Background of the Art The application of thin polymer piezoelectric films, such as polyvinylidene fluoride ("PVDF") to practical transducers has been hindered by the unusual mechanical characteristics of the films compared with conventional piezoelectric materials and the forms in which they have been available. That is, the thinness and the low elastic modulus of the films present new problems in transducer structure, while at the same time these same characteristics, combined with the low mass/area ratios of the films, offer the potential for greatly improved transducer performance in several areas of application.

In applying piezoelectric films to use in electromechanical or electroacoustic transducers, the recent art has arranged the film in a primitive shell configuration, such as a cylinder or a portion of a cylinder, to transform between (1) strain in the film along the uniaxial direction (which corresponds to the largest piezoelectric effect) and tangent to the film surface, and (2) the motion normal to the film surface necessary if direct electroacoustic transduction and the accompanying low mass/area ratio are to be achieved.

In a cylindrical shell, for example, an acoustic pressure difference between the surfaces of the piezoelectric film is supported by the arch of the cylindrical form and is transformed in part into stress and strain tangential to the film and in the uniaxial direction. Because of the electromechanical coupling coefficient k of the film, a signal voltage is generated between the electrodes on the film. Conversely, if an electrical signal is applied to the electrodes, strain is generated by the piezoelectric effect in the uniaxial direction, and the cylinder form of the film changes by deflections normal to its surface, resulting in the output of acoustical energy.

However if the film is very thin and its elastic modulus is low, elastic instability can set in at very low pressure differences, resulting in unacceptable harmonic distortion, failure of the frequency response to be approximately independent of signal level, and lack of reproducibility of performance characteristics in general. For example, an acoustic overload can damage or change the form of the film even to the point of reversing its curvature. For these reasons, the level of elastic stability attainable with this configuration is insufficient for practical use.

In addition, when a voltage is applied to two electrodes of a curved piezoelectric film, the piezoelectric effect gives rise to a variation in the length (or area) of the piezoelectric film. This length variation is substantially linear with the applied voltage. However, owing to the geometry of the piezoelectric film, the conversion of this length, in a direction perpendicular (normal) to the surface of the piezoelectric film, appears to be non-linear, the resultant deformation being dependent on the geometry of the piezoelectric film surface and the amplitude of the length variation (or area variation) of the piezoelectric film. Thus, the geometry of the piezoelectric film is responsible for distortions occurring during conversion of electrical signals into acoustic signals and vice versa.

SUMMARY OF THE INVENTION The present invention provides an electro-acoustic transducer including a substrate and an active element placed on the substrate. The active element forms a catenary curve or a hyperbolic curve to define a sealed chamber between the active element and the substrate. The active element includes a piezoelectric material, such as a polyvinylidene fluoride material. In one embodiment, the substrate further comprises at least one cavity wherein the active element is placed over the at least one cavity to form the sealed chamber. The at least one cavity is, for example, of a substantially concave shape and the catenary curve is formed by the active element at each cavity. A diaphragm is for example bonded to the piezoelectric material.

Alternatively, a diaphragm having a standoff ledge is used to provide the cavity and the active element is placed over the diaphragm.

In another embodiment, at least two active elements, including a first active element and a second active element, are placed on the substrate to provide for discrete multiple transducers. Each of the active elements form catenary curves or hyperbolic curves to define a sealed chamber between the first active element and the substrate and between the second active element and the first active element.

Examples of the more important features of the invention have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS For detailed understanding ofthe present invention, reference should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals: Figure 1 is a side view showing an embodiment of the transducer according to the present invention; Figures 2A-2C are cross sectional views of Figure 1 taken along line A-A, wherein Figure 2B is an exploded cross sectional view; Figure 3 is a top view showing another embodiment of the transducer according to the present invention; Figure 4 is a cross sectional view of Figure 3 taken along line B-B; Figure 5 is a bottom view of still another embodiment of the transducer according to the present invention; Figure 6 is a cross sectional view of Figure 5 taken along line C-C; and Figures 7A-7B are cross sectional views of an embodiment of the transducer according to the present invention wherein multiple discrete active elements are stacked.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The electro-acoustic transducer of the present invention, when excited with an electrical signal of any form or placed in a dynamic pressure field, correspondingly transmits acoustic energy or, conversely, when excited with a form of acoustic energy transfers the acoustic energy into electrical energy, accurately reproducing the waveform in domain.

Referring to Figures 1-4, the electro-acoustic transducer 90 according to the present invention is illustrated including a substrate 100 having at least one active element 104 including a thin sheet or film of a pre-formed, flexible, bi-polar piezoelectric material 114, such as, preferably, a polyvinylidene fluoride ("PVDF").

The thickness of the PVDF material is preferably in the order of 28 microns, although other thicknesses may be used. The substrate 100 includes at least one cavity 106 of a desired shape and depth and is bounded by shoulders 108. The active element 104 forms a catenary curve across the at least one cavity 106 to define a sealed chamber 110 between the active element 104 and the surface 102 of the substrate 100. The substrate 100 is, for example, made of an aluminum material, a carbon fiber material, or other suitable stiff material. The substrate 100 can include any of a variety of shapes, such as, for example, a cylindrical shape (Figures 1 and 2A-2C), or a flat shape (Figures 3 and 4).

