Abstract:
A curve fitted planar loudspeaker minimizes variation and dead zones in the sound field propagated by an array of the loudspeakers. The curve fitted loudspeaker provides a plurality of sound generating transducers. The diaphragm is curved and each adjacent section fits as a section into a larger curve, eliminating or minimizing breaks in the propagated sound filed at the target distance to the target.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to loudspeakers and more particularly to planar or ribbon type loudspeakers fitted to be used in a hanging array of loudspeakers. 
     2. Description of the Problem 
     Electro-dynamic planar loudspeakers are constructed using a thin resistive film diaphragm disposed taut over a thin, rectangular frame. Permanent magnets are mounted within the frame parallel with at least one, and sometimes both, of the major surfaces of the thin film diaphragm. In prior art designs, the bar magnets align with the axis of the direction of elongation of the frame. Typically one major surface of the thin film diaphragm carries electrically conductive traces which are etched from a layer of aluminum applied to the major surface. The conductive traces are positioned with respect to the permanent magnets so that electrical currents applied to the conductive traces result in forces being generated which move the diaphragm. The major surfaces of the diaphragm are enclosed between the magnets adjacent one major surface and, typically, a grate or sound lens adjacent the opposite major surface. 
     The introduction of a varying current to the electrically conductive traces causes motive force to be generated on the thin film conductors, and corresponding movement of the thin film diaphragm in the frame. The diaphragm and electrically conductive traces are of minimal mass to minimize energy expended moving the diaphragm and to optimize response times. A carefully calibrated and even tension should be applied to the diaphragm along its entire perimeter. Consistent spacing between the diaphragm and the magnets contributes to minimization of distortion in sound reproduction. 
     Planar type loudspeakers have a defined acoustical directivity pattern that has a natural application to vertically hung arrays. In such an array a long chain of speakers produces a sound field having broad area coverage. Such arrays are usually constructed to curve rearwardly at the bottom of the curve to produce a shallow J-shaped or fish hook profile. This arrangement orients the lowest loudspeakers in the chain to present their sound emitting surfaces directly toward the portion of the audience sitting below the array. A J-shaped curved array typically has an elongated upper portion which is substantially straight, from which sound is directed outwardly, and a lower, curved portion, from which sound is directed downwardly. 
     It is usually desirable that listeners anywhere in the covered area hear substantially the same sound. Achieving this result is very difficult in practice. One difficulty has been that the lower, curved portion of the J curve array has exhibited excessively large granularity. The term “granularity” is used to refer to the number and size of dead zones vertically disposed in a sound field. With conventional planar loudspeakers, the fewer the number of dead zones, the larger the dead zones tend to be. Fine granularity splits up dead zones to reduce their size below the threshold of human detection. The use of straight planar loudspeakers in the curved portions of a hanging array results in the appearance of obtrusive dead areas in the projected sound field. The lowest practical limit on the length of planar loudspeakers still results in J-curve arrays where the loudspeakers poorly fit the curve of the array. This results in substantial dead spots in the sound field. It is desirable to avoid use of short planar loudspeakers on account of their expense, power handling considerations and the poor low frequency response of very short planar devices. 
     SUMMARY OF THE INVENTION 
     The invention provides a planar loudspeaker. Each loudspeaker has front and back panels. The front and back panels each have front and back major surfaces. The front panel has an aperture connecting the panel&#39;s front and back major surfaces. The aperture allows sound energy to escape. A back panel has front and back major surfaces which are congruent to the front and back major surfaces of the front panel. An aperture connects the front and back major surfaces of the back panel. The front major surface of the back panel is disposed substantially abutting the back major surface of the front panel. A diaphragm is located between the front panel and the back panel and is aligned on the apertures, its major surfaces being parallel to the planes of the apertures. Vibration of the diaphragm produces sound which is emitted through the apertures. The front panel includes at least a first bridge dividing the aperture through the front panel. This bridge or brace is disposed in contact with a major surface of the diaphragm to divide the diaphragm into sections on its front major surface. A corresponding bridge is provided across the aperture on the back panel. Independent circuit traces are disposed on each section of the diaphragm for both major surfaces, corresponding to the sections of the diaphragm. A plurality of bar magnets, arranged in arrays, are disposed one array to each section of the apertures in the sections of the back and, usually, front apertures to complete the transducer assembly. 
