Abstract:
Acoustic devices have members extending transversely of thickness and capable of sustaining bending waves causing consequential acoustic action by reason of areal distribution of resonant modes of natural bending wave vibration consonant with required achievable acoustic action of said member over a desired operative acoustic frequency range, Areal distribution of stiffness including variation(s) therein is used to get desired locations for bending wave transducers and/or good resonant mode acoustic action from inherently unfavourable shapes of members. Members with combined pistonic active drive and bending wave excitement at centers of mass and geometry are featured.

Description:
This application is a continuation of U.S. application Ser No. 09/389,492, filed Sep. 3, 1999 (now U.S. Pat. No. 6,282,298), which is a continuation of International application Ser No. PCT/GB98/00621, filed Feb. 27, 1998, and published in English on Sep. 11, 1998, and a continuation-in-part of U.S. application Ser No. 08/707,012, filed Sep. 3, 1996 (now U.S. Pat. No. 6,332,029). 
    
    
     FIELD OF THE INVENTION 
     This invention relates to acoustic devices capable of acoustic action by bending waves and typically (but not exclusively) for use in or as loudspeakers. 
     BACKGROUND TO THE INVENTION 
     Our co-pending earlier parent application Ser No. 08/707,012 includes general teaching as to nature, structure and configuration of acoustic panel members having capability to sustain and propagate input vibrational energy through bending waves in acoustically operative area(s) extending transversely of thickness usually (if not necessarily) to edges of the member(s). Specific teaching includes analyses of various specific panel configurations with or without directional anisotropy of bending stiffness through/across said area(s) so as to have resonant mode vibration components distributed over said area(s) beneficially for acoustic coupling with ambient air; and as to having determinable preferential location(s) within said area(s) for acoustic transducer means, particularly operationally active or moving part(s) thereof effective in relation to acoustic vibrational activity in said area(s) and related signals, usually electrical, corresponding to acoustic content of such vibrational activity. Uses are also envisaged in that earlier parent application for such members as or in “passive” acoustic devices, i.e. without transducer means, such as for reverberation or for acoustic filtering or for acoustically “voicing” a space or room; and as or in “active” acoustic devices with bending wave transducer means, including in a remarkably wide range of loudspeakers as sources of sound when supplied with input signals to be converted to said sound, and also in such as microphones when exposed to sound to be converted into other signals. 
     Our co-pending US patent application Ser No. 09/246,967, filed Feb. 9, 1999, concerns using features of mechanical impedance in achieving refinements to geometry and/or location(s) of bending wave transducer means for such panel members as or in acoustic devices. The contents of application Ser. Nos. 08/707,012 and 09/246,967 are hereby incorporated herein to any extent that may be useful in or to explaining, understanding or defining the present invention. These applications are collectively referred to herein as the “two prior patents applications.” 
     This invention arises particularly in relation to active acoustic devices in the form of loudspeakers using panel members to perform generally as above (and as may be called distributed mode acoustic radiators/resonant panels later herein), but further particularly achieve satisfactory combination of pistonic action with bending wave action. However, more general or wider aspects of invention arise, as will become apparent. 
     SUMMARY OF THE INVENTION 
     From a first viewpoint, this invention concerns active acoustic devices relying on bending wave action in panel members, particularly providing effective placement(s) for bending wave transducer means different from specific teachings of the two prior patent applications, i.e. other than at location(s) arising from analysis and preference in parent application Ser No. 08/707,012, even including at center(s) of mass and/or geometry rather than off-set therefrom. 
     From a second viewpoint, this invention concerns acoustic devices relying on bending wave action in panel members, particularly providing effective distributions of resonant mode vibration that may be different from what results from specific teachings and preferences of the two prior patent applications even for the same configurations or geometries. 
     From a third viewpoint, this invention concerns acoustic devices relying on bending wave action in panel members, particularly providing effective distributions of resonant mode vibration in panel members of different configurations or geometries from what are regarded as inherently favourable in specific teachings and preferences of the two prior patent applications. 
     It is considered useful to note that effective specific embodiments of this invention utilise panel member(s) intrinsically affording areal distribution of resonant mode vibration components effective for acoustic performance generally comparable or akin to the two prior patent applications, essentially, relying on simple excitement of such intrinsically areally distributed acoustic bending wave action for successful acoustic operation; rather than in any way resembling merely piece-meal provisions for altering intendedly other acoustic action in panel member(s) for which such intrinsic distributed resonant mode action is not even a design requirement indeed, usually where other particular structural etc provisions are made to serve different frequency ranges and/or selectively suppress or specifically produce/superpose vibrations in a panel member that is not intrinsically effective as in two prior patent applications or herein, typically being inherently unsuitable as a matter of geometry and/or location of transducer means. 
