GB2073544A - Electro-acoustic transducer - Google Patents

Abstract

The transducer is particularly useful in a watch. The emission of acoustic waves is caused by vibration of a thin diaphragm 8 occupying virtually the whole of the bottom of the watch case 1. The electromagnetic diaphragm excitation device 10 is disposed at an off-centre position. This not only makes it possible to operate with a small air gap, providing good electromagnetic coupling and therefore a good output, but it also makes it possible to produce a large vibration amplitude at the centre of the diaphragm 8 and therefore a high level of sound intensity for an entirely reasonable rate of consumption. The diaphragm 8 is fixed in a ring 6 which carries the excitation device 10 and which is fixed to the support frame structure 5 of the watch. This modular construction facilitates fitting and interchangeability of the electro-acoustic transducer. <IMAGE>

Description

SPECIFICATION Electro-acoustic transducer The present invention concerns an electroacoustic transducer and, more particularly, although this is not the only field of use thereof, an eiectroacoustic transducer for a timepiece, especially a watch.

The number of functions included in a watch has considerably increased with the appearance of elec tronicwatches. One of the most used and widely appreciated functions is the alarm device which uses a transducer for converting the electrical energy of the battery into acoustic energy.

There are several kinds of electro-acoustic trans duct us for timepieces. First of all, there is the piezoelectric-type transducer which uses a radially polarized ceramic pellet, which expands radially when a voltage is applied to its electrodes. The pellet is secured by adhesive to a cupro-beryllium diap hragm. Expansion of the pellet causes buckling of the diaphragm, which is accompanied by the production of a sound wave. In spite of its simplicity and its attractive price however, this kind of arrangement suffers from the following disadvantages: low acoustic level at low frequencies, poor shock resistance, metallic tone and strong influence of the fitting conditions.

Also known are electromagnetic transducers which are referred to as modular transducers, in which an electro-magnetic device which is disposed in a pot excites a small diaphragm of the same size as the pot. The acoustic response curve of this known transducer is not good, but it can be improved by adding a resonant cavity of greater or lesser size.

Such transducers which are also referred to as buzzers and which were developed for alarms, operate acceptably only at a clearly defined frequency. In spite of their very simple construction and their modular nature, they nonetheless suffer from the following serious disadvantages: very high level of cur rent consumption, poor response at low frequencies and a frequency response curve which is not flat.

Taking into account these defects, such arrangements cannot be used for producing different sounds and accordingly cannot generate different melodies, as is often now described in an alarm.

The object of the present invention is to provide a powerful electro-acoustic transducer which has a low level of power consumption and which has good performance at low frequencies and which is capabie of producing a sound similar to that of a chime, the frequency of which may for example between 500 and 1500 Hz, like the Westminster chime. The invention also seeks to provide a modular trans ducerwhich is easy to assemble and fit in a watch.

These aims are achieved by a transducer according to the present invention, which is defined in claim 1 beiow.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure lisa sectional view of a transducer embodying the invention and its arrangement in a watch, Figure 2 is an exploded view showing the different components of the transducer and the watch in the assembly position, and Figure 3 shows the performance characteristics of the transducer, compared to those of known transducers.

Figure 1 shows a watch case 1 having a bottom la, a peripheral casing ring 1b and a rim 1c. Openings 2 are provided in the bottom la. A glass 4 is mounted in the rim 1c by engagement in a notch therein.

A frame or movement support structure 5 is held in the casing ring 1b by fixing screws 3 (Figure 2). A diaphragm support ring 6 is secured to the structure 5 by a series of screws 7 (see Figure 2). Set in the ring 6 is a thin, circular diaphragm 8 of radius R, which is preferably produced from a non-magnetic material such as cuproberyllium, and which occupies virtually the whole of the bottom of the case 1. The thickness ofthe diaphragm 8 is typically 0.15 mm. The distance Y between the bottom la and the diaphragm 8 is relatively substantial, typically 1 mm. The volume 37 between the diaphragm and the bottom 1a of the watch case forms a resonant cavity forthe transducer.

