Abstract:
A method of making an acoustic device in the form of a loudspeaker drive unit having a resonant panel-form member adapted to provide an acoustic output when excited with bending wave energy, comprising forming a resilient suspension for the panel-form member by moulding and simultaneously fixing the resilient suspension to the panel-form member.

Description:
[0001]    This application is a continuation of PCT/GB99/02268, filed Jul. 29, 1999.  
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to a method of making acoustic devices of the nature of loudspeakers and loudspeaker drive units and comprising resonant panel-form members capable of producing an acoustic output when excited with bending waves, and to acoustic devices made by the method.  
         BACKGROUND ART  
         [0003]    The basic patent teaching for such panel-form acoustic members, known as “distributed mode” or ‘DM’ devices, is in International patent application WO97/09842 of New Transducers Limited.  
           [0004]    It is known to suspend such panel-form members on, for example, a frame or chassis by means of a resilient suspension fixed adhesively to the edge of the member. It is an object of the present invention to facilitate the mounting and suspension of resonant panel-form acoustic members relative to framing which may, for example, be a shallow enclosure or a so-called basket or chassis.  
           [0005]    DM panel-form loudspeakers emit acoustic radiation from both sides of the panel, i.e. are bi-polar, and to facilitate positioning of the loudspeaker in a room, e.g. against a back wall, the rear radiation can be blocked or attenuated by placing the panel in a sealed box with a flexible surround or suspension round the panel perimeter to mount the panel in the box. This prevents any destructive interference from reflecting surfaces behind the panel. The panel surround or suspension may be a strip of flexible foam with adhesive tape on both sides.  
           [0006]    However, in such an arrangement, standing waves may be set up in the rear enclosure which can adversely affect the frequency response of the panel. These standing waves can be damped by filling the enclosure with an acoustic absorber, e.g. of soft foam material, for example flexible polyester or polyether.  
           [0007]    It is another object of the invention to provide a method of making an acoustic device comprising a resonant panel-form member in which a suspension for the panel and an acoustic absorber are combined.  
         SUMMARY OF THE INVENTION  
         [0008]    From one aspect the invention provides a method of making an acoustic device in the form of a loudspeaker drive unit having a resonant panel-form member adapted to provide an acoustic output when excited with bending wave energy, comprising forming a resilient suspension for the panel-form member by moulding and simultaneously fixing the resilient suspension to the panel-form member.  
           [0009]    The moulding may be with the aid of compression and/or heat and using a foamed plastics or rubber as the starting material for the suspension, or may comprise injection moulding of an elastomeric material.  
           [0010]    Fixing of the suspension to the panel-form member may be accomplished with the aid of an adhesive or may involve direct injection of the suspension onto the panel-form member. The resilient suspension may be fixed to a peripheral margin of the panel-form member.  
           [0011]    The method may comprise moulding a marginal portion of a block of soft or resilient foamed plastics or rubber by pressure and/or heat to form the suspension and such that a portion of the block of foamed plastics or rubber forms an acoustic absorber for attenuating or absorbing the acoustic output from one side of the panel-form member.  
           [0012]    Where the suspension is injection moulded, a rigid support member may be co-moulded or integrally moulded or otherwise attached to the resilient suspension at a position spaced away from the edge of the panel-form member to provide a support for fixing the suspension in position, e.g. in an enclosure or in or on a frame or chassis. The rigid support may be in the form of a member extending round the periphery of the panel-form member as a continuous member or may be discontinuous where a suspension in the form of discrete suspension components or elements is required. A continuous suspension may provide an air or dust seal.  
           [0013]    The fixing of the support to the enclosure or frame or chassis may be by clamping, adhesive, fasteners or the like or by a combination of these fixing methods.  
           [0014]    Where the suspension is injection moulded it may abut the edge of the panel-form member or may surround or partially surround the edge of the panel-form member. The resilient suspension may also be such as to surround or at least partially surround the rigid support member. Locating elements in the form of protrusions or recesses or apertures may be moulded into the injection-moulded suspension or may be provided in the rigid support to locate the suspension in the frame, chassis or enclosure.  
           [0015]    The suspension may be formed such that the suspension is an interference fit in or on the frame, chassis or the like and so that tension and/or compression is applied to the suspension when it is mounted to its frame, chassis or the like to enhance the contact therebetween.  
           [0016]    From another aspect the invention is a loudspeaker drive unit made by the method described above.  
           [0017]    From yet another aspect the invention is a loudspeaker comprising a drive unit as defined above. Preferably the panel-form member is constructed generally in accordance with the teachings in International patent application WO97/09842 and counterpart U.S. application Ser. No. 08/707,012, filed Sep. 3, 1996, and is thus a distributed mode device.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    Examples that embody the best mode for carrying out the invention are described in detail below and are diagrammatically illustrated in the accompanying drawings, in which:  
         [0019]    [0019]FIG. 1A is a plan and of a first embodiment of resonant panel-form loudspeaker drive unit according to the invention;  
         [0020]    [0020]FIG. 1B is a sectional view of the drive unit of FIG. 1A taken along line lB-lB;  
         [0021]    [0021]FIG. 2A is an enlarged sectional detail of the drive unit of FIG. 1B;  
         [0022]    [0022]FIG. 2B is a scrap view of a detail in FIG. 2A;  
         [0023]    [0023]FIG. 3 is a scrap sectional view showing a fitting location detail;  
         [0024]    [0024]FIG. 4 is a scrap sectional detail view similar to that of FIG. 2A and showing a variant; and  
         [0025]    [0025]FIG. 5 is a cross-sectional side view of a second embodiment of loudspeaker drive unit.  