     Effective inventive method and means hereof involve areal distribution of variation in stiffness over at least area(s) of such panel member(s) that are acoustically active in relation to bending wave action and desired acoustic operation. As will become clear herein, such variation can usefully be directly related effectively to displacement of transducer means from location as specifically taught in the two prior patent applications to different locations of this invention, and/or, relative to such patent applications, to rendering unfavourable configurations or geometries of panel members more akin to favourable configurations or geometries for acoustic operation involving areal distribution of resonant modes of vibration consequential to bending wave action, and/or with actual resonant mode distribution that may be at least somewhat different, whether due simply to different areal distribution of bending stiffness hereof or to consequential different location(s) for transducer means, or both. 
     Specific teaching of parent application Ser No. 08/707,012 extends to panel member(s) having different bending stiffness(es) in different directions across intendedly acoustically active area(s) that may be all or less than all of area(s) of the panel member(s), typically in or resolvable to two coordinate related directions, and substantially constant therealong. In contrast, advantageous panel member(s) of embodiment(s) hereof have variation of bending stiffness(es) along some direction(s) across said area(s) that is/are irresolvable to constancy in normal coordinate or any direction(s). 
     Areal variation of bending stiffness is, of course, readily achieved by variation of thickness of acoustic panel members, but other possibilities arise, say concerning thickness and/or density and/or tensile strength of skins of sandwich-type structures and/or reinforcements of monolithic structures usually of composite material(s) type. 
     Whilst available practical analysis may not always allow such investigation as precisely and fully to identify and quantify changes in actual areal distribution of acoustically effective resonant mode vibration for panel member(s) hereof—even where having substantially similar geometry and/or average stiffnesses in relevant directions as for specific isotropic or anisotropic embodiments as per parent application Ser No. 08/707,012—practical resulting performance indicates little if any significant diminishing or degradation in achieved successful acoustic performance involving bending wave action, indeed encourages belief in potential even for improving same. Beneficial effects (on areal distribution of resonant mode vibration), of basically favourable configuration/geometry of the two prior patent applications can, however, be substantially retained to very useful extent and effect in two groups or strands of inventive aspects implementing above one viewpoint. 
     One group/strand is as already foreshadowed, specifically providing more convenient location(s) for transducer means in acoustically active panel members or areas thereof having configurations or geometries known to be favourable in isotropic or anisotropic implementations of teachings of two prior patent applications, effectively by displacing what are now called “natural” locations for transducer means (in accordance with these patent applications), to different locations hereof, specifically by either or both of relatively greater and lesser bending stiffnesses to one side and to the other side, respectively, of such natural location(s). Region(s) of greater bending stiffness serve(s) effectively to shift such natural location(s) away from such region(s), typically from said one side towards said other side and region(s) of lesser bending stiffness; region(s) of lesser bending stiffness serving to shift towards own region(s). The other group/strand can be viewed as involving capability only partially to so define same at least notional sub-geometry of larger overall panel member geometry not specifically favourable to good distributed mode acoustic operation as in the two prior patent applications; such sub-geometry being incompletely circumscribed and not necessarily specifically so favourable of itself but the partial definition thereof having significant improving effect on distributed mode acoustic operation, say tending towards a type of configuration or geometry known to include specific favourable ones if not at least approaching such favourable ones; such improving effect being particularly for distributing resonant modes therefor a lower frequencies, but not necessarily (indeed preferentially not) limiting higher frequency bending wave action and resonant mode distribution to such sub-geometry, i.e. allowing such higher frequency resonant mode distribution of vibration past and beyond the partial sub-geometry definition. 
     As to readily achieving required or desired areal variation of bending stiffness panel member(s) can have at least core layer(s) first made as substantially uniformly two prior patent applications, including sandwich structure(s) having skin layers over core layer(s). Variation(s) of thickness can then be readily imposed to achieve desired areal distribution of stiffness(es). For deformable material(s), such as foam(s), such variation of thickness is achievable by selective compression or crushing to achieve desired contouring, say by controlled heating and application of pressure, typically to any desired profile and feasibly done even after application of any skin layers (depending on stretch capability of such skin layer material). Another possibility is for the member to have localised stiffening or weakening, perhaps preferably graded series thereof. For through-cell or honeycomb materials, e.g. of some suitable reticulated section of its cells extending from skin to skin of an ultimate sandwich structure, or rigidly form-sustaining uncrushable composites, variation of thickness is readily achievable by selective skimming to desired thickness contouring/profiling. None of these possibilities involves necessary change of geometrical center, but skimming rather than crushing inevitably results in change of center of mass. Further alternatives for desired thickness/stiffness variation of as-made core(s) will be discussed, including without change of center of mass as can be important for transducer means combining pistonic and bending wave actions, where pistonic action is manifestly best if centered at coincidence of center of mass and geometric center to avoid differential moments due to mass distribution relative to transducer location(s) and/or to unbalanced air pressure effects. 
     Center of mass is, of course, readily relocated, typically to geometric center by selective addition of mass(es) to panel member(s) concerned, preferably without unacceptable effects on desired areal distribution of stiffness, e.g. masses also small enough not unacceptably to affect lower frequency bending wave action and effectively decoupled from higher frequency acoustic action(s), say small weight(s) suitably semi-compliantly mounted in hole(s) in the panel also small enough not unacceptably to affect acoustic action(s). 
