Piezoelectric Polymeric Film Balloon Speaker

This invention relates to piezoelectric polymeric films and more particularly concerns such films which function as mobile and maneuverable speakers when strips or portions thereof are conformably secured to the curved surfaces of an inflated balloon, or the film itself is made to function as the inflated balloon. The metallized coating electrodes of the film are suitably connected to the output of an audio device.

Underwater acoustic transducers employing polymeric piezoelectric film materials are known. In British patent No. 2,120,902, a shell of PVDF material is provided with the usual conductive coatings on each face thereof. When an alternating current of 100 cycles per second is applied to the coatings, the shell vibrates to act as an underwater sound generator.

In U.S. Pat. No. 2,939,970, a spherical microphone assembly includes spherical outer and inner electrodes with a spherical ceramic transducer element therebetween. The assembly may also be used as a loudspeaker. The spherical configuration of the microphone assembly is similar to the balloon shaped speaker of the present invention.

In U.S. Pat. No. 4,284,921, various configurations, including hemispherical, of thermoformed piezoelectric polymeric film materials are disclosed as transducer elements for purposes of receiving and transmitting.

The existing patented devices abovementioned do not suggest the present speaker which is light in weight, maneuverable, deflatable for easy storage and transport, and sufficiently inexpensive to permit its use at entertainment centers and celebrations where tables of guests, for example, could each have an individual balloon speaker with a preferred sound volume, and where the height of the inflatable balloon, filled with helium, for example, could be easily controlled. The electrodes or metallized film coatings of the piezoelectric polymeric film are connected to the output of an audio tape player, radio receiver, phonograph amplifier, and the like, each capable of delivering an adequate signal to energize the piezo film. Conventional amplification and impedance matching devices may be interposed between the audio device and piezoelectric film.

Additionally, the piezoelectric polymeric films of the present invention may function as a highly mobile and maneuverable microphone as well as sender/receiver of ultrasonic signals for communication, surveillance, and range finding purposes, when suitably attached and connected to an inflated balloon.

FIG. 1 is a perspective view, partially diagrammatic, of an embodiment of the present invention, illustrating an inflated balloon with a helical strip of the piezoelectric film secured therearound.

FIG. 2 is a sectional view of FIG. 1 taken along line 2--2 thereof.

FIG. 3 is a view similar to FIG. 1, wherein the piezoelectric film comprises individual strips thereof.

FIGS. 4 and 5 are sectional views of FIG. 3 taken along lines 4--4 and 5--5 respectively.

FIG. 6 is a sectional view, partially diagrammatic, of another embodiment of the present invention.

FIG. 7 is a fragmentary sectional view of yet another embodiment of the present invention.

Generally, polymeric materials are non-piezoelectric. Polyvinylidene fluoride (PVDF) is approximately 50% crystalline and 50% amorphous. The principal crystalline forms of PVDF are the highly polar .beta. form and the non-polar .alpha. form. High piezo response is associated with the polar .beta. form. By carefully controlling process steps to polarize the film, including mechanical orientation and treatment in an intense electric field, a highly piezoelectric and pyroelectric film results. Such a film is commercially available under the trademark KYNAR.RTM., a product of Pennwalt Corporation, Philadelphia, PA., assignee of the present invention.

The procedure for poling is well known in the art and, in the case of dielectric polymer films, generally involves the application of a direct current voltage, e.g., 300 to 2000 kilovolts per centimeter of thickness of polymer film while first heating it to a temperature ranging between just above room temperature to just below the melting point of the film for a period of time and then, while maintaining the potential, cooling the film. Preferred systems for the continuous poling of piezoelectric (or pyroelectric) sensitive polymer film using a corona discharge to induce the piezoelectric charge are described in U.S. Pat. No. 4,392,178 and U.S. Pat. No. 4,365,283.

The invention is not limited to films made of PVDF only, and copolymers of vinylidene fluoride, and copolymerizable comonomers such as tetrafluoroethylene and trifluoroethylene, for example, may be employed.