A metal plate 9 is secured by adhesive or soldered or welded to the diaphragm 8, at a distance T from the centre thereof. The plate 9 is fixed to the inward side of the diaphragm 8, that is to say, the side which faces towards the watch movement 5. The plate 9 is produced from a soft magnetic material. The ratio T/R is advantageously about 4/5.

An electromagnetic device 10 is mounted facing the plate 9, at a distance X therefrom, so as to form an air gap 100. Typically, the width X of the air gap 100 is 0.08 mm when the device is in a rest condition.

The device 10 comprises a brass pot 11 which is secured to the ring 6 by screws 12, a yoke member 13 of high-permeance magnetic material which is driven into the pot 11, and a central core 15 which is terminated by a circular plate 15a and which is made of a material of high magnetic permeability. A coil 16 is disposed around the core 15 on which the coil is held in position by a collar member 17 which is driven on to the end thereof. The device 10 is completed by an axially magnetized annular permanent magnet 14 which is gripped between a peened-in rim at the upper part of the pot 11 and the yoke member 13.

A flexible circuit 18, advantageously comprising Kapton (registered mark of Dupont, Wilmington, Delaware), which is covered with copper on one side, forms the electrical connection between the coil 16 and a printed circuit 20. The circuit 20 is the main circuit of the movement. The circuit 18 and a contact spring 23 are secured to the pot 11 by a rivet 19. By virtue of the pressure that it applies to the circuits 18 and 20, the spring 23 ensures contact between the corresponding conducting portions of the two circuits. An insulating plate 22 prevents any electrical contact between the circuit 18 and the metal parts of the electromagnetic device 10.

Finally, the watch comprises an hour hand 24, a minute hand 25 and a second hand 26.

Figure 2 provides a better illustration of the mimner of fitting the acoustic transducer according to the invention into the watch. The watch case is of conventional nature. The giass 4 is secured by a wedging or pinching action to the rim 1c which is secured to the peripheral edge casing member 1 b by engagement into a notch therein. The bottom la with the openings 2 is also mounted on the peripheral edge casing 1 b by engagement in a notch.

The electromagnetic device 10 is secured to the ring 6 which itself carries the diaphragm 8. The elec tromagnetictransducertherefore forms a modular assembly and it is fixed to the movement frame 5 by the screws 7. The movement frame 5 comprises a cavity 28 in which the electromagnetic device 10 is housed. The electrical connections between the device 10 and the control circuit 29 thereof, which is referred to as the musical circuit, are automatically formed by contact of the flexible circuit 18 (see Figure 1) with the main printed circuit 20, when the device is set in place, as the control circuit 29 is connected to the printed circuit.

The movement further comprises a battery 30 and a two-phase further comprises a battery 30 and a two-phase stepping motor 32. The screws 3 secure the movement frame 5 to the casing member 1b to which there is attached a pendant portion 33 carrying a time setting member 34, and a push button 35.

The mode of operation of the transducer according to the invention will now be described. The musical circuit 29 is triggered either automatically, for example at each full hour, or manually, by pressing the push button 35. The circuit 29 then begins to produce electrical pulses at a given frequency, generating a variable magnetic field in the air gap 100. The magnetic field which is channelled by the pole piece 13 and 15 acts on the magnetic plate 9 which is attracted towards and springs back from the electromagnetic device 10 at the frequency of the pulses applied to the coil 16. The diaphragm 8 which is fixed with respect to the plate 9 vibrates at the same frequency. This produces a sound emission which is amplified by the resonant cavity 37. The openings 2 allow the sounds produced to escape to the exterior.

The ratio between the total area of the openings 2 and the surface area of the bottom 1a has an effect on the level of the sound emission. Experience has shown that maximum sound level is produced when the total area of the openings 2 is substantially equal to 4% of the surface area of the bottom.