     Increasing stiffness in one direction away from or to one side of the ‘natural’ location(s) for transducer means location(s) of the two prior patent applications, or decreasing stiffness in a generally opposite direction or to other side, will result in transducer means location(s) hereof generally in said one direction to said one side, which can advantageously be towards geometric center. Such relative increasing/decreasing of stiffness can be complex as to resulting contouring of the panel member concerning, including tapering down increased thickness/stiffness to edge of the panel member and or sloping up decreased thickness/stiffness, say to have a substantially uniform edge thickness of the panel member. 
     Additionally or alternatively, an inventive aspect of at least the one group/strand is seen in a panel member capable of acoustic bending wave action with a distribution of bending stiffness(es) over its acoustically active area that is in no sense centered coincidentally with center of mass and/or geometrical center of that panel member, though location(s) of acoustic transducer means, whether for bending wave action or for pistonic action or for both, may be substantially so coincident, often and beneficially so. 
     It is noted at this point that there are two ways in which areal distributions of stiffness(es) over a panel member can be considered or treated as centered, one analogous to how center of mass is usually determined, i.e. as putting first moment of stiffness to zero, thus in a sense corresponding to high stiffness (so herein called “high center” of stiffness); the other in an inverse manner, putting first moment of the reciprocal of stiffness to zero, thus in another sense corresponding to weakness or low stiffness (so herein called “low center” of stiffness). In panel members with isotropy or anisotropy as specifically analysed in parent application Ser No. 08/707,012, these notional “high” and “low” centers of stiffness (so far as meaningful in that context) are actually coincident, further normally also coinciding with center of mass and with geometrical center; but, for a panel member with stiffness distribution as herein, these notional “high” and “low” centers of stiffness are characteristically spaced apart and typically further also from center of mass and/or geometric center. 
     Reverting to effective or notional shifting (by beneficial distributions of stiffness(es) hereof) of practically effective location(s) for bending wave action transducer means (from location(s) afforded by preferred teachings/analyses of the two prior patent applications to different location(s) hereof), such shifting can usefully be viewed as towards said “low center” of stiffness which should thus be along same direction as desired notional shifting, and/or away from said “high center” of stiffness that may usefully afford at least a structural design reference position for providing variations of bending stiffness(es) in the desired/required corresponding distribution thereof. Variation of bending stiffness outwards from such “low center(s), to edge(s) of panel member(s) concerned, typically with stiffness(es) increasing to different amounts and/or at different rates in plural directions at least towards “high center(s)”. 
     Feasible structures of honeycomb cellular cored sandwich type can have desired stiffness distribution by reason of contributions of as-made variant individual cell geometries, and without necessarily substantial effect(s) on distribution and center of mass. Thus, desired areal distributions of stiffness(es) are achievable by variations of cells as to any or all of cell sectional area (if not also shape), cell height (effectively core thickness) and cell wall thickness, including with such degree of progressiveness applied to increase/decrease as may be desired/required. Varying bending stiffness(es) without disturbing distribution of mass is achievable in such context, say by varying cell wall thickness and cell height for nominally same cell area, and/or by varying cell area and/or cell height for same thickness of cell walls, and could, of course, be augmented or otherwise affected by skin variations including varying number and/or nature of ply layers. 
     Also, it is seen as inventive for panel members hereof to have at least “low” centers of stiffness(es) and practically most effective drive location(s) that are identified and typified oppositely in terms of minimum and maximum diversity of transit times to panel edge(s) for notional or actual bending Waves considered as started from “low center” of stiffness and from transducer location(s), respectively. 
     Reverting to above second general view, panel members with distribution(s) of stiffness(es) as herein (as might perhaps be called “eccentric”) can have capability applicable to securing that a said panel of some particular given or desired shape (i.e. configuration or geometry) may exhibit practically effective acoustic bending wave action that was not considered achievable hitherto for that particular shape, at least not according to any prior helpful proposition; including not only for unfavourable shapes related to known favourable shapes, but for shapes not so related but treatable as herein to at least approach what would hitherto be characteristic of some particular favourable shape. 
     Indeed, this invention extends to capability of some physically realisable areal distribution of bending stiffness(es) of and for even irregularly shaped panel members capable of bending wave acoustic actions to render such action of satisfactorily distributed resonant mode characteristic, and to afford practically effective location(s) for bending wave action transducer means (including by finite element analysis), even irrespective of and without reference to any envisaged or target shape known to be favourable. Such procedures might proceed to at least some extent pragmatically, by trial and error, as to areal stiffness distributions, but can be helped by analysing same using such as Finite Element Analysis at least in terms of affording useful “low” and “high” centers of stiffness shown herein to have positive (approaching/attracting) and negative (distancing/repelling) location effects on effective location(s) for transducer means within such areal stiffness distribution, whether itself analysable or not. 
     In practice, useful benefits are seen by way of seeking out constructs and/or transforms by which derivation(s) can be made from what is known to be effective for particular panel member geometries and structures to what may, often will, be effective for a different panel geometry/structure, particularly to indicate structural specification for such different panel geometry as to likely successful areal stiffness distribution and as to transducer drive location(s). 
