Abstract:
A spider-less and suspension-less speaker system, having a diaphragm constrained to move in a linear direction and supported by a frictionless or low friction support. A voice coil coupled to an audio input drives the speaker diaphragm. A position sensor determines the physical position of the diaphragm. A control circuit uses the sensed position information to modify the audio signal in such a way that the interaction of the voice coil and the magnetic field both drives the diaphragm to produce an audible output and restores the diaphragm to a neutral position in the absence of a further audio signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the fields of audio loudspeakers, and more particularly to a speaker suspended without a “spider” or “surround,” wherein the motion of the speaker is controlled by an active restoring apparatus. 
     BACKGROUND OF THE INVENTION 
     A representative prior art audio speaker  10  is illustrated in FIG.  1 . Most speakers, like speaker  10 , comprise a diaphragm  12  driven by a voice coil  14 . The voice coil  14  is mounted in a magnetic field created by magnet  22 . The magnet  22  is supported on a back plate  18  and has a top plate  16  and a pole piece  20  forming a magnetic assembly  23 . The diaphragm  12  is supported in such a way that it is free to vibrate in a linear direction, restrained by a spring apparatus. Usually the spring apparatus takes the form of a surround  24  and a spider  26 . When an audio signal is fed to the coil  14 , the diaphragm  12  moves away from a neutral position. The spider  26  and surround  24  act both to constrain speaker to a linear motion and to provide a restoring force, returning the diaphragm  12  to the neutral position in the absence of a displacing force provided by the coil  14 . 
     The spring action in the spider and surround are potentially distorting to sound reproduction, particularly where wide ranges of frequencies or amplitudes are expected. The spider and surround usually act as linear springs over most of their range of motion, coming to a relatively abrupt stop at maximum extension in either the forward or backward direction. In certain situations, particularly with high audio power and at low bass frequencies, the speaker may be driven beyond its physical constraints, striking a hard constraint or straining against the suspension at its limit, a phenomenon referred to as “bottoming.” Bottoming can introduce serious distortion and risk physical damage or deterioration of the speaker. 
     It is an object therefore, of our invention, to provide a speaker system without a spring-type suspension. 
     It is also an object of our invention to provide a speaker with an active restoring apparatus. 
     Another feature or object of our invention is to provide a speaker with a frictionless bearing, that is, negligible friction, constraint, the constraint limiting direction travel of the diaphragm to a linear direction. 
     SUMMARY OF THE INVENTION 
     The objects of our invention have been accomplished by providing a spider-less and suspension-less speaker system, having a diaphragm constrained to move in a linear direction and supported by a frictionless or low friction support. A voice coil coupled to an audio input drives the speaker diaphragm. A position sensor determines the physical position of the diaphragm. A control circuit uses the sensed position information to modify the audio signal in such a way that the interaction of the voice coil and the magnetic field both drives the diaphragm to produce an audible output and restores the diaphragm to a neutral position in the absence of a further audio signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further objects, features and advantages of the present invention will be more fully understood from the following description, taken with reference to the accompanying drawings. 
     FIG. 1 is a cross-sectional plan view of a prior art speaker. 
     FIG. 2 is a cross-sectional plan view of a speaker according to our invention. 
     FIG. 3 is a schematic diagram of a first control circuit. 
     FIG. 4 is a schematic diagram of a second control circuit. 
     FIG. 5 is a schematic of a third control circuit. 
     FIG. 6 is an additional embodiment of the speaker of our invention with an alternative position sensor and support. 
     FIG. 7 is an another embodiment of the speaker of our invention with an alternative position sensor in an alternative support. 
     FIG. 8 is a proportional prospective view of one of the supports of FIG.  7 . 
     FIG. 9 is a series of perspective through sections of speaker diaphragms for use with the speaker of our invention. 
     FIG. 10 is an exploded perspective view of a planar speaker with multiple drivers. 
    
    
     DETAILED DESCRIPTION 
     We will now describe our invention by reference to the drawings. Like numerals are used to designate like parts in all the drawings. 
     FIG. 2 is a cross-sectional representation of an audio speaker  30  according to our invention. The audio speaker  30  comprises a diaphragm  32  supported in a frame  34  by sliding frictionless or low friction bearings  36 . Frictionless bearings  36  constrain the diaphragm to motion in a linear direction. For this purpose, a low friction -inner surface  38  may be provided on the frame  34 . Contacts  40  abut the surface  38 . The contacts  40  may be a block comprised of a suitable material, such as high-density polyurethane or may have a Teflon (trademark) coated surface or other suitable sliding surface. Alternatively, roller or other bearings configured to contact the surface  38  may be used. The bearings  36  may have the capacity to resist torque so that the diaphragm  32  will remain perpendicular to the selected linear direction of motion. 
