EP0067790A1 - Mixed electroacoustic transducer - Google Patents

Abstract

1. An electro-acoustic transducer able to operate as an emitter and as a pick-up of sound waves in the environment and comprising a receptacle (1) having an opening for communicating with said environment, a first diaphragm (12) mounted inside to receptacle, and means (4) for causing said first diaphragm to vibrate in response to an electric energizing signal applied there to when the transducer operates as an emitter, characterized in that it further comprises a second diaphragm (17) able to vibrate under the effect of sound waves and disposed between the opening of the receptacle (1) and the first diaphragm (12), at a distance from the latter, coupling means (19; 28) for imparting the vibrations of said first diaphragm to the second diaphragm when the transducer operates as an emitter and means (16, 25; 26, 25) for producing an output signal in response to vibrations of said second diaphragm produced by sound waves propagating in said environment when the transducer operates as a pick-up.

Description

The present invention relates to a mixed electro-acoustic transducer, that is to say which can function as a transmitter and as an acoustic wave sensor in an ambient medium.

The physical phenomena (electromagnetism, electrostatic, piezoelectricity) applied for the realization of electro-acoustic transducers being reversible, it is in principle possible to use the same device to emit sound waves in response to an electrical signal and vice versa to transform acoustic vibrations in variations of current or electrical voltage. It is clear that in practice, a transducer operating according to a determined principle is not produced in the same way depending on whether it is intended to be used as a microphone or as a speaker. The dimensions, the shape and the arrangement of the elements which constitute it differ in the two cases. For example, to be sufficiently sensitive to sound waves, the membrane of a microphone must be much less rigid than that of a speaker.

Equipping a device with a sound transmitter-receiver unit is therefore equivalent to equipping it with at least one separate speaker and microphone, choosing for each of these transducers the best suited type, taking into account its specific qualities, for the intended application.

This poses a problem when it comes to equipping portable devices whose volume should be as low as possible. This is particularly the case for watches designed to indicate the time or provide other information either in the form of simple monotonic sounds, or in a "spoken" form using words or groups of words memorized and some of which Functions, such as timekeeping corrections, can be controlled directly by voice. The fact of having to provide, for such watches, in addition to the usual organs necessary for the elaboration of time and its display, two independent transducers, goes against the effort currently made to reduce their dimensions.

The invention aims to avoid this drawback by providing an electro-acoustic transducer of low volume, capable of operating selectively in microphone mode and in speaker mode.

Another object of the present invention is to provide a mixed transducer having at the same time the qualities which may be required of a microphone (good sensitivity, sufficiently uniform response curve, wide bandwidth, etc.) and a loudspeaker. small speaker (high sound level, low distortion rate, fairly wide frequency band if necessary, etc.).

• - un réceptacle présentant une ouverture de communication avec le milieu ambiant;
• - une première et une seconde membrane montées à distance l'une de l'autre à l'intérieur du réceptacle, la seconde membrane étant apte à vibrer sous l'effet d'ondes acoustiques et disposée entre la première membrane et l'ouverture;
• - des moyens pour faire vibrer la première membrane en réponse à un signal électrique d'excitation qui leur est appliqué lorsque le transducteur fonctionne en émetteur;
• - des moyens de couplage pour communiquer les vibrations de la première membrane à la seconde membrane; et
• - des moyens pour élaborer un signal de sortie en réponse à des vibrations de la seconde membrane produites par des ondes acoustiques se propageant dans le milieu ambiant lorsque le transducteur fonctionne comme capteur.

These aims are achieved thanks to the fact that the electro-acoustic transducer according to the invention comprises:

• - A receptacle having an opening for communication with the ambient environment;
• - a first and a second membrane mounted at a distance from each other inside the receptacle, the second membrane being capable of vibrating under the effect of acoustic waves and disposed between the first membrane and the opening;
• - Means for vibrating the first membrane in response to an electrical excitation signal which is applied to them when the transducer operates as an emitter;
• - coupling means for communicating the vibrations of the first membrane to the second membrane; and
• - Means for developing an output signal in response to vibrations of the second membrane produced by acoustic waves propagating in the ambient environment when the transducer operates as a sensor.

The coupling means can comprise an air cushion contained in a closed chamber separating the two membranes, this chamber constituting, moreover, a rear acoustic cavity for the second membrane when the transducer operates as an emitter.

This coupling can also be achieved by a rigid mechanical connection between the two membranes.

The means for vibrating the first membrane may consist of an electromagnet.