     In one approach considered inventive herein, useful attention has been concentrated on transducer location(s), including by way of notionally superposing as a target geometry a desired or given configuration of panel member and a subject geometry of a panel member that is known to be effective and for which detailed analysis is readily done or available, so that desired target transducer location coincides with actual preferentially effective transducer location of the subject geometry. Then, a bending stiffness mapping can be made so that, for any or each of selected constructs relative to now-coincident transducer locations of the target and subject geometries, and over such geometries, so that the known/readily analysed bending stiffness of the subject panel structure can be subject to transformation relative to the target geometry to give substantially the same or similar or scaled comparable stiffness distribution as in the subject geometry and acoustically successful bending wave action in the target geometry. Promising such constructs include lines going from coincident transducer locations to/through edges of the target and subject geometries (say as though representing bending wave transits/traverses). Envisaged related transforms depend on relative lengths of the same construct lines in the target and subject geometries, and a suitable relationship, typically involving the quotient of bending stiffness (B) and mass per unit area (μ), i.e. B/μ, for proportionality transforms involving the third and/or fourth powers of such line lengths to edges of target and subject geometries. It is preferred, at least as feeling more natural, for a target geometry to be smaller than a related subject geometry, further preferable for superposition to seek to minimise excess of the latter Over the former, including to minimise transform processing. Whilst generally similar types of target and subject shapes may thus be preferred, or favourable subject geometry closest to unfavourable target geometry, it is seen as feasible for the target geometry to differ quite substantially from any recognisable type of known favourable configuration/structure. 
     It is the case that panels of parent application Ser No. 08/707,012 that are isometric as to areal bending stiffness, and well studied/analysed, are good starting points for subject geometries/structures. Indeed, another construct/transform approach seen as having potential involves seeking to match in the target geometry/structure according to the way that the {now common) transducer location splits bending stiffnesses to each side thereof in the subject geometry/structure. Moreover, similar or related mapping schemes could be used not only as between differing geometry types, but also in the event of wishing or requiring to give to a target geometry of one type such a bending stiffness distribution as to resemble or mimic another type of geometry/configuration, so far as practicable given type of geometry/configuration (e.g. rectangular, elliptical) does have profound influence on actual areal distribution of resonant mode vibration that can be difficult to disturb greatly. 
     For loudspeaker members capable of both pistonic and bending wave types of action, coincidence of location of bending wave transducer means with center of mass and geometric center is particularly effective in allowing a single transducer device at one location to combine and perform both pistonic drive and bending wave excitation. 
     It is, however, feasible to use separate transducers one for pistonic-only action at coincident center of mass/geometric center, and another for spaced location conveniently located as herein for bending wave-only action, though mass balancing may then be required by added masses (if not afforded conjointly with requisite distribution of bending stiffness). 
     A particularly interesting aspect of invention, concerning a single transducer that affords both of pistonic action and spaced bending wave action but at spaced positions, can be used whether spacing is achieved by bending wave transducer location as herein (say to suit convenient transducer configuration) or left as arises without application of above aspects of invention. 
     Generally, of course, application of this invention may involve distributions of mass with center of mass displaced from geometric center and/or any transducer location, or whatever. Indeed, variation(s) of bending stiffness and/or mass across at least acoustically operative area(s) of panel member(s) can be in many prescribed ways and/or distributions, usually progressively in any particular direction to desired ends different from hitherto, and same will generally represent anisotropy that is asymmetric at least relative to geometric center of mass; and application is seen as in parent application Ser No. 08/707,012. 
     Practical aspects of invention include a loudspeaker drive unit comprising a chassis, a transducer supported on the chassis, a stiff lightweight panel diaphragm drivingly coupled to the transducer, and a resilient edge suspension surrounding the diaphragm and mounting the diaphragm in the chassis, wherein the transducer is arranged to drive the diaphragm pistonically at relatively low audio frequencies to produce an audio output and to vibrate the diaphragm in bending wave action at higher audio frequencies to cause the diaphragm to resonate to produce an audio output, the arrangement being such that the transducer is coupled to the center of mass and/or geometric center of the diaphragm and the diaphragm has a distribution of bending stiffness including variation such that acoustically effective resonant behaviour of the diaphragm results (at least preferably being centered offset from the center of mass). 
     The diaphragm may be circular or elliptical in shape and the transducer may be coupled to the geometric center of the diaphragm The diaphragm may comprise a lightweight cellular core sandwiched between opposed skins, and one of the skins may be extended beyond an edge of the diaphragm, with a marginal portion of the extended skin being attached to the resilient suspension. 
     The transducer may be electromagnetic and may comprise a moving coil mounted on a coil former, the coil former being drivingly connected to the diaphragm. A second resilient suspension may be connected between the coil former and the chassis. One end of the coil former may be connected to the diaphragm, and the said second resilient suspension may be disposed adjacent to the said one end of the coil former, and a third resilient suspension may be connected between the other end of the coil former and the chassis. 