     The diaphragm  32  is connected to a voice coil  42  which lies generally within a magnetic field provided by magnet assembly  44 . Magnets  45  and a pole piece  46  are supported on a base  48  of the frame  34 . Preferably underhung construction is used. See, for example, U.S. Pat. No. 5,408,533, the disclosure of which is incorporated herein by reference. The voice coil  42  is connected to an audio signal as illustrated in FIGS. 3 through 5. FIG. 2 illustrates a position sensor  50 . The position sensor  50  may be any suitable sensor capable of determining the relative position of the diaphragm  32  to a reference position. In FIG. 2, an optical sensor  52  is illustrated. The optical sensor  52  is connected by electrical lead  54  to control circuitry to be described in connection with FIGS. 3 through 5. Surface  56  on or near the voice coil  42  either reflects light from the optical sensor  52  or provides its own light source which can be sensed by the optical sensor  52 , whereby the sensor  52  is able to determine the relative location of the diaphragm and to provide a signal representative thereof. 
     The motion of the diaphragm  32  in the selected linear direction is controlled by a control circuit  60 . The position sensor  52  is coupled to the diaphragm  32  and driven by the voice coil  42 . An audio signal from an audio source  62  is directed through the voice coil  42 . Displacement of the diaphragm  32  from the normal position is sensed through the position sensor  52  and, in the embodiment of FIG. 3, is directed through a feedback junction  64  to provide negative feedback. An amplifier or buffer  66  may further be provided between the feedback loop  69  and the coil  42 . The feedback loop  69  substantially modifies the input signal before that signal is fed to the coil  42  so that the magnetic field provides not only the driving force but also the restoring force bringing the diaphragm back to a neutral position. 
     Additional signal processing may also be provided to more fully condition the signal being delivered to the coil  42 . A variable differential amplifier  68  connected in the feedback loop may be used to provide a variable tightness, that is, the magnitude of restoring force added into the signal to return the diaphragm quickly or slowly to the neutral position. By analogy, changing the “tightness” is like varying the spring constant, of the spider or surround, whereby a more or less responsive speaker may be produced to match the quality of the audio signal. In standard speakers, of course, the physical characteristics of the surround or spider cannot be so altered. Additional signal modification may be provided by inserting a feed forward circuit  70  between the feedback connections  64  and coil  42 . The feed forward circuit  70  may be implemented as a digital signal processor using the known mass of the diaphragm, available linear displacement distance, and any frictional losses to calculate a restoring force necessary to counteract the driving force being produced in response to the audio signal. The delay in applying the restoring force is a measure of the responsiveness of the speaker. In contrast to a speaker supported by a spider and surround, the responsiveness can be adjusted to provide varying speaker response. The signal may be critically damped, over damped or under damped, according to taste of a user. Thus the characteristics of the speaker can be controlled, for example, to suit the type of music to be played through the speaker. 
     Between the variable differential amplifier  68  and the feedback connections  64 , a damping circuit  72  may be provided to prevent or restrict overshoot or undershoot of the diaphragm  32 . The damping circuit  72  corrects the motion of the diaphragm  32  for excessive errors, that is, for motion which would fall outside predetermined limits. The circuit  72  may be implemented digitally and may be used without the variable differential amplifier  68 . 
     An alternative control circuit  60 ′ is illustrated in FIG.  4 . In the control circuit  60 ′ of FIG. 4, the feedback junction  64  has been replaced by a differential amplifier  74 . Yet another embodiment of a control circuit  60 ″ is illustrated in FIG.  5 . In this instance, a digital feed forward circuit  76 , similar to the circuit  70  of FIG. 3, performs the function of signal modification. In the digital feed forward circuit  76 , damping feedback and variable tightness may be provided by setting selected parameters, as mentioned above. 
     FIG. 6 illustrates an alternative embodiment of our invention, having endless loop bearings  80  to provide the frictionless or low friction connection between the frame  34  and the diaphragm  32 . The endless loop bearings  80  are elastomeric tube segments, the tube segments having a circumference and a length. The length is perpendicular to the direction of linear translation of the diaphragm  32 . Each of the endless loop bearings  80 ,  82  have an outer surface and are connected to the diaphragm. The frame  34  has a surface  38  parallel to the direction of linear translation of the diaphragm  32 . The outer surface of the endless loop bearing  80 ,  82  is in rolling contact with the surface  38  of the frame  34 . The endless loop bearings may be connected lengthwise at a first line of contact  88 ,  90  to the frame and may also be connected lengthwise at a second line of contact  84 ,  86  to said diaphragm. The second line of contact is spaced apart from the first line of contact. Preferably, the second line of contact  84 ,  86  is equidistant around the circumference from the first line of contact  88 ,  90 . We prefer to provide a plurality of endless loop bearings, spaced around the diaphragm, and more preferably at least three loop bearings spaced around the diaphragm to stabilize the diaphragm and allow motion only in a selected linear direction. In the embodiment of FIG. 6 an alternative position sensor is also shown. A miniature accelerometer  92  is mounted on or near the diaphragm so as to move with the diaphragm. Piezoelectric accelerometers are known. See, e.g. U.S. Pat. No. 4,727,584 and U.S Pat. No. 5,014,703. The output of the accelerometer is directly related to acceleration, rather than position as such. The output must, therefore, be integrated twice to give position information. 
