Abstract:
An ultra low frequency transducer or subwoofer for automotive speaker systems with a polygonal, concave diaphragm. The periphery preferably is sized to be substantially coextensive with the front of the speaker housing or cabinet. In this way, air displacement in the subwoofer is maximized for a given size of enclosure, and thus so is the loudness of the speaker. The transducer typically includes a frame or basket with a polygonal front for supporting the periphery of the diaphragm. The preferred diaphragm includes a convex surround with pleated corners. For increased durability, trusses are formed in the diaphragm.

Description:
This application claims priority to U.S. provisional application Ser. No. 60/160,959, filed Oct. 22, 1999, entitled “Ultra Low Frequency Transducer,” now expired. This application is a continuation of co-pending application Ser. No. 10/404,468, filed Apr. 1, 2003, entitled “Ultra Low Frequency Transducer and Loud Speaker Comprising Same,” which is a continuation of application Ser. No. 09/610,600, filed Jul. 5, 2000, entitled “Ultra Low Frequency Transducer and Loud Speaker Comprising Same,” now U.S. Pat. No. 6,611,604. The contents of all these prior applications and patent are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to ultra low frequency transducers for use as subwoofers for automotive speaker systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a right frontal perspective view of the loud speaker of the present invention. 
         FIG. 2  is a front elevational view of the ultra low frequency transducer. 
         FIG. 3  is a longitudinal sectional view of the ultra low frequency transducer of the loud speaker of  FIG. 1 . 
         FIG. 4  is a rear perspective view of the basket of the ultra low frequency transducer. 
     
    
    
     SUMMARY OF THE INVENTION 
     The present invention further comprises a loud speaker comprising a driver assembly supported inside an enclosure. In this loud speaker, the enclosure has a square diaphragm opening, and the diaphragm has a square periphery shaped to conform to the opening in the enclosure. There is a flexible surround between the periphery of the diaphragm and diaphragm opening in the enclosure. The driver assembly is supported in the enclosure and comprising a permanent magnet and a reciprocally supported voice coil, the voice coil connectable to a signal source, whereby the driver assembly is adapted to convert signals from the signal source into reciprocating axial motion of the voice coil, the diaphragm is configured to produce effectively only ultra low frequency sounds in the range of from about 20 Hz to about 80 Hz in response to signals in that frequency received by the voice coil. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In automotive speakers, space is extremely limited. The need to minimize the size of the speaker is constantly at battle with the need to maximize sound and performance of the speakers. This is especially true in the case of subwoofers, where ultra low frequency sounds are produced. 
     Speaker loudness is a function of the volume of air displaced by movement of the diaphragm, which is in turn a function of the stroke or excursion of the diaphragm and its diameter. The lower range of the typical stereo speaker or woofer only operates down to about 40-80 Hz. However, with modern recording techniques, musical recordings generate much lower signals to as low as 20 Hz or below. There is a need for a subwoofer that can satisfactorily reproduce these ultra low range signals. 
     The present invention is directed to an ultra low frequency transducer, that is, a transducer capable of effectively reproducing only about the lowest two octaves of music, from about 20 Hz to about 80 Hz. The transducer of the present invention has a concave diaphragm (or cone) with a rectangular or square periphery. The rectangular shape maximizes the diaphragm area relative to the enclosure, which is also usually rectangular, and thus the air displacement in the speaker. 
     Turning now to the drawings in general and to  FIG. 1  in particular, there is shown therein a loud speaker in accordance with the present invention and designated generally by the reference numeral  10 . The speaker  10  comprises an enclosure  12  with a frontal opening  14 . The enclosure houses a transducer  16  with a diaphragm  18  having a periphery  20  sized to be substantially coextensive with the opening  14 . As best shown in  FIG. 2 , the frontal opening  14  of the enclosure preferably is rectangular and more preferably the opening is square. Similarly, the periphery  20  of the diaphragm  18  is rectangular and more preferably is square and is about the same size as the frontal opening  14 . As used herein, “rectangular” means a parallelogram with a right angle, and square means a rectangle with four equal sides. 