     The end of the coil former adjacent to the panel diaphragm may be coupled to drive the panel diaphragm substantially at one point. Conical means may be connected between the coil former and the panel diaphragm for this purpose. 
     The coil former may comprise a compliant section radially offset from a rigid section to drive the diaphragm pistonically and to provide offcenter resonant drive to the diaphragm. 
     In other aspects the invention provides a loudspeaker comprising a drive unit as described above; and/or is a stiff lightweight panel loudspeaker drive unit diaphragm adapted to be driven pistonically and to be vibrated to resonate, the diaphragm having a center of mass located at its geometric center and a center of stiffness which is offset from its center of mass. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary specific implementation is now illustrated/described in/with reference to accompanying diagrammatic drawings, in which: 
     FIGS. 1A-D are plan and three outline sectional views indicating desired positioning of bending wave transducer location of an acoustic panel member, including and achievement by compressing deformable core material or by profiling core or composite material; 
     FIGS.  2 A,B,C are outline overall plan view and core sectional views for an elliptical acoustic panel member hereof; 
     FIGS.  3 A,B,C are similar views of another elliptical panel member hereof; 
     FIGS.  4 A,B,C indicate a acoustic panel member of unfavourable circular shape rendered more favourable by part-elliptical grooving/slotting, and model distribution graphs without and with such grooving/slotting; 
     FIGS.  5 A,B,C are diagrams useful in explaining possible mappings/constructs/transforms for deriving stiffness distribution for desired or target geometry for a rectangular panel member and a sectional/profile representation of results; 
     FIGS.  6 A,B,C are outline graphs of interest relative to useful methodology including of FIG. 5; 
     FIGS.  7 A,B are sectional side and plan views of one embodiment of loudspeaker drive unit of the present invention; 
     FIGS.  8 A,B are sectional side views of another loudspeaker drive unit and a modification; 
     FIGS.  9 A,B are sectional side view of a further loudspeaker drive unit and modification; 
     FIGS.  10 A,B are a perspective view of a loudspeaker drive coupling or actuator for spaced application of pistonic and bending wave action, and detail of mounting to a diaphragm/panel member; and 
     FIGS.  11 A,B show relationships for such actions and crossover. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring first to FIG. 1A, a substantially rectangular acoustic distributed mode panel member  10 A is indicated as though resulting directly from teachings of the two prior patent applications, thus having its “natural” location  13  for bending wave transducer means spaced from its geometrical center  12  and off true diagonal shown dashed at  11 . In application of the present invention, however, the transducer location  13  is to be at the geometric center  12  of the panel member  10 A, i.e. effectively to appear shifted along the solid line  15 , which is achieved by appropriate areal distribution of bending stiffness of the panel member. To this end, the bending stiffness is made relatively greater and lesser to one side (right in FIG. 1A) and to the opposite side (left in FIG. 1C) of the geometric center  12  and the “natural” transducer location  13 , specifically in opposite directions along the line  15  and its straight-line extensions  15 G and  15 L, respectively. 
     FIG. 1B is an outline section along the line  15  including extensions  15 G and  15 L, and indicates the same situation as FIG. 1A, i.e. “natural” transducer location  13 B likewise spaced from geometric center  12 B of distributed mode panel member  10 B, see projection lines  12 P,  13 P. FIG. 1B gives no details for the actual structure of the panel member  10 B; but does indicate the alternatives of being monolithic, see solid outer face lines  16 X,Y, or being of sandwich type, see dashed inner face lines  17 X,Y indicating skins bonded to an inner core  18 , typically (though not necessarily) of cellular foam type or of honey-comb through-cell type. 
     FIG. 1C indicates use of a core  18 C of material that is deformable, specifically compressible in being capable of crushing to a lesser thickness, as is typically of many foamed cellular materials suitable for distributed mode acoustic panel members and assumed in FIG.  1 C. Such crushing is indicated by thickness of the core  18 C diminishing from right to left in FIG. 1C, and its cells going from roundedly fully open ( 19 X) to flattened ( 19 Y). It is not, of course, essential for those cells to be of the same or similar size, or of regular arrangement, or be roundedly fully open at maximum thickness (suitable foam materials often being of partially compressed foamed type). The core  18 C is further shown with facing skins  17 A,B. It is feasible, even normal, for the core material  18 C to be deformed to the desired profile before bonding-on the skins  17 A,B—but not essential so long as the panel member  10 C is good for distributed mode acoustic action if compressively deformed with the skins  17 A,B attached. Resulting greater and lesser thickness of the core  18 C and the panel member  10 C will correspond with greater and lesser bending stiffness; and the indicated profile of progressive thickness, thus stiffness, variation is such as to cause coincidence of the transducer location  13 C with the geometric center  12 C, see arrow  13 S and circled combined reference  12 C, 13 C. Crushing deformation will normally be done with thermal assistance and using a suitably profiled pressure plate. There will be no change to the center of mass of the panel member  10 C, i.e. center of mass will remain coincident with the geometric center  12 C, now also coincident with the transducer location  13 C. 
