Vacuum stressed polymer film piezoelectric transducer

#The present invention relates to tranducers, particularly to electro-acoustic transducers suitable for use as loudspeakers.

#There has been considerable interest recently in the so called "flat" loudspeakers, which take up less space in a domestic environment, and which can, in certain circumstances, have good acoustical properties. The term "flat" in relation to loudspeakers refers to the external shape of the loudspeaker, and is taken to mean one in which the thickness is very much less than the other dimensions. Typically, the thickness may be 20% of the next larger dimension, or less in the case of the so-called "picture frame" loudspeakers which are intended to be hung on a wall.

#Known such "flat" loudspeakers utilise a stiff, light, thin diaphragm produced from material such as, for example, expanded polystyrene. The high mechanical Q of such materials, however, is very difficult to damp, and the break-up modes of such a diaphragm give sound reproduction having a very marked colouration. Such colouration is a disadvantage when it is desired to faithfully reproduce the sounds represented by electrical signals sent to the loudspeaker.

#One embodiment of the invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

#FIG. 1 is a schematic cross-sectional view of a transducer formed as an embodiment of the present invention; and

#FIG. 2 is an enlarged cross-sectional view, in diagrammatic form, of a part of a diaphragm suitable for use in the embodiment of FIG. 1.

#Referring now to the drawings, it will be seen that the embodiment shown comprises a sealed, flat, hollow structure which has as part of one of its major surfaces, a diaphragm including a polymer having piezo-electric properties. The diaphragm 1 is made from a film polymer with piezo-electric properties, such as, for example, polyvinylidene fluoride. The diaphragm is supported all around its periphery by a supporting framework 4 which, together with side walls 5a, and a rear wall 5b forms an enclosure 6. The diaphragm 1 is larger than the area defined by the supporting framework 4 and the enclosure 6 is evacuated through a suitable valve outlet such as that shown diagrammatically by the reference numeral 8. The evacuation of the enclosure 6 causes the diaphragm 1 to bow inwardly to adopt an ovoid or part-spherical shape as indicated in FIG. 1.

#Coated on each side of the diaphragm 1, as shown more particular in FIG. 2, are two electrodes, 2, 2' which are secured to the diaphragm by means of vacuumdeposition. The electrodes 2, 2' must be very light, typically less than half of the mass of the diaphragm. Electrical connection is made to the electrodes on either side of the diaphragm 1 by means of terminals 3, 3' which provide for an electrical signal to be applied to each side of the diaphragm from an amplifier or voltage generator 9.

#The level of the vacuum in the interior 6 of the enclosure formed by the side walls 5a, the rear wall 5b and the diaphragm 1 is such that the transmission of acoustic energy from the diaphragm rearwardly into the interior of the enclosure is substantially reduced. If the enclosure is not entirely airtight and allows a slow drift of air molecules into the interior 6 the outlet 8 may also be provided with a vacuum pump in order to maintain the evacuated space at the desired reduced pressure.