     The planer loudspeaker is elongated from end to end and conforms locally to the curve of a J-hook vertical array of planar loudspeakers in which it is used. The front and back panels are shaped from end to end in their direction of elongation to conform to a curve. The bar magnets in each section being are oriented parallel to a bridge bordering the section. The bar magnets associated with the front panel, when used, being are narrower than those disposed in the sections of the aperture through the back section and are stepped with respect to one another to maintain spacing from the diaphragm. 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a side view of a stacked ribbon array of planar loudspeakers. 
         FIGS. 2A-C  are diagrams illustrating sound field coverage from ribbon arrays of loudspeakers using conventional loudspeakers and curve fitted planar loud speakers of the present invention. 
         FIG. 3  is an exploded view of a planar loudspeaker in accord with the invention. 
         FIG. 4  is a front view of view of a planar loudspeaker according to the invention. 
         FIG. 5  is a view of the planar loudspeaker with its front panel removed to expose a diaphragm set on the frame of the loudspeaker. 
         FIG. 6  is a cross sectional view taken along section lines  6 - 6  in  FIG. 4 . 
         FIG. 7  is a cross sectional view taken along section lines  7 - 7  in  FIG. 4 . 
         FIG. 8  is an illustration of a conductor trace pattern as used on diaphragms for planar loudspeakers of the invention. 
         FIG. 9  is a block diagram of a circuit for controlling segments of a planar loudspeaker element of the present array. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1  an array  10  of planar loudspeakers, assembled as and hung in a J-curve, is illustrated. Ribbon array  10  comprises both flat curve planar loudspeakers  12  used in a straight, long throw, portion  16  in the upper part of the ribbon array  10  and curve fitted planar loudspeakers  14  used in the medium and short throw portions  18 ,  20 , filling out the lower portion of the array. The planar loudspeakers  12 ,  14  are connected end to end in a strip to construct the array. Alternatively, a frame may be used to support the loudspeakers  12 ,  14  in such an end to end arrangement and relieve the loudspeaker units themselves of the need to be constructed to support the weight of the portion of array  10  suspended from a particular unit. Array  10  is typically used in an arena type setting, hanging from its upper end from the roof or rafters over the floor space of the arena. It is intended to fill efficiently the space of the arena with a sound field, with areas underneath array  10  being taken care of by the short throw section  20 , intermediate locations being covered by the medium throw section  18  and more distant areas covered by long throw section  16 . 
       FIGS. 2A-C  illustrate sound fields  23 ,  25 ,  27  produced by arrays of loudspeakers. Sound field  23  is generated by an array of conventional planar loudspeakers  12  hung to conform to a uniform curve  22 . A plurality of substantially evenly spaced dead zones  30  (shown by hatching) of uniform width appear in the sound field  23 . Sound field  25  is produced by an array of conventional loudspeakers  12  hung to conform to a progressive curve. Sound field  25  exhibits fewer dead zones  31 , which are progressively more widely spaced, but which grow progressively in width from the top of the field to the bottom. In essence, the dead zones are collected and grouped vis-a-vis sound field  23 . Lastly, sound field  27  represents the result of using a hanging array of curve fitted loudspeakers  14  in a progressive curve. While dead zones exist, their fine granularity is such as to keep them from being noticed by the average human listener. As will become clearer later in this discussion, the dead zones have in effect, been finely broken up into many small pieces. 
       FIG. 3  is an exploded view of a curve fitted planar loudspeaker  14  in accordance with a preferred embodiment of the invention. Arrows A and B indicate the direction of elongation of the element. The more widely spaced edges of the loudspeaker  14  fall adjacent to the ends of neighboring loudspeakers  12 ,  14  in array  10 . A rear panel  34  attaches to the back of front panel  30  with a diaphragm  32  fitted between the panels and held across aperture  49  and a corresponding rear panel aperture. Front panel  30  includes a center aperture  49  between its major front and rear surfaces from which sound escapes. First, second and third diaphragm braces  50  extend across center aperture  49  from elongated side to elongated side, dividing aperture  49  into four sections. The braces  50  (and  52 ) also abut the diaphragm  32  into sections, allowing each section to vibrate independently. 
     Rear panel  34  is similar to front panel  30 . The rear panel  34  is also elongated and has a center aperture  51  connecting its front and back major surfaces. First, second and third diaphragm cross braces  52  extend between the elongated sides of the aperture, are oriented perpendicular to the direction of elongation of the panel and are aligned on the cross braces  50  of the front panel  30 . Diaphragm  32  is also held taut over aperture  51 . Panels  30 ,  34  are preferably milled aluminum but may be stamped in some applications. 