     Where core density contribution is small, i.e. bending stiffness is dominant, the linear factor of core mass contribution may be neglected and the desired areal thickness distribution may be achieved by shaping the thickness of an isotropic core of polymer foam or fabricated honeycomb sandwich or monolithic without skin and a core; and any such structure can be fabricated, machined or molded as desired herein. 
     FIG. 1D shows distributed mode acoustic panel member  10 D with progressive relief of its lower surface so that its thickness reduces with similar profile to that of FIG.  1 C. Such profile might be somewhat different for the same intended effect, i.e. achieving coincidence of transducer location  13 D with geometric center  12 D, say depending on material(s) used for the panel member  10 D. Such materials may be monolithic reinforced composites or any kind of cellular, typically then as a skinned core, including of honey-comb type with through-cells extending from skin-to-skin. The foamed-cell-like indication  19 Z of FIG. 1D could correspond with use of foamed material that is by choice not crushed or is not suitable for crushing; but is intended to do no more than indicate that there is no significant change of density. There must, of course, then be a change in the distribution of mass and the center of mass of the panel member  10 D as such will be spaced from the geometric center, generally in the direction of arrow CM. In order to achieve coincidence of overall center of mass with geometric center  12 D, the panel member  10 D is shown with at least one additional balancing mass  22  indicated mounted in preferably blind receiving hole  23 , further preferably by semi-compliant means  24 , say in a suitable mechanically or adhesively secured bush or sleeve, so that its inertial compress is progressively decoupled from the panel member  10 D at higher frequencies of desired vibration distribution. There may be more than one balancing mass ( 22 ), say in a less than 180° locus through the notional extension line  15 L, or some other array disposition, and need not all be of the same mass, say diminishing in mass progressively away from the line  15 L. At simplest, the thickness may be simply tapered along through the section of FIG. 1B, though a more complex taper is normal, including to a common equal edge thickness and/or progressively less away from the line  15 - 15 G, L. Geometric relations of bending frequency to size are used need to be taken into account. For any given shape, increasing its size lowers the fundamental frequencies of vibration, and vice versa. Effective shift of preferential transducer location can be seen as equivalent to shortening the effective panel size in relation bending along the direction of such shift. 
     Turning to FIGS. 2A-C and  3 A-C, all panel members are shown as being of generally elliptical shape, those referenced  20 A,  30 A being isotropic, thus showing coincidence at  25 ,  35  of geometrical center and center of mass. To the extent meaningful for isometric panel geometries and structures, distributions of stiffness will, of course also be centered at  25 ,  35 —whether as to “high center” (stiffness as such) or as to “low center” (softness or compliance). In addition, FIGS. 2A,  3 A show at  26 ,  36  one preferentially good or best location (as in parent application Ser. No. 08/707,012) for a bending wave action transducer and operative for desired resonant mode acoustic performance of the panel member  20 A,  30 A, say as or in a loudspeaker. 
     Turning to FIGS. 2B, C and  3 B, C the center positions of the panels  10 B  20 B,  30 B are now labelleds  25 ,  26  and  35 ,  36  and still correspond to both of geometric center and center of mass ( 25 ,  35 ), but now also further to acoustically effective bending wave transducer location ( 26 ,  36 ). Compared with FIGS. 2A,  3 A the transducer locations  26 ,  36  have effectively been displaced by a distribution of bending stiffness(es), hereof, and accompanying displacements of “high and “low” centers of stiffness, are indicated  27 ,  28  and  37 ,  38  as generally oppositely relative to the geometric centers  25 ,  35 . This different asymmetric stiffness distribution is shown achieved by progressive changes to cells  29 ,  39  particularly as to their heights, thus thickness of the panel members  20 A,  30 A; but also as to their areas and population density (see FIGS. 2B, C), or as to their areas and wall thicknesses but not their population density (see FIGS. 3B, C) thereby achieving desired distribution of stiffness without at least operatively significant disturbance to distribution of mass, thus center of mass is now coincident with both geometric center and transducer location ( 25 ,  26 ;  35 ,  36 ). 
     There are further feasible approaches to varying stiffness(es), thus areal distribution; say by introducing out-of-planar formations, such as bends, curves etc affecting stiffness in generally understood ways; or such as grooves, slots or scorings in surfaces to reduce stiffness or rib formations to increase stiffness, including progressively by spaced series of such provisions, say along the line extensions  15 G, L of FIG. 1A (not shown, but computable using such as Finite Element Analysis). 
