Abstract:
The present invention provides an electro-acoustic transducer including a pair of piezo-electric plastics film diaphragms coupled in a push-pull manner and so arranged as to form a lens-like configuration by a body of a light fibrous material therebetween. 
     The diaphragms are coupled in push-pull manner, and the transducer is especially suitable for use in a telephone instrument.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electric acoustic transducers, and particularly to a transducer employing a piezo-electric polymer diaphragm as the active element. 
     2. Description of the Prior Art 
     A telephone subscriber&#39;s instrument usually employs a carbon microphone transmitter and a rocking armature electro-magnetic receiver. Whilst such a combination is satisfactory in operation the necessity of manufacturing two different types of transducer results in relatively high production costs. Furthermore both types of transducer must be made available for repairs and maintenance of the telephone equipment. 
     Piezo-electric plastics film has recently become available, and this film can be electrically polarised and provided with surface electrodes such that a change in the linear dimensions of the film induces a potential difference between the electrodes and vice versa. In particular, electrically polarised polyvinylidene fluoride (PVDF) film shows this property. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided an electro-acoustic transducer including a pair of piezo-electric plastics film diaphragms coupled in a push-pull manner and so as to form a lens configuration by a body of light fibrous material therebetween. 
     According to the invention there is further provided an electro-acoustic transducer including a piezo-electric plastics foil multilayer diaphragm assembly, in which the diaphragm has lenticular portions the two convex surfaces of which each comprise one or more layers of the plastics foil, in which each said layer has surface electrodes, the material of the foils being electrically polarised so that movement of the diaphragm generates potential differences between the electrodes of each of the foils, and vice-versa, and in which the electrodes of the layers of the two surfaces are coupled so that the two sets of foils operate in a push-pull manner. 
    
    
     Embodiments of the invention will now be described with reference to the accompanying drawings in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a piezo-electric transducer assembly embodying the invention; 
     FIG. 2 is an enlarged cross-section of part of the diaphragm of the transducer of FIG. 1; 
     FIG. 3 shows a multilayer diaphragm construction embodying the invention; 
     FIG. 4 shows a multi-lenticellular diaphragm construction embodying the invention; 
     FIG. 5 is a cross-section of a transducer employing the diaphragm construction of FIG. 4; 
     FIGS. 6 and 7 are cross-section and plan views respectively of a transducer intended for use as a telephone receiver; and 
     FIG. 8 is a cross-section of a transducer fitted with a composite diaphragm. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, the transducer assembly includes a pair of annular baffle members 11 and 12 having a pair of piezo-electric plastics diaphragms 13 and 14 clamped therebetween. The diaphragms are electrically polarised so that they are in a `back to back` configuration and are stretched into a lenticular form by a body 15 of light fibrous material trapped between the diaphragms 13 and 14. This diaphragm can conveniently be rectangular in plan, although in other cases, e.g. when used in a telephone instrument, it could be circular. The filling 15 is preferably synthetic fibre monofilament material. 
     As shown in FIG. 2, each plastics diaphragm is provided with electrodes 21 and 22 which electrodes are interconnected such that the diaphragm operate in a push-pull manner to maximise their output. Application of an alternating voltage to the electrode causes the diaphragms to expand and contract so as to generate a corresponding audio signal. Conversely, vibration of the diaphragm by an audio signal causes the generation of a corresponding alternating voltage. 
     The above arrangement is intended specifically for use as a microphone, in which case it is useful to reduce the acoustic impedance of the diaphragm to a value comparable with the free-air load impedance. When a head receiver is to be considered, the acoustical load impedance to be taken into account is that of the ear which is several orders of magnitude higher than that of free air. There is then a mechanical advantage in increasing the acoustical impedance of the transducers, and this is achieved in the manner shown in FIG. 3. 
     FIG. 3 shows a high output diaphragm arrangement in which each lenticular shell 31 of the diaphragm assembly comprises successive layers 32 of piezo-electric plastics film each provided with electrodes (not shown) and interconnected so that the layers of each shell operate in unison, the two shells operating in push-pull manner. Each shell may have as many as ten layers, the layers being separated by thin layers of air, which are equivalant acoustically rigid couplings of the axial movements of individual diaphragms. 
     An alternative construction is shown in FIG. 4 in cross-section through the diaphragm. In this embodiment the diaphragm 41 is formed from a sheet of PVDF pressed into an overall part-spherical form and additionally is further formed into a number of small part-spherical cells, 42. Each small cell moves individually as a unit up to the higher telephonic frequencies of about 3 kHz and the multiplicity of small cells moves as a whole by the stiffening of the diaphragm into an overall part-spherical curvature. The PVDF is polarised to be piezo-electric, and electrodes are applied to each side of the whole diaphragm. 
     FIG. 5 shows a transducer fitted with a multi-spherical diaphragm of the type shown in FIG. 4. The diaphragm 51 includes a diaphragm of the type shown in FIG. 4 with a simple-part-spherical diaphragm, and the whole is clamped between a pair of annular baffle members 52 and 53. These are mounted between a perforated mounting plate 54 and a perforated front cover 55 in a plastics housing 56. Conductive leads 57 couple the diaphragm electrodes to terminals 58 in the housing. A pressure equalising tube 59 may also be provided through the housing wall. In a preferred embodiment the individual cells of the diaphragm are approximately 5 mm in diameter and the whole diaphragm is moulded from 10 micron thick PVDF film. 
     Such a diaphragm arrangement can be used in the manner shown in FIG. 1, i.e. with a &#34;filling&#34; of the fibrous material. 
     FIGS. 6 and 7 show a transducer arrangement intended for use as a telephone receiver. In this arrangement the capacitance of the configuration of FIG. 1 has been reduced with little or no loss of electro-acoustic efficiency. The domed diaphragm 61 is passive and may be made from a polycarbonate or unplasticised PVC. A rectangular flat strip 62 of PVDF material is mounted on an annular ring 63. After assembly the strip 62 is bowed by the curvature of the dome 61. The contact between the diaphragm dome 61 and the strip 62 stretches the latter slightly. On application of a signal voltage to the electrodes (not shown) of the PVDF strip 62, one polarity increases the length of the strip relaxing the force on the diaphragm 61 while the opposite polarity decreases the length increasing the force on the diaphragm. 
     FIG. 8 shows a transducer fitted with a sandwich type diaphragm. The planar diaphragm assembly 81 is mounted in a housing 82 and includes an expanded polystyrene or microporous polypropylene sheet 83 to each face of which a layer 84 of PVDF material is bonded, e.g. by an adhesive. The PVDF layers 84 are oppositely polarised so that they operate in push-pull manner to bow the diaphragm when a signal is applied.