotation of the magnets and mass. Mounted in an ear cup in a plane parallel with the wearer&#39;s sagittal plane, these embodiments produce torsional rotation of the ear cup cushion against the wearer&#39;s skin. Multiple magnets and coils may be used in place of a single electromagnetic element. 
         [0023]    In some embodiments, the module may be made of compliant materials suitable for direct skin contact. The skin-facing portion of the housing may be comprised of a stretchable cover. The magnets underneath this cover may be embedded in a puck comprised of compliant elastomer. The puck may be suspended on a layer of ferrofluid. The upper cover may be sealed at the perimeter to a lower cover to provide an impermeable compliant housing that holds the puck and ferrofluid in proximity to a coil. The underlying coil itself may be embedded in a compliant elastomeric material so that the entire module is compliant. 
         [0024]    Planar motion of the module may be provided by various arrangements of magnets and coils. In some embodiments, a mass may be urged laterally by a magnet that is polarized along the axis of motion. To reduce the module&#39;s thickness, the lateral dimension of the magnet may be elongated, fitted with flux guides, and may be driven by an elongated oval coil that operates within an air gap defined by the flux guide. In other embodiments, the mass may be urged laterally by several magnets polarized along the motion axis, arranged side-by-side, and situated on the one edge of the mass. In still other embodiments, a long thin magnet polarized through the thickness direction may lie within a coil. Movement of the magnet within the coil may be coupled to the mass by brackets, and the motion of the magnet within the tube may be guided by ferrofluid bearing. 
         [0025]    In some embodiments, the module can be provided with a clear plate that enables viewing of the motion within it. The module may be mounted in an ear cup with a window that provides a view of the motion inside the module. The ear cup may include a retaining element for the module. 
         [0026]    In some embodiments, the complaint module may be integrated directly into cushions on the headphone bow, so as to apply vibratory shear tractions to the skin. In other embodiments, one or more of the modules may be mounted on movable armatures fixed to the ear cup and or bow of the headphones. The armatures may include rotational and prismatic degrees of freedom, and may be spring loaded to oppose the module to the skin, and may also be electromechanically actuated to produce a massaging motion on the skin of the scalp or face. The armature may include routing for electrical leads of the coil and/or an electrode that makes contact with the skin. The electrode may provide a means of recording electrical potential on the body surface, and/or for electrical stimulation of the wearer. 
         [0027]    Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
         [0028]    The present invention accordingly comprises the features of construction, combination of elements, and arrangement of parts all as exemplified in the constructions herein set forth, and the scope of the invention will be indicated in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    For a fuller understanding of the inventive embodiments, reference is had to the following description taken in connection with the accompanying drawings in which: 
           [0030]      FIG. 1  shows an exploded view of a prior art headphone set having axial shaker suspended on a spiral-cut spring; 
           [0031]      FIG. 2A  shows a perspective view of a prior art headphone illustrating an axial shaker orientation that stimulates the skin by plunging the ear cup against the side of the head; 
           [0032]      FIG. 2B  shows a graph illustrating the excess bass audio apparent generated by the prior art headphones of  FIG. 2A ; 
           [0033]      FIG. 3A  shows a chart illustrating two physical bounds on the force output of an electromagnetic vibration module arising from limitations on the space available for translating the mass and from the limited force output of the coil; 
           [0034]      FIG. 3B  shows a cross-sectional view of an exemplary module including an arrangement of a coil, magnets and a suspended inertial mass obeying the constraints illustrated in  FIG. 3A , in accordance with some embodiments; 
           [0035]      FIG. 3C  shows a cross-sectional view of the module of  FIG. 3B  with exemplary magnetic flux lines superimposed thereon, in accordance with some embodiments; 
           [0036]      FIG. 4A  shows a perspective view of an exemplary damped planar electromagnetic module, in accordance with various embodiments described herein; 
           [0037]      FIG. 4B  shows an exploded view the module of  FIG. 4A , in accordance with various embodiments described herein; 
           [0038]      FIG. 5A  shows an exploded view of an exemplary headphone showing the orientation of the module in the ear cup, in accordance with various embodiments described herein; 
           [0039]      FIG. 5B  shows a perspective view of a user wearing the headphone of  FIG. 5A  and illustrates how the motion axis lies parallel to the side of the user&#39;s head, in accordance with various embodiments described herein; 