     FIG. 4A shows another application of into-surface grooving, slotting or scoring, specifically to improving distributed mode bending wave action for an acoustic panel member  40  that is actually of a configuration or geometry, namely circular, that is known to be unfavourable as a distributed mode acoustic panel member, especially with central location of exciting transducer means. This known unsatisfactory performance capability is indicated by the modal frequency distribution indicated in FIG. 4B as will be readily recognised and understood by those skilled in the art, specifically corresponding to concentric vibration patterning. Profound improvement on what is shown in FIG. 4C has been achieved by grooving, slotting or scoring as indicated at  45  in the form of part of an ellipse, i.e. in a class of configurations/geometries known to include some highly favourable as distributed mode acoustic panel members (as in FIGS. 2,  3  above), though not actually according to such a known favourable particular ellipse. However, effect on lower frequency modal action is markedly better distributed than the symmetry of simple centrally excited circular shapes, and higher frequency modal action is able to extend past and beyond the open ends of the groove  45 . The shape of the groove  45  was developed using Finite Element Analysis, see indicated complex element patterning, such techniques being of general value to detail implementation of teachings hereof. Lesser arcuate formations asymmetrically spaced relative to center of a circular panel member have also shown promise, and should be readily refined by further Finite Element Analysis. 
     FIGS. 5A, B indicate constructs and transforms much as discussed above, specifically shown for rectangular target ( 51 A, B) and subject ( 52 A, B) configurations/geometries. Construct lines  53 A, B processed according to different lengths and desired/required bending stiffnesses show highly promising effectiveness of the approach at least as applied to shapes of the same rectangular type. The methodology of FIG. 5B is particularly attractive in that the subject configuration/geometry  52 B is efficiently constructed from the target configuration/geometry  51 B placed at one corner by extensions from that corner so that a preferential transducer location  54 B of a well-understood and analysed isometric shape  52 B simply coincides with geometrical center of the target shape  51 B. FIG. 5C indicates a typical section through target member  50  of target shape  51 A resulting from methodology according to FIG.  5 B. 
     Inspection of the B/μ quotient or the B and/or μ parameter values, specifically alone with the other held constant, in the various radial directions  53 B, and mathematical mapping from panel of shape  52 B to panel of shape  51 B, allows distribution of stiffness hereof to be computed in those directions ( 53 B) further using a power relation including fourth power of length and second or third powers of thickness depending on whether bending stiffness required is of skinned core sandwich panel or an unskinned monolithic solid composite structure. 
     FIG. 6A shows ratiometric results of length mapping for FIG. 5B methodology, and FIG. 6B shows how required (target) bending behaviour is related to the ratiometric results of FIG.  6 A and relative to material properties, specifically stiffness alone involving fourth power of length (solid line), thickness of a sandwich structure involving a square power (dotted line), and thickness of a monolith structure involving a {fraction (4/3)} power (dashed line). For a sandwich structure, skin stiffness (tensile strength) would also involve fourth power of length; and skin thickness a {fraction (4/3)} power. FIG. 6C shows modal density mapping with 3% damping for a target square panel member, without bending stiffness distribution hereof, a subject 1.134:1 aspect ratio isometric panel member of parent application Ser No. 08/707,012, i.e., involving adjustment relative to one side difference only; and the square panel improved by bending stiffness distribution according to skin parameters, specifically thickness (h) and Young&#39;s modulus (E). 
     Referring to FIGS. 7A and 7B, a loudspeaker drive unit comprises a chassis  71  in the form of an open frame shaped as a shallow circular basket or dish having an outwardly projecting peripheral flange  71 F pierced with holes whereby the drive unit can be mounted on a baffle (not shown), e.g. in a loudspeaker enclosure (not shown) in generally conventional fashion. The chassis  71  supports a transducer  72  in the form of an electrodynamic drive motor comprising a magnet  73  sandwiched between pole pieces  74 A,B and affording an annular gap in which is mounted a tubular coil  75  former carrying a coil  75 C which forms the drive coupling or actuating movable member of the motor. 
     The coil former is mounted on resilient suspensions  76 A,B at its opposite ends to guide the coil former  75  for axial movement in the gap of the magnet assembly. One end of the coil former  75  is secured, e.g. by bonding  77 , to the rear face of a lightweight rigid panel  70  which forms an acoustic radiator diaphragm of the loudspeaker drive unit and which comprises a lightweight cellular core  70 C, e.g. of honeycomb material, sandwiched between opposed front and rear skins  70 F,R. The panel  70  is generally as herein taught, specifically with distribution of bending stiffness affording coincidence of center of mass and preferential bending wave exciter location at its geometric center. In the example shown, the front skin is conveniently of conventional circular form integrating with the contour and in some cases blending in effective operation with the surround/suspension. The rear skin is chosen to be rectangular to form a composite panel compliant with distributed mode teaching (it may be driven directly by the differential coupler of FIGS.  10 A and  10 B). 
     For a simple central, or central equivalent drive the distributed mode panel section will be designed with preferential modal distribution as per the invention herein generated for example by control of areal stiffness, so as usefully to place the modal driving point or region at or close to the geometric and mass center. Thus good modal drive at higher frequencies and pistonic operation at lower frequencies is obtained for a conventional style of driver build and geometry. 
     The front facing skin  70 F of the panel  70  is extended beyond the edge of the panel and its peripheral margin is attached to a roll surround or suspension  77  supported by the chassis  71  whereby the panel is free to move pistonically. The transducer  72  is arranged to move the panel  70  pistonically at low frequencies and to vibrate the panel  70  at, high frequencies to impart bending waves to the panel whereby it resonates as discussed at length above. 