     With reference now also to  FIGS. 3 and 4 , the construction of the preferred transducer will be described. As indicated, and as best seen in  FIG. 3 , the diaphragm  18  is concave, that is, the inner aspect or body portion  22  is generally dome-shaped. As used herein, “concave” is relative to the front of the enclosure and denotes a shape in cross-section that has a depth. Thus, “concave” would include conical or frusto-conical. 
     The diaphragm  18  preferably is made of a special talc-filled polypropylene material that is very stiff and highly internally damped. The internal damping controls standing waves on the diaphragm, while the stiffness suppresses flex during large excursions. The preferred diaphragm is resistant to pollution, sunlight and moisture. 
     The diaphragm in a subwoofer is subject to a great deal of stress because of the high excursion. For this purpose, the diaphragm  18  is formed with reinforcing trusses, one of which is designated by the reference numeral  26  ( FIG. 2 ). Preferably, the trusses are contours formed into the diaphragm when it is made. 
     In the preferred embodiment, the diaphragm  18  includes a surround  30  extending around the periphery  20 . The surround  30  conforms to the shape of the periphery of the diaphragm  18  and therefore also is rectangular. The ideal configuration of the surround  30  is convex in cross-section. As used herein, “convex” is relative to the front of the speaker. The surround  30  preferably is made of a flexible, but shape-sustaining material, and usually is not the same as the more rigid material used for the main body  22  of the diaphragm  18 . 
     In the preferred embodiment, the diaphragm edge is stepped down to form a platform or flange  32  to which the surround is glued. The stepped down flange  32  adds to the rigidity of the diaphragm  18 , and serves as a convenient point of attachment, usually by stitching, for the inner edge of the surround  30 . 
     To improve the performance and durability of the surround  30 , the corners are provided with one or more radially positioned ribs or pleats, one of which is designated by the reference numeral  34 . While the pleats  34  shown are outwardly extending, the pleats alternately may be depending grooves. Further, the number, spacing, width, and cross-sectional shape of the pleats may be varied. The pleats allow the surround to collapse and expand uniformly as the diaphragm  18  reciprocates. This reduces distortion and buckling of the surround  30 , particularly at the corners. 
     With continued reference to  FIGS. 3 and 4 , the transducer  16  comprises a frame or basket  36  that supports the components of the transducer. The basket  36  has a front  38  and a rear  40  connected by a plurality of struts  41  therebetween. The front  38  preferably is provided with a planar edge  42  having a recess  44  adapted to receive an outer flange  46  on the surround  30 . In this way, the periphery  20  of the diaphragm  18 , by means of the flange  46  on the surround  30 , is supported on the front  38  of the basket  36 . Preferably, the basket is made of die cast aluminum. The cast aluminum basket is more rigid than stamped steel. 
     More particularly, in a manner that will be described hereafter, the diaphragm  18  is thereby supported on the basket  36  for axial, reciprocal movement inside the enclosure  12 . Though not shown herein, a gasket may be attached over the flange  46  of the surround  30  and under the front edge  42  of the basket  36 . A preferred gasket material is Rubatex brand, which is composed of foam rubber, chopped and glued, to provide an excellent seal to many surfaces. 
     As best seen in  FIG. 3 , the center portion  48  of the diaphragm  18  preferably is frusto-conically shaped and extends inwardly. The diaphragm  18  is operatively associated with a suitable driver assembly  50 , yet to be described, also supported by the basket  36 . To that end, the open edge of the diaphragm center  48  is attached circumferentially to the closed end of a voice coil former  52  in a known manner. A Kapton brand voice coil former is preferred because it is lightweight, strong, and retains its shape at high temperatures. 
     While the surround  30  attached to the front  38  of the basket  36  is the upper support for the moving system, a spider  54  preferably provides the lower support. A progressive spider may be used to reduce stress on the glue joint at the former  52 , and force it to the outer regions. The spider  54  also provides soft mechanical limiting and centering as it extends radially between the former  52  and adjacent annular section  56  of the basket  36 . 
     A voice coil  58  is supported on the former  52  in a known manner. A longer voice coil  58  is desired as it gives the driver greater excursion capability to move more air and produce more low bass. A preferred coil for the present invention comprises a 4-layer round wire. The wire insulation and coil adhesives are designed to handle the heat associated with high power operation. 