           [0040]      FIG. 5C  shows that the measured acceleration of the exemplary headphone of  FIG. 5A  at various frequencies is approximately uniform over the range 40-200 Hz, in accordance with various embodiments described herein; 
           [0041]      FIG. 6A  shows an exploded view of an exemplary suspension, in accordance with various embodiments described herein; 
           [0042]      FIG. 6B  shows a detailed perspective view of a portion of the exemplary suspension of  FIG. 6A , in accordance with various embodiments described herein; 
           [0043]      FIG. 7  shows a perspective view of a portion of an exemplary module, in accordance with various embodiments described herein; 
           [0044]      FIG. 8A  shows an exploded view of an exemplary torsional module, in accordance with various embodiments described herein; 
           [0045]      FIG. 8B  shows a schematic view of the torsional module of  FIG. 8A  illustrating the action its flexures, in accordance with various embodiments described herein; 
           [0046]      FIG. 8C  shows a perspective view of a user wearing headphones incorporating the torsional module of  FIG. 8A  and illustrates exemplary rotational motion in a plane parallel to the side of the user&#39;s head, in accordance with various embodiments described herein; 
           [0047]      FIG. 9A  shows an exploded view of an exemplary compliant vibration module, in accordance with various embodiments described herein; 
           [0048]      FIG. 9B  shows a cross-sectional view of the compliant vibration module of  FIG. 9A , in accordance with various embodiments described herein; 
           [0049]      FIG. 10A  shows an illustrative two-dimensional finite element analysis of a coil carrying a current in the magnetic gap formed by a single magnet and flux guides, in accordance with various embodiments described herein; 
           [0050]      FIG. 10B  shows a perspective view of an exemplary module having multiple cylindrical coils in circular magnetic gaps driving magnets coupled to an inertial mass, in accordance with various embodiments described herein; 
           [0051]      FIG. 11  shows a perspective view of an exemplary elongated version of the coil and gap, driving an elongated magnet and mass, in accordance with various embodiments described herein; 
           [0052]      FIG. 12  shows a cross-sectional view of an exemplary housing with elements to guide the lateral translation of the mass and magnets as they are driven by the coil(s) at one end, in accordance with various embodiments described herein; 
           [0053]      FIG. 13A  shows a perspective view of an exemplary vibration module, in accordance with various embodiments described herein; 
           [0054]      FIG. 13B  shows an exploded view of the module of  FIG. 13A  illustrating the suspension and attachment to the housing, in accordance with various embodiments described herein; 
           [0055]      FIG. 14A  shows a perspective view of an exemplary headphone ear cup with retaining features for a vibration module, in accordance with various embodiments described herein; 
           [0056]      FIG. 14B  shows an exploded view of the headphone ear cup of  FIG. 12A , in accordance with various embodiments described herein; 
           [0057]      FIG. 14C  shows a perspective view of a user wearing headphones including the headphone ear cup of  FIG. 12A , in accordance with various embodiments described herein; 
           [0058]      FIG. 15A  shows a perspective view of a user wearing an exemplary headphone with multiple vibrating cushions situated on the headphone, in accordance with various embodiments described herein; 
           [0059]      FIG. 15B  shows a cut-away cross-sectional view of a portion of the headphone of  FIG. 15A , in accordance with various embodiments described herein; 
           [0060]      FIG. 16A  shows a perspective view of a user wearing an exemplary headphone with armatures that position vibrating elements, in accordance with various embodiments described herein; 
           [0061]      FIG. 16B  shows an exploded view of the armatures of  FIG. 16A  illustrating degrees of freedom afforded by an example of an armature, in accordance with various embodiments described herein; 
           [0062]      FIG. 16C  shows an exploded view of an exemplary positioner with vibration element and electrode, in accordance with various embodiments described herein; and 
           [0063]      FIG. 17  shows a perspective view of another exemplary positioner, in accordance with various embodiments described herein. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0064]    Various embodiments for providing damped electromagnetically actuated planar motion for audio-frequency vibrations are disclosed herein. The force output across a frequency range of a tactile transducer used for this purpose is limited by the space available for moving the internal mass and the peak force of the actuator causing the movement.  FIG. 3A  shows chart  30  illustrating these two physical bounds on the force output of an electromagnetic vibration module arising from limitations on the space available for translating the mass and from the limited force output of the coil. For an electromagnetic actuator, these limits may be termed travel limit  31  and coil limit  32 , respectively. If the system is not underdamped, the output of the transducer can be described by a curve in region  33 , below these limits  31  and  32 . 