     The arrangements shown in FIGS. 8A and 8B are generally similar to that described above, except that in these cases the chassis  81  is even shallower, the motor  72  is largely outside the chassis  81 , and the coupler/actuator coil former  85  extends into the chassis with consequent modification of its suspension  86 . Modification of FIG. 9B involves use of a smaller neodymium motor  82 N and sectional end reduction  85 A of the coil former  85 . 
     The arrangements shown in FIGS. 9A and 9B are very similar to those shown in FIG. 8A and 8B except that the extended end  95 A, B of the coil former  95  is formed with a single or double conic section, the pointed end  95 P of which is attached to the rear face of the lightweight rigid panel diaphragm  90  at the geometric center thereof. 
     FIGS.  10 A,B show a diaphragm coupler/actuator  100 , conveniently a coil former of a drive motor (not shown), having a major arcuate peripheral part  108  of its drive end, which is adapted to be attached ( 107 ) to a rigid lightweight panel  100  made of a semi-compliant material; and with arcuate peripheral part  109  of the same end rigid. The drive applied to the panel  100  will be pistonic at low frequencies through both of the arcuate peripheral end parts  108 ,  109 . At high frequencies the coupler/actuator will excite bending wave action by the minor part  109 , thus vibrational energy in the panel  100  at a position offset from the axis of the coupler/actuator  105 . By its semi-compliant nature, the major arcuate peripheral end part  108  will be substantially quiescent at high frequencies. Thus the true actuation position of the drive is frequency dependent even though applied in the same way and by the same means  105 . 
     The simple illustrated case of one direct coupling section and one semi-compliant section may be extended to multiple firm contact points and more complex semi-compliant arrangements, e.g. two or more preferential distributed mode panel member transducer locations may be involved. The semi compliant section may be tapered or graded, or plurally stepped in thickness or bulk property, to provide a gradation of coupled stiffness interactively calculated with the panel acoustic performance criteria to improve overall performance, whether with a distributed mode acoustic panel with bending wave transducer location spaced from geometric/mass center to suit convenient structure for the coupler/actuator  105 , or with the latter suited to such as transducer locations of the two prior patent applications. 
     Such differential frequency coupler ( 105 ) can be used with the usual motor coil employed in electrodynamic exciters. While such coupler  105  may be a separate component of predetermined size or diameter, it is convenient to see its application as part of the attachment plane of a motor coil of similar diameter, which may as indicated above be chosen to encompass one or more of the preferential drive transducer locations of a distributed mode acoustic panel member, specifically at and excited by rigid end part(s)  108  as intended higher frequency response is by bending mode vibration in a distributed mode acoustic panel diaphragm member  100 . At lower frequencies the semi-resilient parts/inserts  108  become more contributory, and progressively bring the whole circumference of the actuator/coupler  105  into effect for balanced, center of mass action, thus satisfactory pistonic operation at low frequencies. The fundamental bending frequency of the panel member  100  and the resilience of the coupler/actuator part(s)  108  are chosen to allow for satisfactorily smooth transition in acoustic power from the pistonic to the bending vibration regions of the frequency range. Such transition may be further aided by plural stepping of the part(s)  108 , or by tapering as indicated at  108 A. 
     Understanding operation of this coupler  108  is aided by FIG. 11A outlining intended variation of velocity applied to the acoustic panel, including in the region of crossover. At low frequencies the semi compliant part(s)  108  contribute effective power to the panel member  100  in a balanced pistonic manner. That piston like action decays with increasing frequency as the mechanical impedance of the vibrating panel member  100  becomes predominant and is excited at preferential eccentric position(s). Thus the active velocity contribution at higher frequencies arises from the rigid, offset sector(s) of the coupler. 
     FIG. 11 b  further shows displacement of effective variation of pistonic drive and distributed mode excitation points with frequency. At low frequencies the pistonic drive point is predominantly at the center and center of mass. With increasing frequency there is a transition to a bending wave excitation point offset from the center, aligned by suitable choice of panel design and also complex coupler actuator diameter and parts geometry to drive at or close to the preferred distributed mode point for satisfactory favourable distribution of vibration modes. 
     In above FIGS.  7 A,B bending wave transducer means of this type with an overall diameter in the range 150 to 200 mm would operate “natural” transducer location(s) of a distributed mode panel member of satisfactory bending mode performance commencing in the range 150 Hz to 500 Hz. Pistonic operation will be effective from lower frequencies, eg from 30 Hz for a suitable acoustic mounting, and would decline in its upper range as the panel member enters the bending mode range. 
     The differential frequency capability of couplers of this invention allows subtle refinements to use of distributed mode acoustic panel members. For example, in a given panel a change in the driving point with frequency may be found desirable for purposes of frequency control seen in particular applications, such as close to wall mounting in small enclosures and related response modifying environments. More than one grade and/or size/area of semi-compliant parts or inserts may be used on suitable geometries of coupler effectively to gradually or step-wise move between more or most effective drive point of the modal pattern with frequency, and advantageously modify the radiated sound.