     Current carrying circuits are conventionally provided using electrical traces etched on one or both of the major surfaces of the diaphragm  32 . More exotic systems may be used as they become available. Electrical connection terminals  48  are provided on the outer major surfaces of front panel  30  and rear panel  34  for electrical traces located on both major surfaces of the diaphragm  32 . These provide points for the external connection of electrical leads and for connection to the conductive traces  40  disposed on both major surfaces of diaphragm  32 . Diaphragms can be divided into multiple sections, and not just the four sections illustrated. Each section has one or two conductive traces  40 . Where there are two conductive traces  40  they are provided front and back on the diaphragm. 
     Diaphragm  32  is a conventional thin film made of a synthetic polyamide or a similar material. Electrical traces  40  are disposed on both the front and back major surfaces of the film including front surface  44 . Traces  40  are replicated four times on both major surfaces of diaphragm  34  (only the front surface  44  traces are shown, those of the rear surface being identical). Each conductive trace  40  is provided with its own set of electrical terminals  42  allowing each trace to be independently energized. Conductive traces  40 , when located in a magnetic field and energized by an electrical current, move and in doing so impart motion to diaphragm  32 . The magnetic field is supplied using bar magnets on both sides of diaphragm  32 . Magnets  37 ,  39  are provided in four sets  36 ,  38  of four magnets each, corresponding to the conductive traces  40  laid out on the diaphragm, front and back. The magnets in each set are elongated neo or bar magnets, laid parallel to one another and extending between the elongated sides of the center apertures  49 ,  51 , in parallel to the diaphragm braces  50 ,  52 , respectively. The bar magnets of each of sets of magnets  36  placed in front of diaphragm  32  are placed in the subapertures of center aperture  49  defined by diaphragm braces  50 . Four bar magnets  37  are illustrated in each subaperture, the magnets being oriented parallel to the braces  50 . However, a different magnetic topology could be used and a greater or fewer number of magnets could be present. Gaps between the magnets  37  provide outlets for the sound generated by movement of diaphragm  32  and define an acoustic lens across the aperture. 
     Otherwise magnets  39  are disposed with respect to rear panel  34  and the rear major surface of diaphragm  32  just as magnets  37  are with respect to the other side of the diaphragm. Magnets  39  fitted to the subdivisions of aperture  51  are substantially wider than magnets  37  with correspondingly smaller gaps between the magnets. Magnets  37  along front panel  30  are preferably narrower and shallower than magnets  39  supported along the back panel  34 . The gaps between magnets along the front panel  30  thus are wider and shallower. This arrangement improves the forward off axis dispersion of the sound field, particularly helping off axis linearity of high frequencies in the sound field. 
     Referring to  FIG. 4 , a front plan view of curve fitted loudspeaker  14  illustrates its front major surface  54 . An acoustic lens  56  forward from diaphragm  40  is defined by the braces  50  and permanent magnets  37  disposed in and across the center aperture of front panel  30  in front of the diaphragm. Acoustic lens  56  is in effect a grating over the sound generating element of curve fitted planar loudspeaker  14 , defining a plurality elongated slots  60  over each of four sections of the front major surface  44  of diaphragm  40 , as defined by the location of conductive traces  40 . Electrical connection points  46  are provided on the surface and connect through front panel  30  into contact with electrical leads on diaphragm  32 . 
     Referring to  FIG. 5 , curve fitted planar loudspeaker  14  is illustrated with front panel  30  and forward magnet arrays  36  shown removed to expose diaphragm  32  positioned on back panel  34 . Four conductive traces  40 , which function as diaphragm voice coils, are positioned centered between the diaphragm braces  52  and/or the outside ends of back panel  34 , with one trace from each major surface of the diaphragm positioned in front of one each of the arrays  38  of permanent magnets  39 . 