         [0065]    The travel limit obeys the equation: 
         [0000]        F   max   =mx   max (2π f ) 2   (1)
 
         [0000]    where: 
         [0066]    F max =[N], maximum force 
         [0067]    x max =[m], space in package available for displacement 
         [0068]    m=[kg], mass in motion 
         [0069]    f=[Hz], frequency 
         [0070]      FIG. 3B  shows an exemplary vibration module  300  obeying the constraints illustrated in  FIG. 3A , in accordance with some embodiments. In particular,  FIG. 3B  illustrates how travel limit  31  and coil limit  32  apply to embodiments of the present invention, which may generally include moving mass  304 , oppositely polarized magnets  302   a  and  302   b  (collectively oppositely polarized magnets  302 ), coil  307 , flux guides,  308 , and housing  305 . 
         [0071]    In one particular example, travel limit  31  for vibration module  300  may be calculated for moving mass  304  having a mass of 0.015 kg that can undergo a maximum displacement of ±0.002 m (x max ) before contacting the wall of housing  305 . In this example the product of mass and available displacement are (0.015 kg)·(0.002 m)=3E-5 kg·m. To maximize force, the product of mass and available travel should be maximized. The higher the frequency of interest, the greater the acceleration that is possible, up to some limit imposed by the actuator. For an electromagnetic actuator, this coil limit  32  typically reflects the maximum current I that can be put through the copper windings. There are also an instantaneous limit associated with the power supply and a longer term limit—typically seconds to minutes—associated with overheating the coil. In some embodiments, the mass times the displacement may be, for example, 1×10 −5  kg-m or greater. 
         [0072]      FIG. 3C  illustrates the parameters that affect the coil limit. In particular, oppositely polarized magnets  302  produce a magnetic field B transecting coil  307  formed from a wire of length l. The Lorentz force F arising from the current transecting the magnetic field is: 
         [0000]        F   max   =i   max   ∫d{right arrow over (l)}×{right arrow over (B)}   (2)
 
         [0000]    where: 
         [0073]    F max =[N], maximum force 
         [0074]    i max =[Amp], current limit of supply, or thermal limit 
         [0075]    l=[m], wire length 
         [0076]    B=[Tesla], magnetic field strength 
         [0077]    Force output may be maximized by arranging coil  308 , magnets  302 , and flux guides  308  to steer maximum magnetic flux B through coil  307  cross-section carrying current I, and to provide a low-resistance path for heat out of the coil so that current I max  does not produce an unacceptable temperature rise. For illustration, a practical coil limit of 1 N Force is assumed in  FIG. 3A . Together the travel limit and coil limit define the maximum steady-state force output of a critically damped transducer. 
         [0078]      FIGS. 4A and 4B  show, respectively, a perspective and exploded view of an exemplary damped planar electromagnetic vibration module (vibration module  400 ), in accordance with various embodiments described herein. In some embodiments, vibration module  400  may be generally flat or planar so that it can easily be incorporated into the ear cup of a headphone, and provide a reciprocating force along axis  401  orthogonal to the thinnest dimension of the vibration module. 
         [0079]    As shown in  FIG. 4B , a pair of oppositely polarized magnets  402  can be held by a retainer  403  in a pocket or depression formed in mass  404 , which may be suspended on flexures  406  within a frame or housing  405 . Flexures  406  provide for movement of inertial mass  404  and magnets  402  along axis  401 , which may be orthogonal to the thinnest dimension of the vibration module. Lateral forces can be imparted to magnets  402  by virtue of a Lorentz force generated by passing current through an coil  407 , which is depicted in  FIG. 4B  as an elongated coil of conductive wire. Upper flux guide  408 , which may be a piece of iron, or other suitable ferromagnetic material, adhered to or otherwise placed in close proximity to coil  407 , can guide the magnetic flux and act as a heat sink and means of retaining coil  407  in place within housing  405 . For example, magnetic flux guide  408  can retain coil  407  in slot  409  formed in top plate  405   a  of housing  405  so that coil  407  is fixed with respect to frame  405 . In some embodiments, a portion of the housing (e.g. top plate  405   a  in the embodiment depicted in  FIG. 4 ) supporting the coil (e.g. coil  407 ) can be a printed circuit board with components to provide low-pass filtering of an audio signal and/or power amplification for driving the coil. 