Furthermore, it is possible to use an electret sheet as the second membrane, the means for producing the output signal then comprising a front electrode and a rear electrode arranged on either side of the electret sheet, l one of the electrodes being integral with the latter, these means delivering a signal in voltage representative of the vibrational movements of the electret sheet.

• - la figure 1 représente schématiquement, en coupe, un premier mode de réalisation possible du transducteur lorsque celui-ci comprend une feuille d'électret comme seconde membrane, des électrodes avant et arrière comme moyens d'élaboration du signal de sortie, des moyens d'excitation électromagnétiques de la première membrane et dans lequel le couplage entre les deux membranes est assuré par un coussin d'air; et
• - la figure 2 est une coupe schématique d'un second mode de réalisation avec toujours les mêmes éléments, certains étant réalisés et agencés différemment, et dans lequel le couplage entre les deux membranes est assurée par une liaison mécanique rigide.

The invention will be better understood from the following description of two possible embodiments of the mixed transducer which is the subject thereof. This description refers to the attached drawing in which:

• - Figure 1 shows schematically, in section, a first possible embodiment of the transducer when the latter comprises an electret sheet as a second membrane, front and rear electrodes as means for developing the output signal, means d electromagnetic excitation of the first membrane and in which the coupling between the two membranes is ensured by an air cushion; and
• - Figure 2 is a schematic section of a second embodiment with always the same elements, some being made and arranged differently, and in which the coupling between the two membranes is provided by a rigid mechanical connection.

The mixed electro-acoustic transducer shown diagrammatically, in section, in FIG. 1, comprises an open housing 1, formed of a cylindrical side wall 2 and of a bottom 3 each made of an insulating material. The bottom 3 of the housing carries the fixed part of an electromagnet generally designated by 4. This fixed part comprises a yoke 5, made of soft magnetic material, having a base 5a surmounted by a core 5b which surrounds a coil 6. A cylindrical part 5c of the cylinder head, which surrounds the coil 6 has its edge covered by an annular polarizing magnet 7. Two holes 8a, 8b passing through the base 5a of the cylinder head 5 and the bottom 3 of the housing allow the passage of the coil wire 6. Each of the ends 9a, respectively 9b, of this wire is welded to a metal pad 10a, respectively 10b, carried by the external face of the bottom 3 of the housing, this bottom can advantageously consist of a printed circuit board from which the metallized layer has been partially removed. On the metal areas 10a, 10b are also welded the ends of two wires 11a, 11b by which an electrical excitation signal can be brought to the coil 6.

In front of the fixed part of the electromagnet 4 is placed a first metallic membrane 12, for example made of steel or a beryllium-copper alloy, which carries, on its rear face, facing the bottom of the case, a disc 13 made of soft magnetic material, constituting the movable armature of the electromagnet 4. The metal membrane is fixed by its periphery, for example by welding, on a support 14 made of an electrically conductive material such as steel or the brass and bearing on an insulating ring 15, itself supported by the base 5a of the cylinder head 5, so that the armature 13 is a short distance from the upper face of the core 5b of the electromagnet when the membrane 12 is at rest. The function of the insulating ring 15 which separates the support 14 from the membrane 12 of the base 5a and from the cylindrical part 5c of the cylinder head 5 will be indicated below.

In this first embodiment, the transducer comprises, in addition to the assembly which has just been described, the general structure of which is that of an electromagnetic type loudspeaker, and still inside the housing 1, elements of an electret, condenser microphone. These elements include an electrode rear or counter-electrode constituted by a rigid plate 16 made of an electrically conductive material, for example nickel silver. This plate 16 has at its periphery, a flange 16a projecting from the bottom side 3 of the housing and bearing on the support 14 so that the plate 16 is spaced from the front face of the metal membrane 12 while being electrically connected to the support 14 and incidentally, but not necessarily in this embodiment, to this first membrane 12.

The microphone part also comprises, as a membrane, an electret sheet 17 placed in front of the counter electrode 16. The electret sheet 17 has its front face oriented towards the opening of the housing, entirely covered by a metallic deposit 25 forming a front electrode and is highly tensioned and fixed on a metallic annular ring 18 situated on the side of the metallization, so that there is an electrical connection between the ring 18 and the metallic layer 25; this connection can be achieved using for example as a fixing means a conductive adhesive.

