FR2521381A2 - Acoustic transducer using metallised piezoelectric polymer films - where two films are joined at their central zones to increase output signal without reducing resonant frequency - Google Patents

Abstract

Parent patent describes a transducer using polymer films, the edges of which are fixed in a hollow support so the film edges are separate from each other. In this addn., two films are used which each have a concave surface (a) with a flat middle zone (a1), and a convex surface (b) with a flat middle zone (b1). The two flat convex zones (a1,b1) of the two films are joined together, but the edges of the two films are kept separate by the hollow support. The result is a simple and expensive transducer.

Description

 The present invention, like main patent application No. 80 01469, relates to an acoustic transducer comprising a hollow support and a plurality of metallized piezoelectric polymer films operating as an oscillator.

Acoustic transducers using piezoelectric elements as an oscillator are known in the literature. For example American patents
No.3,832,580 and No.3,792,204 describe transducers using a single piezoelectric film, an article of
Tamura et al presented in 1978 at the conference "Acoustical Society Meeting" in Honolulu describes a pair of piezoelectric films stretched on the lower and upper surface of a cushion of polyurethane foam. U.S. Patent No. 3,832,580 describes the use of a plurality of suspended piezoelectric elements in various configurations.

 For a given AC voltage, a piezoelectric film transducer typically produces a lower amplitude than that produced, with the same voltage, by other types of transducers such as electro-dynamic transducers.

This lower voltage sensitivity can lead to an undesirably low amplitude for certain applications, such as telephone receivers, where the signal voltage is low. On the other hand, piezoelectric film transducers used as microphones or transmitters produce a lower output voltage for a given sound pressure than other types of transducers, such as electret capacitors. Such low output voltages require the use of relatively high cost gain amplifiers
The invention described in the main patent application overcomes these drawbacks by increasing the sound amplitude without the use of an electronic amplifier.

 The acoustic transducer according to the main patent application comprises a hollow support and a plurality of metallized piezoelectric polymer films functioning as an oscillator mounted at their peripheries on the hollow support so that their peripheries are separated and they are physically connected in the vicinity of their center a at least one adjacent film.

 The transducer thus obtained is simple and of a very advantageous cost price.

 The present invention proposes to further increase the level of the output signal of such a transducer without reducing its resonant frequency by extending the physical connection between two adjacent films further than the vicinity of their centers.

 The acoustic transducer according to the invention is characterized in that said films have the form of flattened truncated cone, one truncated cone being inverted with respect to the other and that said films are physically connected to the small bases of truncated cones .

 By physically connecting two adjacent films of such a transducer by a surface extending beyond the vicinity of the center, the surprising and unexpected result was obtained of increasing the level of the output signal without practically decreasing the resonant frequency of the transducer.

 According to a variant according to the invention, the acoustic transducer is designed to produce sound and the films are electrically connected in parallel with their selected polarities so that said films move in the same direction when they are electrically excited.

 According to another variant in accordance with the present invention, the transducer is designed to convert the sound into an electrical signal and the films are electrically connected with their selected polarities so that the voltage or the output current of each of said films is added. when said films oscillate.

 The accompanying drawing shows, by way of example, an embodiment of the invention.

 Figure 1 is a vertical section of an acoustic transducer according to the invention.

 Figure 2 is an electrical diagram of said transducer.

 In Figure 1 are shown the central portion 10 and the side portion 12 of a microphone. The films in the form of flattened cone trunks 14 and 16 are connected to their small base by epoxy adhesive 18 or are thermally welded and mounted at their periphery on a cylindrical support 20 between rings 22 and 24, 24 and 26 respectively. The films 14 and 16 consist of layers 28 of polyvinylidene or polarized copolymer, 9 microns thick which are metallized by a layer 30 of gold with a thickness of 200 μm. The metallization stops 1 a short distance from the edges of the film and possibly does not cover the central part.

 The films are polarized to obtain a high piezoelectric coefficient in the two directions (X and Y) of the film surface (commonly denoted d31 and d32) so that the films deform symmetrically which effectively improves the result. The polarization vectors 43 of the films 14 and 16 are aligned perpendicular to the surfaces of the films, and the films are mounted such that the two vectors are in opposite directions. The films are 2.5 cm in diameter and their edges 32, 34 are separated by 0.1 millimeter.

 In FIG. 2 is shown the electrical mounting in series of the films 14 and 16. For a given sound pressure level, the output voltage of the microphone comprising two films is practically double that of a microphone comprising a single film. In vibration, the voltage generated by the two films is added in series.

 All the combinations and variants described in the main patent application are applicable to the present invention.

Claims (7)

 1. Acoustic transducer comprising a hollow support and a plurality of metallized piezoelectric polymer films operating as an oscillator, said films being mounted at their peripheries on the hollow support so that their peripheries are separated and being physically connected in the vicinity of their center, characterized by the fact that said films have the shape of a flattened truncated cone, one truncated cone being inverted relative to the other and that said films are physically connected to the small bases of the truncated cones.  2. A transducer according to claim 1, characterized in that the small bases of the truncated cone are glued with adhesive epoxy.  3. Transducer according to claim 1, characterized in that the small bases of truncated cone are welded by thermal fusion.  4. Transducer according to any one of claims 1 or 2, characterized in that it is designed to convert sound into an electrical signal and that said films are electrically connected in series with their polarities selected so that the voltage of output of each of said films is added when said films oscillate.  5. A transducer according to any one of claims 1 or 2, characterized in that it is designed to convert sound into an electrical signal and that said films are electrically connected in parallel with their selected polarities so that the current produced by each of said films adding when said films oscillate.  6. Transducer according to any one of claims 1 or 2, characterized in that it is designed to produce sound and that said films are electrically connected with their selected polarities so that said films move in the same direction when electrically excited.  7. Transducer according to any one of claims 1 4, characterized in that the surface of the small base of each frustoconical film is approximately one third of the surface of the large base.