         [0080]    In some embodiments, movement of the mass  404  and magnets  402  may be damped by thin layer of viscous ferrofluid  410  retained in a gap between the magnets  402  and bottom plate  405   b  of housing  405 . An additional lower magnetic flux guide  408   b  may be provided to counterbalance the attractive force drawing magnets  402  toward upper flux guide  408   a . Current may be routed to coil  407  using conductive leads  407   a . In some embodiments conductive leads  407   a  may be soldered to solder pads  405   aa  formed on an accessible surface of housing  405  (e.g. a top surface of top plate  405   a  as shown in  FIG. 4B  or any other outer surface). Leads from a power source (not shown) may also be attached to solder pads  405   aa  in order to electrically couple the power source to coil  407 . 
         [0081]      FIG. 5A  shows an exploded view of exemplary headphone set  50  illustrating the orientation of vibration module  500  in the ear cup, in accordance with various embodiments described herein. Vibration module  500  is depicted mounted so as to occupy relatively little of the thickness of ear cup  51  and to provide a reciprocating force in an axis  501  substantially orthogonal to the thinnest dimension of the vibration module. Vibration module  500  can be situated behind audio driver  52  and sound baffle  53 , which may be mounted on the headphone bow  54 . Providing vibration modules that generate damped electromagnetically actuated planar motion for audio-frequency vibrations can advantageously speed a user&#39;s reaction time by adding tactile sensations to audio provided by the headphone set. The vibrations can also help to preserve the user&#39;s hearing by lowering the user&#39;s preferred acoustic listening level. 
         [0082]      FIG. 5B  shows a perspective view of a user wearing the headphone of  FIG. 5A  and illustrates how the motion axis lies parallel to the side of the user&#39;s head, in accordance with various embodiments described herein. As shown in  FIG. 5B , a time-varying voltage can produce forces and accelerations in a plane parallel to the side of the headphone wearer&#39;s head along axis  501  labeled “x,” though one skilled would appreciate that the forces and accelerations directed along a different axis, such as the axis labeled “y,” for example, lying substantially in the same plane, may also be suitable for providing skin tractions that are perceptible as vibration while producing minimal excess sound. 
         [0083]      FIG. 5C  shows a chart  50   c  of experimental results of the measured acceleration of the exemplary headphone of  FIG. 5A , in accordance with various embodiments described herein. In particular, chart  50   c  demonstrates that the measured acceleration of the ear cup along axis  501  is substantially uniform over the range 40-200 Hz. To characterize the frequency response, sinusoidal voltage (V vibrate ) ranging from 20 to 200 Hz was applied to one of the conductive leads  55  attached to the coil of vibration module  500  while the other lead was held at ground potential (GND) as shown in  FIG. 5A . 
         [0084]    Below approximately 40 Hz, in sub-resonance frequencies  502 , the output of vibration module  500  is constrained by the “travel limit” (e.g. travel limit  31  of  FIG. 3A ) because as voltage is increased, the mass (e.g. mass  304  of  FIG. 3B ) travels farther, and increasing the voltage too high results in the travel exceeding x max  and causes the mass to come into contact with the frame (e.g. housing  305  of  FIG. 3B ), producing an undesirable acoustic knocking sound. Above approximately 40 Hz, the system response is constrained by the “coil limit” (e.g. coil limit  32  of  FIG. 3A ) where increasing the voltage eventually produced an undesirable increase in coil temperature. The viscosity and volume of the damping fluid (e.g. viscous ferrofluid  410  of  FIG. 4B ) in vibration module  500  were adjusted to damp resonance that would be evident at 30-50 Hz, to achieve the relatively uniform, non-peaked, response evident in  FIG. 5C  between 40 and 200 Hz in range  503 . The absence of resonant peak in the response makes it possible to reproduce the tactile component of a musical experience with previously unattainable high fidelity. 