As shown in Figure 1, the edge of the rear face of the electret sheet 17 is applied against the periphery of a recess 16b of large area but shallow that has the plate 16. The depth of the recess 16b is provided to allow the second diaphragm of the transducer, that is to say the electret sheet, to vibrate freely but also so that the distance between the latter and the rear electrode 16 remains very small. Note that the electret sheet 17 electrically insulates the metal layer 25 and the ring 18 from the counter-electrode 16.

Furthermore, the metal plate 16 is pierced with several holes 16c which can be distributed over a circle, at a distance sufficient from the center of this plate not to be almost blocked by the second membrane when the latter is subjected to vibrational movements which bring practically its central part into contact with the rear electrode 16. These holes 16c ensure communication between the 'recess 16b and the space between the plate 16 and the first membrane 12 so as to provide between the two membranes a closed chamber 19 containing an air cushion through which these two membranes are coupled. This chamber 19 also serves as a rear acoustic cavity for the second membrane 17 when the transducer operates as a receiver.

The transducer, shown in Figure 1, also includes a metal cover 20, made for example of aluminum, which covers the top of the housing as well as its side wall 2 by bearing directly on the metal ring 18. The cover 20 has several lugs 20a folded over the bottom 3 of the housing, which keep it in place. At least one of these lugs 20a is in contact with a third conductive track 21 carried by the bottom of the housing and on which a ground wire 22 is welded. It is clear that, when the transducer is mounted in an apparatus, the wire 22 and the conductive track 21 are superfluous when the cover 20 is in contact with a part forming part of the mass of this device.

Furthermore, the bottom 20b of this cover 20 which covers the opening of the housing 1 is pierced with holes 20c, for example five in number and distributed as shown in the figure, that is to say one in the center and four arranged in a circle around the central hole which put the chamber 23 between the bottom 20b of the cover and the second membrane 17 in communication with the environment.

Finally the bottom 3 of the housing is pierced with a hole 3a through which passes a wire 24 for signal output for the microphone, the end of which is welded to the support 14 of the first membrane. This wire 24, like the wires 11a, 11b for supplying the coil 6 and the ground wire 22, is connected to an electronic circuit, not shown, associated with the transducer.

When the electro-acoustic transducer which has just been described functions as a sensor, only the microphone part works in the manner of a conventional electret microphone, that is to say that, when the acoustic waves to be captured penetrate the chamber 23 through the orifices 20c of the cover, the second membrane 17 is subjected to vibrations. This has the effect of varying the potential difference existing between the front electrode 25 and the rear electrode 16 which form a variable capacitor permanently charged under the effect of the electric field created by the electret sheet in its vicinity. The low voltage signal thus produced is applied to the gate of a field effect transistor forming part of the electronic circuit associated with the transducer, to which the microphone output wire 24 is connected, in order to be amplified to be then used for specific purposes. Providing an insulating ring 15 between the support 14 of the first membrane, which electrically connects the rear electrode 16 to the output wire 24, and the yoke 5 of the electromagnet, rather than directly supporting this support on the cylinder head, significantly reduces the stray capacitances to which the microphone output signal is very sensitive.

When the transducer is used as an emitter, an excitation current, corresponding to the acoustic signal to be emitted, is sent, by the wires 11a and 11b, into the coil 6 of the electromagnet; an attractive or repulsive force which follows the variations of the excitation current acts on the armature 13 and thus vibrates the first membrane 12. The movements of the first membrane are transmitted, by the air cushion contained in the closed chamber 19, to the second membrane 17 which then emits an acoustic wave, the latter escaping through the holes 20c of the cover 20.

It is clear that means will be provided in the electronic circuit to avoid, in this operating mode, the presence of an output signal on the wire 24 or to block such a signal. As the second membrane must be necessarily flexible and thin, therefore of low equivalent mass, to be as sensitive as possible to external acoustic waves when the transducer is used as a microphone, the response curve of the assembly formed by the two membranes coupled by the air cushion is very close to that which would be obtained if the first membrane itself emitted an acoustic wave in response to the excitation signal, both with regard to the bandwidth and the value of the sound pressure obtained.

The mixed transducer is therefore practically equivalent to separate microphone and loudspeaker while having the advantages of a smaller footprint and a lower cost price, taking into account the fact that a single housing and a single cover are necessary.

