Abstract:
The invention consists of an integral transducer for a sound radiating diaphragm which may consist of a piezoelectric material, a substrate or a spacer and electrical connector means for a wire harness or other electrical connection. The substrate is used to enhance the motion of the piezoelectric element by spacing the piezoelectric element from the diaphragm. The substrate is larger in area than the piezoelectric element. The transducer system acts to impart motion to a diaphragm. The transducer comprises a piezoelectric element subject to displacement by applied electric potential that has a top side, an under side and an outer perimeter; a substrate that is joined to the underside of the piezoelectric element, and means to apply electric potential to the piezoelectric element.

Description:
REFERENCE TO EARLIER FILED APPLICATION 
     This application is a continuation-in-part of earlier filed application Ser. No. 08/554,049, filed Nov. 6, 1995, now U.S. Pat. No. 5,838,805. 
    
    
     BACKGROUND ART 
     Loudspeakers employing a piezoelectric transducer capable of propagating surface acoustic waves to drive a diaphragm have been proposed as an alternative to moving coil loudspeakers. Such a device was described by Martin in U.S. Pat. No. 4,368,401 and later Takaya in U.S. Pat. No. 4,439,640. Both inventions dealt with attaching a disc shaped piezo to a diaphragm. Martin&#39;s device used a thick glue layer (10 to 50% of the carrier plate thickness) between a carrier plate and the piezo ceramic. The adhesive layer served to attenuate resonance. Any displacement in the piezoelectric is directly related to the applied electrical potential. 
     One disadvantage to utilizing transducers employing a piezoelectric element is that such materials are very costly and that a substantial expense would be involved to utilize a sufficiently sized piezo electric material to drive large diaphragms. Another disadvantage is that piezoelectric materials are as a rule comparatively brittle and do not deform well. Consequently, if one attempts to have piezoelectric materials conform to the curvature of an irregularly shaped diaphragm they may shatter or break, resulting in necessary expense. 
     Therefore it would be advantageous to attempt to reduce the cost of using piezoelectric elements in a transducer and to adapt them is such a way to a diaphragm so as to reduce the possibility of having the piezo be damaged. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention involves a transducer which is utilized to drive a diaphragm, in particular a comparatively large diaphragm. The transducer is comprised of a piezoelectric layer, (or a layer of some other material covered with a layer of piezo-electric material) capable of propagating flexural acoustic waves, which piezoelectric material typically is a flat layer placed on top of a substrate layer which has essentially the same degree of rigidity (as measured by its Young&#39;s modulus and thickness) as the piezoelectric electric material, but has more rigidity than the diaphragm material so that when the substrate material is distorted by the motion of the piezoelectric material the diaphragm will move accordingly. In this regard, the thickness of the substrate may be optimized to the properties of the piezoelectric material. The substrate will be larger in surface area than the piezoelectric element in order to impart motion to a larger area of the diaphragm. The invention also comprises utilizing multiple transducers on a single diaphragm to extend the frequency range. In this case larger transducers would be used to produce low frequencies and smaller transducers would be used to produce higher frequencies. The use of multiple transducers serves to increase the motion imparted to the diaphragm and, hence, the volume or loudness of the sound. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates one embodiment of a transducer according to the present invention. 
     FIG. 2 illustrates possible shapes of piezoelectric elements utilized in the present invention. 
     FIG. 3 illustrates another embodiment of a transducer of the present invention in which the piezoelectric element is utilized in conjunction with motion couplers. 
     FIG. 4 illustrates a further embodiment of a transducer of the present invention in which the piezoelectric element is shown as being utilized in conjunction with motion couplers in another manner. 
     FIG. 5 illustrates another embodiment of the present invention in which two transducers are connected to each other via a mechanical connection. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates one embodiment of transducer design  10  of the present invention. A piezoelectric element  11  is placed on top of a substrate  12  which has a larger surface area than the piezoelectric layer. The piezoelectric layer may be bonded to the substrate by any suitable material. 