         [0085]    It will be evident to one skilled in the art that the embodiment of the vibration module presented in  FIGS. 3A-4B  is a particular, non-limiting example, meant merely to illustrate an exemplary vibration module that could be employed in accordance with various embodiments of the present invention. Additional exemplary vibration module embodiments will now be presented, each of which may be configured to produce appropriately oriented motion in a headphone as shown in  FIGS. 5A-5C . 
         [0086]      FIG. 6A  shows an exploded view of vibration module  600 , in accordance with various embodiments described herein. Vibration module  600  is substantially similar to vibration module  400 , except that it is equipped with an alternative suspension system for accurately locating and spacing the suspended mass within the housing. In particular, vibration module  600  includes mass  604  to which flexures  606  are bonded on opposite ends, so as to suspend the mass within housing  605 . Flexures  606  engage holes  605   ab  and  605   bb  in top plate  605   a  and bottom plate  605   b , respectively. The pocket in the mass  604  may be equipped with bottom  608 , embodied in  FIG. 6A  as a thin plate bonded to the mass. The magnet pair and portions of the housing are omitted in this instance for clarity. 
         [0087]      FIG. 6B  shows a detailed perspective view of a portion of flexure  606 , in accordance with various embodiments described herein. Flexure  606  may include projecting tabs  606   a  that engage holes  605   ab  and  605   bb  in the top and bottom plates, to provide alignment of the plates and set the size of the gap between them. Flexures  606  may also have shoulders  606   b  that provide clearance for flexing member  606   c  to prevent contact between of the flexing member  606   c  and top plate  605   a  and bottom plate  605   b  as mass  604  travels within housing  605 . 
         [0088]      FIG. 7  shows a perspective view of a portion of exemplary vibration module  700 , in accordance with various embodiments described herein. Vibration module  700  includes oppositely polarized magnets  702  coupled to (e.g. affixed with an adhesive to) suspension base member  711 . Flexures  706  may be formed integrally with or otherwise coupled to suspension base member  711 . Mass  704  may be arranged and coupled to suspension base member  711  (e.g. at an end of the suspension base member  711  opposite magnets  702 ). In some embodiments, mass  704  may be or include a battery for powering vibration module  700 . The portion of vibration module  700  depicted in  FIG. 7  may be enclosed in a housing, not shown (e.g. housing  405  of  FIG. 4 ). 
         [0089]      FIG. 8A  shows an exploded view of exemplary torsional vibration module  800 , in accordance with various embodiments described herein. Vibration module  800  is a rotational analog of the linearly traveling vibration module examples disclosed thus far. As shown in  FIG. 8A  two pairs of oppositely polarized magnets  802  and two inertial masses  804  may be coupled to a disk  812  suspended on flexures  806  that allow torsional rotation of the disk about central hub  812   a . The ends of the hub may be coupled to front housing member  805   a  and back housing member  805   b . Coils  807  can be retained in slots of front housing member  805   a  and either coupled to or brought into close proximity to magnetic flux guides  808   a . Magnetic flux guides  808   b  may also be provided on back housing member  805   b.    
         [0090]      FIG. 8B  shows a schematic view of torsional vibration module  800  illustrating the action of flexures  806 , in accordance with various embodiments described herein. In particular,  FIG. 8B  illustrates the action of flexures  806  as they deflect from an initially straight position  806 - 1  to a deflected position  806 - 2  as disk  812  rotates about hub  812   a.    
         [0091]      FIG. 8C  shows a perspective view of a user wearing headphone set  80  incorporating torsional vibration module  800  and illustrates exemplary rotational motion in a plane parallel to the side of the user&#39;s head, in accordance with various embodiments described herein. Rotation of the masses on central disk  812  produces counter rotation of the ear cup about the axis of the hub along rotational path  801  labeled “θ.” The motion lies in the plane parallel to the side of the user&#39;s head, producing skin tractions perceptible as vibration, without causing a change to the volume of air inside the ear cup, thus minimizing unwanted sound. This particular embodiment of the rotational system has twice the number of coils and magnets of the linear systems illustrated previously, but produces the same general effect. Accordingly, one skilled in the art may appreciate that any number (N=1,2,3 . . . ) of actuator elements can provide equivalent or similar results. Likewise, it should be apparent to one skilled in the particular shape of the sectors housing masses  804  and magnets  802  may be varied, such that other shapes, such as half-circular sectors, can perform in an equivalent or similar manner to the explicitly disclosed embodiments. 