     Referring to  FIG. 6 , curve fitted planar loudspeaker  14  is illustrated in cross section from end to end in the direction of elongation of the loudspeaker. Curve fitted planar loudspeaker  14  bulges outwardly along its forward surface toward the center along its longitudinal axis, as may be seen by comparison to a phantom reference line A. The bulge is shaped to fit the curve of a hanging array  10  at the point at which the loudspeaker  14  is to be fitted. The bulge or curve of loudspeaker  14  may be chosen to fit uniform or progressive curves of varying radii. The division of planar loudspeaker  14  into divisions meets the objective of keeping the diaphragm  32  substantially evenly spaced relative to each array of magnets  36 ,  38 . Diaphragm braces  50 ,  52  help support the curve of diaphragm  32  from end to end. The diaphragm braces or bridges  50 ,  52  locally maintain a consistent location of the diaphragm relative to the magnets  37 ,  39 . The perimeter edges of the diaphragm  32  are held between front and back panels  30 ,  34 . Magnets  37 ,  39  are arrayed in a stepped pattern to keep their spacing from diaphragm  32  even. This results in the magnetic field through the diaphragm  32  also being substantially even. For planar loudspeakers  12 , which are flat, the division of diaphragm  32  into sections may still be utilized to allow shaping of the resultant sound field. 
       FIG. 7  illustrates curve fitted planar loudspeaker  14  in cross section from side to side, in which magnets  37 ,  39  extend substantially across apertures  49 ,  51 .  FIG. 8  illustrates a representative conductive trace  40  as might be applied to either major surface of diaphragm  32 . 
       FIG. 9  illustrates audio input signal conditioning circuitry  58 . Pairs of conductive traces  40 , aligned from front and back along the length of diaphragm  32 , are driven synchronized with one another, but may be driven independently from adjacent pairs of conductive traces. Thus each section of the four sections of a single curve fitted planar loudspeaker  14  can be, but is not required to be, independently driven. Each of the four sections of a single curve fitted planar loudspeaker  14  is represented by four speaker transducers (S 1 -S 4 )  92 A-D. Typically a single audio input signal is phase or time delayed by an amount calculated to meet audio or psychoacoustic objectives. For example, it may be desired that projected sound be synchronized when it intersects a boundary, such as the seating area on the floor of a large auditorium. As an alternative, it may be desirable to shape a sound field by compressing or expanding it in a vertical plane. 
     Generation of sound may be initiated electronically upon microprocessor  62  receiving a trigger signal from a source of operational inputs  84 . The source of the audio input signal is a signal source  61 , which may a sound reproduction system, the mixed output of a plurality of microphones, or a blend of sources. Signal source  61  provides an audio input signal to an adjustable amplifier  70  under the control of processor  62 . Processor  62  controls the output amplitude to achieve the preferred sound pressure level at the target distance for the particular curve fitted planar loudspeaker  14 . The target distances for individual sections of a single planar loudspeaker  14  should be so close as to not necessitate individual amplitude adjustment for each section of the planar loudspeaker. However, if this not the case, the gains for the final drive amplifiers  90 A-D could be made adjustable. 
     Following preamplification, the audio input signal is converted to a digital signal by analog to digital converter  72  to ease further modification of the signal. A variety of modifications to the signal are possible, but the only one illustrated for the preferred embodiment is a series of dynamic delay elements  76 A-D. The four dynamic delay elements  76 A-D are parallel elements and effectively divide the digitized audio input signal into four channels corresponding to the four sections of the planar loudspeaker  14  with slight phase or time delay differences between the four channels. In the preferred embodiment it may be desirable that every listener in an auditorium hear exactly the same thing at the same moment. While segmentation of the drivers in the planar loudspeaker  14  of the invention allows the planer to be fitted to a curve, it also allows fine adjustment of sound front generated by a vertical array of planar loudspeakers built in accordance with the invention. 
     Each of the phase differentiated, digitized, audio input signals are then applied to a different one of the four analog to digital converters  85 A-D to provide an analog drive signal for each of speaker transducers  92 A-D, respectively. These signals are amplified by application to amplifiers  90 A-D before application to speaker transducers  92 A-D. 
     It is not necessary that all loud speakers in an array be driven synchronously. Nor is necessary that all segments of a particular planar loudspeaker be driven synchronously. Segmentation of the conductive traces and providing differentiated sound driver facilities for each segment or speaker drive channels allows each segment to be independently controlled. Circuitry to effect such operation can take a number of different forms. With the planar loudspeakers arrayed aligned with the alignment of the segments within each planar, sound generation can be readily steered upwardly, downwardly, vertically collapsed, or vertically expanded. In addition, digital signal processors can be programmed in a number of different ways to implement equivalent circuits to that illustrated in  FIG. 9 . 
     The present invention provides a curve fitted planar loudspeaker which can be used in an elongated, curved array of such speakers to produce sound field that do not exhibit breaks to the average auditor. 
     While the invention is shown in only a few of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.