     The substrate will have a larger surface area than the piezoelectric element in order to impart motion to a larger area of the diaphragm than if the substrate alone was attached to the diaphragm. This will result in cost savings since lesser amounts of the costly piezoelectric material need be utilized. The substrate will have a rigidity no greater than the rigidity of the piezoelectric element but greater than the rigidity of a diaphragm to which the substrate will be attached. 
     Many materials may be advantageously be used for the substrate. These materials include steel, aluminum, brass, copper, and other metals, plastics, composite materials, etc. Brass is a preferred material for the substrate because of its low cost, environmental resistance, ease of bondability and because its Young&#39;s modulus of elasticity is similar to that of certain piezoelectric materials, such as PZT (lead-zircon-titanate). The transducer will also include means to apply electric potential to the piezoelectric element, which in the depicted embodiment comprises a connector  13  for a wire harness which is optionally attached to and extends from the edge  14  of substrate  12 . FIG. 1 also illustrates electrical leads  15  from the piezoelectric element  11  to connector  13 . 
     Substrate  12  will be attached directly, on the side opposite to the side that is attached the piezo element, to a diaphragm (not shown). The substrate and perhaps the piezoelectric element may be preformed, or otherwise configured, to conform to the curvature, or other shape, of the sound radiating diaphragm to which the substrate is attached. In a preferred embodiment, for maximum efficiency and minimum distortion both the mechanical and electrical impedances of the transducer should be matched. That is, the mechanical impedance of the transducer should be matched to that of the sound radiating diaphragm while the electrical impedance of the amplifier that drives the transducer should be matched to that of the transducer when it is radiating sound. In another embodiment, the transducer may also be covered with a conformal coating to provided electrical insulation and environmental resistance. In addition, the piezo element may consist of two or more layers arranged on top of one another and electrically connected in an alternating fashion to enhance the motion of the piezoelectric element. 
     FIG. 2 illustrates examples of possible shapes for the piezoelectric element. The element may be made in a variety of shapes, such as square, rectangular and round. Irregular shapes may also be used to minimize resonances on the transducer itself and/or to extend the frequency range. To accomplish the latter goal, elliptical, semi-elliptical, truncated rectangular and truncated square shapes, etc. may be used. 
     FIG. 3 illustrates another embodiment of a transducer of the present invention in which piezoelectric element  20 , which in the illustration has a rectangular shape (although any other shaped piezoelectric element can be utilized in this embodiment) is coupled on, most preferably, all its sides  21 , 22 ,  23  and  24  with motion couplers  25 ,  26 ,  27 ,  28  to further ensure the coupling of the motion of the piezoelectric element to substrate  29  by provide a coupling transition to the substrate, to which piezoelement  20  is bonded and positioned on top of, in all directions of movement. If desired, the motion couplers may be attached only to certain sides of the piezoelectric element. By providing a coupling transition to the substrate it will be further insured that the motion of the piezoelectric element will be coupled to the sound radiating diaphragm (not shown). This is accomplished by tightly coupling, preferably, both the transverse and lateral motions of the piezoelectric element, first to the motion couplers, with the end result that the motion will thereafter be passed through the substrate to the sound radiating diaphragm. The motion couplers will also be attached to the substrate. It has been discovered that the use of the motion couplers will increase the loudness of the sound produced by the sound radiating diaphragm and extend the bass sound produced to lower frequencies. 