         [0092]    Thus far, several rigid embodiments in accordance with the present invention have been disclosed. However, compliant constructions suitable for direct skin contact are also contemplated as falling within the scope of the invention.  FIG. 9A  shows an exploded view of exemplary compliant vibration module  900 , in accordance with various embodiments described herein. Vibration module  900  can include a planar pair of oppositely polarized magnets  902  embedded in a compliant puck  904  supported on a layer of ferrofluid  911 , where both puck  904  and ferrofluid  911  are trapped between two impermeable elastic membranes. The compliant materials used in formation of compliant vibration module  900  may have an elastic modulus of less than 50 MegaPascal. 
         [0093]    Lower membrane  905   b  provides a stationary platform for movement, whereas the upper membrane  905   a  moves with the puck  904  and may optionally be corrugated to easily afford lateral movement of puck  904 . The upper and lower membranes may be sealed at the circumference, for example by a heat sealing process for thermoplastic elastomers, by adhesive or solvent bonding, or any other suitable bonding method. As before, the magnets are urged laterally by current passed through coil  907 . In this embodiment, the coil  907  can be enclosed in a compliant stage  905   c  so as to provide a supporting stage for movement of the puck  904 . 
         [0094]    Applying time-varying signals to lead  907   a  of coil  907  with respect to lead  907   b  produces time-varying forces on the puck  904 , and corresponding lateral accelerations of upper membrane  905   b  coupled to it. Upper membrane  905   b , in turn, may be placed in direct contact with the wearer&#39;s skin or may be integrated with the cushion fabric in contact with a wearer&#39;s skin. 
         [0095]      FIG. 9B  shows a cross-sectional view of compliant vibration module  900 , in accordance with various embodiments described herein. As illustrated in  FIG. 9B , current I flows through coil  907 , urging magnets  902  laterally. Relative movement between the compliant upper membrane  905   a  and stage  905   c  is facilitated by the ferrofluid layer  911 . The seal at the circumference of vibration module  900  is evident where the lower membrane  905   b  contacts upper membrane  905   a.    
         [0096]    Although examples so far have focused on vibration modules incorporating planar pairs of magnets, embodiments of the present invention are also contemplated having alternative arrangements between magnet and coil. Several exemplary embodiments are shown in  FIGS. 10A-13 . 
         [0097]      FIG. 10A  shows an illustrative two-dimensional finite element analysis of coil  1007  carrying a current in the magnetic gap formed by a single magnet  1002  and flux guides  1008 , in accordance with various embodiments described herein. Magnet  1002  has magnetic flux that is guided by magnetic flux guides  1008  through an air gap in which coil  1007  carries current I. The generated Lorentz force urges coil  1007  in direction  1001   a  and the rest of the components illustrated in  FIG. 10  in direction  1001   b , opposing direction  1001   a.    
         [0098]      FIG. 10B  shows a perspective view of exemplary vibration module  1000 , in accordance with various embodiments described herein. Vibration module  100  can include multiple drivers including cylindrical coils in circular magnetic gaps driving magnets coupled to an inertial mass  1004 . In some embodiments, one or more of these drivers may be situated along one edge of mass  1004 , so that applying time varying voltage to coils  1007  generates Lorentz force on the magnets  1002  and flux guides  1008  and thereby urges mass  1004  to move along axis  1001  lying substantially in the plane of the vibration module. If coils  1007  are fixed to a housing (omitted for visual clarity) the magnets, flux guide, and inertial mass translate with respect to the housing. 
         [0099]      FIG. 11  shows a perspective view of exemplary vibration module  1100  having a coil and gap structure that is integral and elongated with respect to the coil and gap structures of vibration module  1000 , driving an elongated magnet  1102  and mass  1104 , in accordance with various embodiments described herein. The resulting geometry uses an elongated oval coil  1107  arranged in the air gap of an elongated flux guide  1108 . As with the previously disclosed embodiments, time-varying voltage sweeping current through coil  1107  urges the magnet, flux-guide, and inertial mass laterally along an axis  1101  in the plane of the module. 