     Yet another embodiment of our invention is illustrated in FIG.  7  and FIG.  8 . In this embodiment, endless loop bearings  94 ,  96  comprise an elastomeric tube segment  108  (see FIG. 8) having an inner surface  110 , an outer surface  112 , a circumference and a length  113  perpendicular to the direction of linear translation of the diaphragm. There is a plurality of rectangular prisms  114  on the outer surface. Each of the rectangular prisms  114  has a rectangular base  116  and a height  118  perpendicular to the base. Preferably, the height is parallel to the length of the tube  108 . On the inner surface  110  of the tube  108  there are trapezoidal prisms  120 . Each of the trapezoidal prisms  120  have a trapezoidal ase  122  and a height  124  perpendicular to the base  122  and preferably parallel to the length  113  of the tube  108 . A selected rectangular prism  102  (see FIG. 7) of the rectangular prisms  114  is connected to the diaphragm  32 , but the endless bearings  94 ,  96  are not connected to the frame  34 . Rather the bearings oscillate in a cavity formed by the inner surface  38  of the frame and a first flange  98  and a second flange  100 . Adjacent the flanges, the bearing folds back on itself as seen in FIG.  8 . The trapezoidal prisms come together while the rectangular prisms spread apart. Between the flanges, the rectangular prisms lie adjacent each other and keep the bearing relatively straight. FIG. 7 also illustrates an alternative position sensor, namely, a linear variable displacement transducer  103 . The transducer  103  comprises a plunger  104  and a coil  106 . The relative position of the plunger within the coil  106  changes the mutual reactance of the coil in a manner directly related to the position of the plunger. Since the plunger  104  is coupled to the diaphragm, the position of the diaphragm is also known. A linear variable displacement transducer is available from Macro Sensors, Inc., for example. 
     Standard audio speakers, such as the speaker  10  of FIG. 1, often have conical diaphragms to resist the spring action of the surround or spider without physically distorting the diaphragm. Since the speaker of our invention does not have a spring-type support, alternative shapes for the diaphragm can more easily be used. These shapes can cause the audio wave front produced by the speaker to more closely represent a point source of sound, thus further reducing distortion. For example, instead of a conical diaphragm, a planar diaphragm  130 , as shown in through section in FIG. 9, might be used. Alternatively, a domed diaphragm  132 , convex in a direction of maximum sound propagation, could be used. In such a diaphragm, the center of curvature of the diaphragm would be the apparent point source of the sound. A similar effect, but with a flatter profile for the diaphragm, might be achieved by a diaphragm  134  having the configuration of a Fresnel lens. Other diaphragm configurations could, of course also be used with the speaker of our invention. 
     Our invention may be particularly useful for large speakers. In large speakers, the spider or surround tends to distort the shape of the speaker, that is, there is more motion near the attachment point for the speaker diver and less motion at the edges, near the spider or surround. Moreover, it is difficult to use multiple drivers in a single speaker, because the drivers are not exactly in phase with one another. Since the drivers of our invention are position-controlled, multiple in-phase drivers can be attached to a single speaker diaphragm. For example, an exploded perspective view of a multiple-driver speaker  140  is illustrated in FIG.  10 . The multiple driver speaker  140  comprises a larger diaphragm  142 . A planar diaphragm is illustrated but other configurations, as explained above, could also be used. The rectangular shape shown in FIG. 10, which may be on the order of a meter by two meters, may be replaced by other configurations, for example a polygon such as an octagon, or a circular or elliptical configuration. Around an edge  144  of the diaphragm  142  a plurality of bearings  146  are mounted. These may be endless loop bearings like those described above in connection with FIG. 6, FIG. 7 or FIG. 8. A frame  148  supports a plurality of tracks  150  for the bearings  146 . Side walls  152 ,  154  guide the bearings  146  to keep the motion of the diaphragm linear. End walls  156 ,  158  perform the function of the flanges  98 ,  100  described in connection with FIG. 7 above. 
     A plurality of drivers  160  are attached to the diaphragm. Each of the drivers  160  is of the type described above and has a displacement sensor, such as the optical sensor  52 , accelerometer  92  or linear variable displacement transducer  103 . Each is connected through a control circuit such as circuit  60 ,  60 ′ or  60 ″. Support brackets  162  in the frame  148  connect to the drivers  160  and provide mechanical support for the divers  160 . Because the drivers are position sensitive, they are in phase with each other and can be used to drive a very large diaphragm. Because the diaphragm is not connected to the frame by a spider or surround, the diaphragm is not distorted by the differing distances of the various drivers  160  from the edge  144  of the diaphragm  142 . 
     The foregoing examples of embodiments of our invention should be deemed exemplary only. Those skilled in the art will recognize that changes and modifications could be made in the design or construction without departing from the scope or teachings of our invention. It is intended, therefore, that the scope of our invention should be defined by the accompanying claims.