     
    
     DETAILED DESCRIPTION  
       [0026]    In FIGS. 1A and 1B, a loudspeaker drive unit  10  comprises a shallow generally rectangular frame or basket  11  for a rectangular distributed mode acoustic panel  12  generally as taught in WO97/09842 and U.S. Ser. No. 08/707,012. The panel  12  is suspended in the frame  11  by way of a resilient suspension  13  extending round the edge of the panel  12 . In this embodiment the frame  11  has a perforated base lB, and sides  11 S terminated by upper and lower outwardly extending flanges  11 U,  11 L. Internally the frame  11  is formed with an interior shelf  11 P. The acoustic panel  12  has inertial vibration exciters  14 A,  14 B mounted thereon to impart bending wave energy into the panel.  
         [0027]    Turning to FIGS. 2A and 2B, the resilient suspension  13  comprises relatively higher and lower compliance parts  21  and  22  which may be of an elastomer and of a rigid plastics, respectively. Lower compliance part  22  is in the form of a continuous ring-like rigid support member spaced from the edges of the panel  12 . The higher compliance part  21  connects the rigid support member  22  and the panel edge  12 E, and as shown is moulded to embrace both the member  22  at  21 A and at  21 B,C and the panel edge  12 E at  21 X and  21 Y. The suspension part  21  has an intermediate web  21 W contributing substantially to achieving the desired overall compliance of the panel suspension.  
         [0028]    It is preferred and advantageous for the web  21 W to have slight angling as moulded, as shown in dotted lines at  21 Z in FIG. 2B, thus canting the suspension support  22 , as shown in dotted lines at  22 Z. Recovery of this angling  21 Z, shown as more or less total but not necessarily so in FIG. 2A, when the suspension  13  is located in the frame  11  assists in achieving the desired intimacy of contact, if necessary with sealing, between the suspension and the frame with some element of compression of the part  21 A. The suspension  13  may be located in the frame  11  on brackets  23  provided on the shelf lip.  
         [0029]    Deformability of the material of the part  21  of the suspension assists self-locating assembly in the frame and its resilience assists retention after assembly, though the additional use of an adhesive or clamping is possible.  
         [0030]    The association of the parts  21  and  22  is conveniently achieved in a moulding operation which simultaneously incorporates the support  22  into the part  21  and which moulds the part  21  onto the panel edge  12 E to form a strong bond therebetween.  
         [0031]    As shown in FIG. 3, it can be useful to have mechanical latching between the suspension  13  and the frame, e.g. by forming projections  21 M formed on the suspension part  21 A and recesses  11 H through the frame sides  11 S. The projections  21 M are shown as wholly in the part  21 A, but could follow corresponding projections on the support  22 . In practice it may be preferred to form the suspension  13  with indentations and to form corresponding projections on the frame  11 .  
         [0032]    Turning to FIG. 4, it is noted that thinning of the suspension web  21 W is shown, typically to as little as 0.25 millimeter or less. Such thickness is, of course, readily adjustable by way of mould inserts. Desired air leakage may be afforded by slits (not shown) in the web  21 W and/or by full height interruptions (not shown) of the part  21 A, either or both also readily achieved by mould inserts as may be desired. FIG. 4 also has dashed indication of optional edge or near-edge rebating of the panel  12  to form groove  12 G on its underside and into which the suspension  13  can be moulded at  21 K.  
         [0033]    [0033]FIG. 5 shows an embodiment of resonant panel-form loudspeaker drive unit  10  generally in accordance with the teaching in WO97/09842 and U.S. Ser. No. 08/707,012, and in which a resonant panel-form member  12 , with an attached exciter  14 , is mounted in a shallow enclosure  24  to contain acoustic radiation from the rear face of the panel, the arrangement being such that the panel suspension  13  and an acoustic absorber  25  in the enclosure  24  are integrated. Soft plastics foam used as the acoustic absorber  25  is compressed to the required thickness round the perimeter of the panel to form the flexible suspension  13  whereas the central region of the foam plastics filling the enclosure  24  remains uncompressed allowing it to form an effective acoustic absorber.  
         [0034]    A manufacturing process for this unitary absorber/suspension is as follows:  
         [0035]    1. Apply a layer of adhesive (e.g. epoxy, acrylic or cyanoacrylate) around the perimeter of the panel  12 .  
         [0036]    2. Cut a piece of uncompressed foam  25  to the required size.  
         [0037]    3. Position the foam  25  in one half of the press tool and the panel  12  in the opposite half.  
         [0038]    4. Close the press and apply heat and pressure suitable for the foam material to compress to form a suspension  13  at its margin and the adhesive to cure and form an effective bond between panel and suspension material.  
         [0039]    5. Remove component from press tool.  
         [0040]    6. The component comprising the panel, acoustic absorber and suspension can then for example be adhesively bonded or mechanically clamped to the back box or enclosure  24  to form a complete unit.  
         [0041]    The embodiment of FIG. 5 thus provides an arrangement whereby the foam surround and acoustic absorber are incorporated into a single item which can be easily formed and bonded to the panel in a simple press tool. This new design simplifies the manufacturing process for a DML panel used in a closed back loudspeaker design.