         [0100]      FIG. 12  shows a cross-sectional view of an exemplary housing  1205  with elements  1206  that guide the lateral translation of mass  1204  and magnets  1202  as they are driven by the coil(s)  1207  at one end, in accordance with various embodiments described herein. Housing  1205  may be a suitable housing for vibration modules  1000  and  1100  illustrated in  FIGS. 10B and 11 . Coil  1207  may be fixed to a wall of housing  1205 . When current is passed through coil  1207 , magnet  1202 , flux guide  1208 , and inertial mass  1204  are urged laterally along axis  1201  that lies in the plane of the vibration module. In this embodiment, the movement of inertial mass  1204  can, for example, be guided by linear glides  1206  rather than flexures. However, a person of skill in the art would recognize that a variety of suspensions lie within the scope of the present invention, and that comparable results may be achieved with flexures, a ferrofluid, bushings, and even ball bearings provided that they are pre-loaded and packed with viscous grease so as not to rattle audibly when reciprocated at frequencies in the 20-200 Hz range. 
         [0101]      FIG. 13A  shows a perspective view of yet another exemplary vibration module  1300 , in accordance with various embodiments described herein. Vibration module  1300  includes thin magnet  1302  polarized along the thin axis. It operates in the center of a long coil with an oval cross section  1307 . The flat sides of the oval carry current I running transverse to the flux of magnet  1302 , and therefore generates a force perpendicular to both the current and the magnetic flux. That is, the Lorentz force urges magnet  1302  in a direction aligned with its long axis  1301 , and urges coil  1307  in the opposite direction. Magnetic flux guide  1308  provided concentrically outside coil  1307  can improve orientation of the magnetic flux. Bracket  1303  can couple movement of magnet  1302  to inertial mass  1304 . 
         [0102]      FIG. 13B  shows an exploded view of vibration module  1300  illustrating an exemplary suspension and attachment to housing  1305 , in accordance with various embodiments described herein. Flexures  1306  can be attached to inertial mass  1304  so that inertial mass  1304  may move with respect to housing  1305 . In some embodiments, housing may be provided with mating surface  1305   a  that may be coupled to magnetic flux guide  1308  provided around coil  1307  so that coil  1307  is fixed with respect to the housing. A second bracket  1303   b  for translating the motion of magnet  1302  to inertial mass  1304  is shown. Also shown is the axis of motion  1301  of inertial mass  1304 . 
         [0103]      FIG. 14A  shows a perspective view of an exemplary headphone ear cup  141  with retaining features  142  for holding a vibration module, in accordance with various embodiments described herein. Although clips are depicted in  FIG. 14A , other suitable retaining features, such as adhesives and fasteners, for example, may be substituted. 
         [0104]      FIG. 14B  shows an exploded view of the headphone ear cup  141 , in accordance with various embodiments described herein. In particular,  FIG. 14B  depicts an embodiments of the present invention in which movement of the inertial mass is visible through a wall of headphone ear cup  141 . In this embodiment, the back plate  1405  of the vibration module  1400 , is formed from a transparent material, such as glass or transparent plastic, for example, and headphone ear cup  141  is provided transparent window  141   a . Together, back plate  1405  and transparent window  141   a  afford a view of the moving inertial mass  1404   
         [0105]      FIG. 14C  shows a perspective view of a user wearing headphone set  140 , including headphone ear cup  141 , in accordance with various embodiments described herein. As shown in  FIG. 14C  the edges of window  141   a , on which a visual design  141   b  is optionally displayed, the movement of inertial mass  1404  and/or other components of vibration module  1400  are visible. That is, a viewer may be provided a clear optical path so that vibration of vibration module  1400  within ear cup  141  is visible when the vibration module is worn on a user&#39;s head. 
         [0106]      FIG. 15A  shows a perspective view of a user wearing an exemplary headphone set  150  with multiple vibrating cushions  152 , in accordance with various embodiments described herein. In particular, vibrating cushions  152  are provided on headphone bow  153  to produce tangential tractions on the wearer&#39;s skin at multiple locations. 