A plurality of cavities 106 of a substantially concave shape and bounded by shoulders 108 are included on a surface 102 of the substrate 100. The active element 104 forms a catenary curve across the plurality of cavities 106 to define a plurality of sealed chambers 110 between the active element 104 and the surface 102 of the substrate 100. The cavities 106 are positioned relative to each other and the active element 104 is, for example, hermetically sealed at the shoulders 108 by a suitable means such as an adhesive or a clamp, placed around the entire periphery of each cavity 106 to form an enclosed or sealed chamber 110 between the active element 104 and the surface 102 of the substrate 100. The transducer 90 includes, for example, unpressurized air in the sealed chamber 110.

In the embodiment illustrated in Figure 2A, the plurality of cavities 106 are formed on the surface 102 of the substrate 100. In another embodiment, illustrated in Figure 2C, a compliant diaphragm 112, having a standoff ledge for forming the shoulders 108, also forms the plurality of cavities 106 and is bonded to the piezoelectric material 114. In the embodiment illustrated in Figure 2B, a compliant diaphragm 112, pre-formed in a catenary curve over each cavity 106, is bonded to the piezoelectric material 114. Additionally in each embodiment, a protective layer over the piezoelectric material 114 can be used to protect the active element 104.

Preferably, the size of the active element 104 is sufficient to cover the entire surface 102 of the substrate 100. The active element 104 is placed on the substrate 100 over the cavities 106 and is hermetically sealed at the shoulders 108 by a suitable means, such as an adhesive or a clamp, placed around the entire periphery of the shoulders 108 to form the sealed chamber 110 between the active element 104 and the substrate 100.

Electrical connectors 116 and 118 are coupled to the piezoelectric material 114 using, for example, a piezoelectric film tab 120. The electrical connectors 116 and 118 receive or transmit electrical signals from or to the piezoelectric material 114. The application of an electrical signal across the electrical connectors 116 and 118 results in a change in length in the piezoelectric material 114 or film as well as a change in the thickness of the piezoelectric material 114. The change in length of the piezoelectric material 114 when suspended across a fixed length results in a change in volume in the medium in which the transducer 90 operates. Conversely, a change in the pressure field in which the traducer 90 operates produces a change in the length of the piezoelectric material 114 or film, resulting in an electrical signal produced when the capacitance of the bi-polar piezoelectric material 114 is shorted across a known resistance.

Figures 5 and 6 illustrate another embodiment of a transducer 130 according to the present invention wherein the active element 104, including the piezoelectric material 114, forms a hyperbolic curve to define the sealed chamber 110 between the active element 104 and the substrate 100. In the embodiment illustrated, the substrate 100 has a substantially hyperbolic curved shape and the active element 104 is placed over an inner surface 126 of the substrate 100 to form the sealed chamber 110. The electrical connectors 116 and 118 are coupled to the piezoelectric material 114 using, for example, a piezoelectric film tab 120. The electrical connectors 116 and 118 receive or transmit electrical signals from or to the piezoelectric material 114. In the embodiment illustrated in Figures 5 and 6, the medium in which the transducer 130 operates is allowed to flood both sides of the active element 104.

The shape of the piezoelectric material 114 across the sealed chamber 110 is important in the performance of the transducer 90 of the present invention. The piezoelectric material 114 is shaped substantially in the form of a catenary curve or a hyperbolic curve. The apex 124 of the catenary curve or hyperbolic curve of the piezoelectric material 114 provides for a focused direction for the change in length and thickness or strain of the piezoelectric material 114. The smaller the angle of the apex 124 of the catenary curve shape or the hyperbolic curve shape of the piezoelectric material 114, the more directionally focused the change of the piezoelectric material 114 becomes, providing for less distortions in converting acoustic signals into electrical signals and vise versa. The catenary curve and the hyperbolic curve shaped active elements of the present invention provide for greater signal reliability than the typical cylinder shaped active element transducer since, for a given surface area, the volume displaced by the catenary curve and the hyperbolic curve shaped active elements is greater than that of the cylinder shaped active element.

The use of the active element formed into a catenary curve or a hyperbolic curve can also be used to provide for multiple discrete transducers on a substrate as illustrated in Figures 7A and 7B by stacking the active elements. For example, at least two active elements, a first active element 132 and a second active element 134, are placed on the substrate 100. Each of the first and second active elements 132 and 134, respectively, form a catenary curve or a hyperbolic curve (not shown) to define a first sealed chamber 136 between the first active element 132 and the substrate 100, and a second sealed chamber 138 between the first active element 132 and the second active element 134. Electrical connectors (not shown) are coupled to the first and second active elements 132 and 134, respectively, to provide for multi-channel data acquisition. Alternatively, the output response from the first active element 132 is combined with the output response from the second active element 134 to provide for a combined output response having a reduced distortion.

While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly it is to be understood that the present invention has been described by way of illustrations and not limitations.