Referring now to FIG. 1, inflated balloon 10 is provided with an helical strip of piezoelectric polymeric film material, typically PVDF, secured therearound. Balloon 10 is suitably rubber or polyester and may have a diameter approaching 1 inch, but preferably will be about 1 to 3 feet in diameter since the curvature of such sized balloons provide quality reproduction of the audio signals. The balloon need not be spherical but should provide curved surfaces.

If balloon 10 has a diameter of about 2 feet, then helical strip 12 will typically be about 1 to 3 inches wide with similar spacings between turns. It is not intended that strip 12 and spacings between turns be limited to the widths abovementioned since cost and quality considerations will normally dictate the total area of the piezoelectric PVDF film to be secured to any balloon, it being understod that the cost of the balloon speaker will rise as the amount of PVDF film used thereon increases. It should also be understood that the amplitude of the sound transmitted by the balloon film might not be sufficiently audible if the area of the film is excessively reduced.

PVDF film may be suitably secured to balloon 12 by double-sided tape, for example, a pressure-sensitive spray adhesive, and the like.

Stopper 14, typically rubber, permits balloon 10 to remain inflated.

Referring additionally to FIG. 2, the output of audio device 16, typically a tape player, radio receiver, phonograph amplifier, and the like, is capable of delivering an adequate signal to PVDF film 12 by means of metallized surface coatings 18 and 20 via conductors 22 and 24 respectively. Initially, the output of audio device 16 may be amplified, and by suitable transformer means (not shown), the impedances of the amplified output and the speaker film matched. The electromagnetic energy outputted from audio device 16 produces mechanical stresses on PVDF film 12 which, in turn, retransmits the original audio signals.

In FIGS. 3, 4 and 5, the PVDF film may be identical to PVDF film 12 of FIG. 1, but in the form of individual strips 26A through 26E, for example. Each strip 26A-26E will have its outer surface coating 28 and inner surface coating 30 electrically serially connected to its adjacent strip by means of connectors 32 and 34 respectively. Connectors 32 and 34 may comprise copper tape, Mylar with conductive ink deposited thereon to provide an electrical connection, conductive adhesives, and the like. Audio signals from the output of audio device 16 are supplied to PVDF film 26A through conductors 22 and 24. Since strips 26A-26E are serially connected, each of the strips contributes to the output of audio energy from balloon 10.

In FIG. 6, PVDF film 38 comprises the balloon, along with its metallized surface coatings 40 and 42. Stopper 14 maintains the balloon in an inflated state. Output of audio device 16 is connected to the coatings, as described above. It is to be understood that in each of the present embodiments, it may be desirable if a suitable amplifier (not shown) receives the audio output signals from amplification thereof prior to the amplified signals being connected to the film coatings. Of course, matching of impedances, as aforediscussed, may be effected after amplification but prior to the connections to the film coatings. Amplification and impedance matching means are not shown or further described herein. A skilled audio artisan could readily employ such means, if desired.

In FIG. 7, PVDF film 44 with coatings 46 and 48 is adheringly disposed interiorly balloon 10. The usual electrical connections from the audio device are made to the coatings.

Fabrication of the speaker balloons of FIGS. 6 and 7 is within the skill of the balloon manufacturing art.

The thickness of the piezoelectric polymeric film used in the present invention may range between about 6.mu. to 110.mu., and preferably 20 to 50.mu. whereas the metallized film electrode coatings will typically be about 6-8.mu. in the thickness. The coatings may be conveniently deposited on the piezoelectric polymeric film by a conventional silk screening process, for example, the silk-screening conductive ink comprising a finely divided electrically conductive metal, suitably silver, nickel or copper embedded within a polymer matrix.

The strips of FIGS. 1 and 3 may be adhered to the curved surfaces of the balloon's interior.

Balloon speakers, in accordance with the present invention, having a diameter of only about 6 inches, for example, produced faithful retransmission of the audio signals at decibel levels higher than typical normal home listening volumes.

The helical strip of film need not have equal spacings between turns; nor is it required that the individual strips have equal spacings therebetween. The strips of film may be disposed asymetrically around or within the balloon.