     The coil  58  is attached by tinsel leads  59  which connect to terminals  60  supported on the annular section  56  of the basket  36 . The external leads  61  connect the terminals  60  to a signal source  61 , such as the radio or disk player in an automobile (see  FIG. 1 ). 
     As the tinsel leads  59  must absorb the abuse of motion as well as transfer large currents to the voice coil  58 , it will be desirable to use two braided leads. An annular cone brace  63  may be included to provide additional support for the cone, which must also endure repetitive wide excursions. 
     Preferably the transducer  16  comprises an inverted structural dome or dust cap  64  that extends across the body portion  20  of the diaphragm  18  and encloses the voice coil former  52 . Integration of the concave body  22  of the diaphragm  18  with the concave dust cap  62  in a locking action is advantageous as it increases the rigidity of the diaphragm to further inhibit diaphragm flex. This integration creates a piston action that is more effective, especially at high sound pressure levels (SPL). 
     The voice coil former  52  is telescopically received over the pole piece  66 . The pole piece  66  preferably is tubular, the hollow center forming a pole piece vent  68 . The vent  68  will enhance the speaker&#39;s compliance by relieving pressure from under the dust cap  64 . Otherwise, this pressure might impede diaphragm movement at low frequencies and generate noise as air rushes through the voice coil gap. This is especially important in long throw drivers. A screen  69  may be placed over the vent hole to keep foreign material from entering the inner motor structure. 
     The transducer preferably comprises a back plate  70  in form of an annular ring. More preferably, the pole piece  66  and the back plate  70  are integrally formed in one piece of metal to improve magnetic field strength as well as heat transfer. It is advantageous to dye the back plate  70  and pole piece  66  black to increase heat transfer. In the preferred practice of this invention, the back plate  70  is formed with a “bumped” portion  72  to allow the voice coil former  52  maximum movement without ever contacting the back plate. The lower windings can be damaged and cause driver failure if contact occurs. 
     A permanent magnet  74  is supported between the back plate  70  and a top plate  76 . A suitable magnet is a ceramic ferrite magnet. The magnet  74  should be optimized for the performance of each driver assembly  50 . The size of the magnet does not always relate to the amount of bass produced. Too large of a magnet structure can over dampen and restrict diaphragm motion, reducing efficiency and low response characteristics. 
     The top plate  76  engages the lower surface of the rear end  78  of the basket  36 . Thus, the pole piece  66 , back plate  70 , magnet  74  and top plate  76  are rigidly attached to the basket  36 , while the voice coil former  52  and attached diaphragm  18  are supported in the basket for axial, reciprocal movement. A soft rubberized magnet boot  79  is desirable in many applications, as it will protect the magnet  74  from chipping and adds to the appearance of the driver assembly  50 . 
     The annular surface  80  of the magnet  74  and the annular surface  82  of the top plate  76  are spaced radially from the voice coil  58  providing a gap  84 . When the coil  58  is energized by a signal from the signal source  62 , a magnetic field is created in the gap  84  between the voice coil former  52  and the magnet  74  causing the former and the diaphragm  18  linked to it to reciprocate axially and produce sound. The top plate  76  focuses the magnetic field in the gap  84 . The top plate  76  may also be dyed black to improve heat transfer. 
     As shown in  FIG. 2 , the pole piece  66  preferably is extended above the top plate  76  to create a more uniform magnetic field to provide linear diaphragm travel. This reduces distortion caused by nonlinear movement. The longer pole also helps keep the voice coil cooler by not allowing the upper coil windings to radiate heat across to each other. This is desirable considering because heat buildup can cause speaker failure. 
     Now it will be appreciated that, in the ultra low frequency transducer of this invention, the size of the diaphragm and surround can be maximized to the size of the opening in the enclosure. This provides greater air displacement than the largest round diaphragm that would fit in the same enclosure. As a result, the diaphragm size and thus the displacement volume are increased as compared to conventional round diaphragm designs. 
     Changes can be made in the combination and arrangement of the various parts and elements described herein without departing from the spirit and scope of the invention as defined in the following claims.