     FIG. 4 illustrates a further embodiment of a transducer of the present invention in which the piezoelectric element  41  is shown as being utilized in conjunction with motion couplers in another manner. In this embodiment, the outer perimeter  42  of piezoelectric element  41  is completely surrounded by a single motion coupling plate  43 . Motion coupling plate  43  has a hole, which in the depicted embodiment is in its center, which is cut out in order to accommodate the presence of piezoelectric element  41 . Piezoelectric element  41  must fit the hole in motion coupling plate  43  very snugly so that the piezoelectric element  41  will be bonded at its edges  42  to the edges of the hole in motion coupling plate  43 . In general, motion coupling plate  43  should be of the same thickness as the piezoelectric element  41 . Piezoelectric element  41  and motion coupling plate  43  are both bonded to the underlying substrate  45 . The material of the motion coupling plate  43  and the substrate  45  may be of the same material or different materials such that the motion of the piezoelectric element  41  is not substantially restricted. One advantage of this concept is that less parts are involved and hence the transducer is more readily adaptable to being mass produced. 
     The transducer of the present invention will of course, when attached to a diaphragm, form a loudspeaker. FIG. 5 illustrates another embodiment of the present invention in which more than one integral transducer, in this case a pair of transducers  51  and  52 , which are constructed in accordance with the present invention, are attached to the same diaphragm  53 . It has been discovered that using more than one transducer in conjunction with the same diaphragm will create a stereo sound image, and will also increase the loudness and/or extend the frequency range. To create such an effect the two (or more) transducers should be separated from one another by a stereo-sound effect producing distance such that a stereo sound effect can be produced when the transducers are electronically driven by a stereo sound signal. If the two transducers are too close to one another a stereo sound effect will not be apparent. The preferred distance by which the transducers should be separated will depend on the size, material of construction and configuration of the speaker. FIG. 5 illustrates a further embodiment of the present invention, in which transducers  51  and  52  are connected to each other via a mechanical connector  54 . It has been shown that, when such a mechanical connection is employed, the quality of the stereo effect produced will be enhanced and the overall quality and volume of the sound will be improved. In one embodiment tested, the mechanical connector was a metal beam of 0.02 inch thick sheet steel and was one inch wide. The length of the mechanical connector should be such that some outward force is exerted on the integral transducers. Of course, other materials of construction and/or other dimensions of mechanical connector  54  may be utilized. In another embodiment, when more than one transducer is utilized in conjunction with a particular diaphragm, the mechanical connector may be an integral part of the transducers. For example, the substrate may be made continuous between the transducers to form the mechanical connection. Alternatively, the motion couplers described above may be formed into an integral mechanical connection. For larger diaphragms, more than two transducers may be so utilized. When more than two transducers are utilized it is preferred that they be utilized in pairs, preferably with the transducers in each pair being connected to each other by a mechanical connector. 
     As indicated, the piezoelectric material typically is in the form of a plate that is placed on top of a substrate plate which has essentially the same degree of rigidity (as measured by its Young&#39;s modulus and thickness) as the piezoelectric electric material. In this regard, attention should be paid to the extension stiffness (K), represented by K=EA/L=wt/l, wherein E=Young&#39;s modulus of elasticity; A=cross sectional area of the plate; l=length of the plate; w=width of the plate; t=thickness of the plate. For a unit length and width of a plate, the extensional stiffness becomes K=Et. 
     Therefore, there are two parameters, E=Young&#39;s modulus of elasticity; and t=thickness of the layer, that may be used to match the stiffness or rigidity of the piezoelectric material with those of the substrate and motion coupler layers. To couple the motion of the piezoelectric material to the substrate and motion coupler layers the stiffness of all layers (or just the piezoelectric element and substrate when motion couplers are not utilized) should be substantially the same and certainly with an order of magnitude. That is, the extensional stiffness of the piezoelectric material under electric stimulation should be substantially equal to the extensional stiffness of the substrate and (when utilized) the extensional stiffness of the motion couplers. 
     The piezo transducers of the present invention can be utilized as part of a signal reproducing device in which conventional electronics (not depicted) may be utilized to support and/or drive the transducers. Included among such conventional electronics are signal generating devices (i.e. input devices) such as tape players, phonograph players, radios, compact disc players, telephones, video sources, and so forth; signal processing devices such as graphic equalizers, digital signal processing units, tone control, loudness controls, preamplifiers, etc. and audio amplifiers. 
     The forgoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and, accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.