Using a thin diaphragm which is of large area and which is excited in an off-centre manner makes it possible on the one hand to operate with a small air gap x, thus providing good electromagnetic coupling and therefore a good output, and on the other hand, in a region in which the diaphragm is sufficiently stiff to avoid the danger of sticking. This arrangement also makes it possible, by a sort of acoustic lever effect, to provide a large amplitude in respect of vibratory movement at the center of the diaphragm and therefore a high level of sound intensity, for an entirely reasonable current consumption rate. In addition, as the diaphragm is thin, it has a low degree of inertia and therefore provides a good acoustic response to the frequency produced by the coil.

Figure 3 shows different response curves in respect of the sound intensity SPL of an acoustic transducer in dependence on the excitation frequency (f). Reference 41 denotes the curve of a device embodying the invention, reference 42 denotes the curve of a piezoelectric-type transducer, while reference 43 denotes the curve of a modular electromagnetic transducer or buzzer. The efficiency of the device according to the invention is particularly noteworthy, at low frequencies, which permits the device to be satisfactorily used, in particular for bells or chimes of the Westminster type.

The structural arrangements of the abovedescribed transducer provide many advantages.

First of all, fixing the diaphragm 8 in place by a peening or crimping action makes it possible to minimize variations in production. In addition, as all the magnetic components are fitted in a pot 11 which surrounds them, the pot 11, together with the ring 6 and the diaphragm 8, forms a modular assembly which can be readily assembly to the movement by means of screws. Moreover, as the pot 11 is fixed with respect to the fixing ring 6, this guarantees an air gap of reproducible size, in production. In addition, the modular construction of the abovedescribed acoustic transducer provides interchangeability in response of all components, and easy and rapid assembly. Finally, the ring 6 and the diaphragm 8 thereof ensure that the movement5 is sealed.

Among possible modifications, the magnetic plate 9 may be omitted and the thin diaphragm 8 is itself then made of a magnetic material.

Claims (12)

1. An electro-acoustictransducer comprising a thin diaphragm, an electromagnetic device which is magnetically coupled to a position on side diaphragm to cause vibration of said diaphragm, and support means supporting the diaphragm and the electromagnetic device, wherein the electromagnetic device is magnetically coupled to a position of the diaphragm which is spaced from the centre of the diaphragm.

2. A transducer according to claim 1, wherein the diaphragm is substantially circular and the coupling position on the diaphragm is disposed at a distance from the centre of the diaphragm which is substan- - tially equal to 4/5 of the radius of the diaphragm.

3. Atransducer according to claim 1 or2, wherein the support means comprise a ring in which the diaphragm is fixed and which also carries the electromagnetic device.

4. Atransducer according to claim 1,2 or 3, wherein the support means include a bottom plate which is provided with through openings and which is disposed facing the diaphragm.

5. Atransducer according to claim 4, wherein the bottom plate is the bottom of a timepiece case and which covers virtually the whole of the diagram.

6. Atransducer according to claim 4 or 5, wherein the openings occupy substantially 4% of the surface area of the bottom plate.

7. Atransducer according to claims 3 and 5 in combination, wherein the ring is fixed to the ssup port frame structure of the timepiece.

8. Atransducer according to any of claims 1 to 7, wherein the electromagnetic device comprises a centrai core terminated by a circular plate, a coil embracing the core, a yoke member disposed around the coil, and an axially magnetized cylindrical permanent magnet which is disposed coaxially with the core.

9. A transducer according to claims 3 and 8, in combination, further comprising a pot of nonmagnetic material containing the electromagnetic device, the pot being fixed to the ring.

10. Atransducer according to any of claims 1 to 9, wherein the diaphragm is of non-magnetic material and carries a plate of magnetic material, which is disposed facing the electromagnetic device.

11. A watch incorporating a transd ucer accord- ing to any of claims 1 to 10.

12. A watch incorporating atransducersubstantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.