         [0107]      FIG. 15B  shows a cut-away cross-sectional view of a portion of headphone set  150 , in accordance with various embodiments described herein.  FIG. 15B  illustrates headphone bow  153  and a compliant vibration module  1500 , which may be similar to compliant vibration module  900  of  FIGS. 9A and 9B , embedded in a cushion formed from foam member  154  and cover  155 . The cushion may be attached (e.g. with adhesive  156 ) to headphone bow  153 . In this embodiment, the movement of the compliant puck within vibration module  1500  causes shear movement  1501  of the cushion cover where it rests on the wearer&#39;s skin or hair. 
         [0108]      FIG. 16A  shows a perspective view of a user wearing an exemplary headphone set  160  with armatures  166  that position vibrating elements  162 , in accordance with various embodiments described herein. As shown in  FIG. 16A , one or more positioners  166  may be provided to adjust the locations of the vibrating elements  162  with respect to headphone bow  164  and ear cup  161 , so as to provide vibrations at various locations on the wearer&#39;s skin. 
         [0109]      FIG. 16B  shows an exploded view of armatures  166  of  FIG. 16A  illustrating degrees of freedom afforded by an example of an armature, in accordance with various embodiments described herein. Here, a vibrating element, such as compliant vibration module  1600  (which may be similar or identical to compliant vibration module  900  of  FIGS. 9A and 9B ), is positioned so as to impose tangential shear tractions on the wearer&#39;s skin. The vibration axis may be chosen to lie primarily parallel to the user&#39;s sagittal plane (that is parallel with, but not necessarily coincident with the side of the user&#39;s head), to minimize unwanted movement toward and away from the user&#39;s ear, to minimize unwanted sound. 
         [0110]    As further shown in  FIG. 16B , armature  166  may provide a surface  166   a  that supports vibration module  1600  and also affords lateral movement  1601  over the surface of the user&#39;s skin, for example by rotation  1601   a  about a rotational degree of freedom provided by a pivoting base  166   b . Armature  166  can also provide rotation  1601   b  about a second degree of freedom by virtue of a hinged connection  166   c  between armature  166  and armature base  166   b  that allows movement that accommodates the variable height of the user&#39;s skin with respect to the positioner base  16   b  where it connects to headphone  160 . 
         [0111]      FIG. 16C  shows an exploded view of positioner  166 , in accordance with various embodiments described herein. In particular,  FIG. 16C  illustrates how electrical leads for the vibrating element  1600  may be routed through it, and how it affords a mounting point and electrical connection  166   d  for an optional skin-contact electrode  166   e . Electrode  166   e  may, through an independent electrical lead  166   f  source or sink current independent of any time-varying voltage applied to the lead  166   g  of the vibrating element. 
         [0112]    The skin contact electrode thereby provides a means of stimulating the wearer, for example to provide transcranial direct current stimulation. Because vibration masks pain, the pain commonly associated with electrical stimulation through the skin can be avoided. The electrode can also provide a one or more sensors for recording electrical potentials on the surface of the wearer&#39;s body, for example signals arising from the wearer&#39;s electroencephalogram, indicating brain activity, or the electrooculogram, indicating eye orientation, or the wearer&#39;s electromyogram indicating contraction of the facial muscles, the conductivity of the user&#39;s skin, indicating sweating, or any other electrical potentials on the surface of the wearer&#39;s body. 
         [0113]      FIG. 17  shows a perspective view of another exemplary positioner  176 , in accordance with various embodiments described herein. Positioner  176  can have an extensional degree of freedom  1701  that affords radial positioning of the skin contact point with respect to the positioner base  176   b . Additional flexibility is optionally imparted to the orientation of the skin contact point by elastic pillars  176   g  that join the support for the vibrating element to the positioner. It is clear to one skilled in the art that these various degrees of freedom in the positioner may be passive, spring loaded, or electromechanically actuated to provide a massaging motion by positioning a vibration module over a desired location on a user&#39;s body. 
         [0114]    It should be understood that the aspects, features and advantages made apparent from the foregoing are efficiently attained and, since certain changes may be made in the disclosed inventive embodiments without departing from the spirit and scope of the invention, it is intended that all matter contained herein shall be interpreted as illustrative and not in a limiting sense. 
         [0115]    It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention that, as a matter of language, might be said to fall there between.