Naturally the nature and dimensions of the membranes 12 and 17, the volume of the chamber 19, which directly influences the sensitivity and resonant frequency of the transducer, the number, the dimension and the distribution of the holes 16c of the plate 16 and possibly other parameters, such as the volume of the chamber 23, the number, the size and the arrangement of the orifices 20c of the cover, which can intervene with regard to the characteristics of the transducer, must be chosen according to the acoustic signals that this transducer will have to pick up or emit and the qualities that will be required of it, taking into account the use that will be made of it .

FIG. 2 schematically shows in section another possible embodiment of a mixed transducer always using the essential elements of a loudspeaker of the electromagnetic type and of an electret microphone. Certain constituent parts of the transducer of FIG. 1 which are identical or with minor modifications of adaptation, in this second embodiment are designated by the same references. The microphone counter-electrode here consists of the first metallic diaphragm 26 which, in this case, must necessarily be electrically connected to its support 14. This first diaphragm, which always carries on its underside the armature 13 of the electromagnet 4, has holes 26a connecting the intermediate space between the two membranes 17 and 26, very reduced in this embodiment, with the enclosed space defined by the upper face of the fixed part of the electro -magnet 4, the support 14 of the first membrane 26 and this first membrane itself, so as to constitute a rear acoustic cavity 27 of sufficient volume for the second membrane 17.

Furthermore, coupling means are provided to ensure a rigid mechanical connection between the two membranes. They consist of a metallic spacer 28, fixed on one side to the front face of the first membrane 26, in the center thereof, and on the other side, to the rear face of the electret sheet 17, this fixing can be done for example by gluing or welding; in the latter case, a metallization must naturally be provided at the center of the rear face of the second membrane 17. The two membranes then no longer form a single organ vibrating at the frequency of the excitation signal when the transducer operates as an emitter.

This solution has notably as advantages, compared to that shown in FIG. 1, a lower volume of the transducer and a slightly improved sound level, the damping of the vibrations of the second membrane by the air cushion contained in the chamber 19 (Figure 1) being avoided here.

Note that the fact of securing the two membranes in this way does not affect the operation of the transducer in microphone mode. On the contrary, this makes it possible to obtain a better frequency response for the high frequencies.

It is clear that the invention is not limited to the exemplary embodiments which have just been described. For example, the electromagnet can be replaced by a piezoelectric crystal fixed on or coupled to the first membrane. Similarly, the use of the elements of an electret microphone, although advantageous, is not imperative; these can be replaced, for example, by the conductive membrane and the counter electrode of a condenser microphone, with however the disadvantage of the need for the application of a DC voltage between the two when the transducer operates as a sensor.

It is therefore possible, without departing from the scope of the invention, to produce the mixed transducer by combining the elements of simple transducers of different types. Several combinations are possible. The choice of one or the other of them naturally depends on the use to which the transducer is reserved.

Furthermore, it is clear that, when the transducer is fitted to a device, a conductive and perforated wall thereof can act as a cover. In an electronic watch, for example, the transducer will be mounted so that the metal ring 18 bears against the watch case provided with holes at this location.

In addition, the housing can be replaced by any other receptacle adapted to receive the other elements of the transducer. This receptacle can consist, for example, of a cavity inside an insulating part, this part being intended to support or contain not only the transducer but also other devices.

Claims (11)

1. Electro-acoustic transducer capable of operating as a transmitter and as an acoustic wave sensor in an ambient environment characterized by the fact that it comprises: - a receptacle (1) having an opening for communication with said ambient medium; - a first and a second membrane (respectively 12; 26 and 17) mounted at a distance from each other inside said receptacle, said second membrane being capable of vibrating under the effect of acoustic waves and disposed between said first membrane and said opening; - means (4) for vibrating said first membrane in response to an electrical excitation signal which is applied to them when the transducer operates as an emitter; - coupling means (19; 28) for communicating the vibrations of said first membrane to said second membrane; and - means (16.25; 26.25) for producing an output signal in response to vibrations of said second membrane produced by acoustic waves propagating in said ambient medium, when the transducer operates as a sensor. 2. electro-acoustic transducer according to claim 1, characterized in that said coupling means comprise an air cushion contained in a closed chamber (19) separating said first and second membranes (respectively 12 and 17), said closed chamber also constituting a rear acoustic cavity for said second membrane when the transducer operates as an emitter. 3. electro-acoustic transducer according to claim 2, characterized in that said second membrane is constituted by an electret sheet (17) having a front face facing the opening of the receptacle (1) and that the means for producing said output signal comprise a front electrode (25) and a rear electrode (16) arranged on either side of the electret sheet, one of said electrodes being integral with the latter and forming with the other electrode a variable capacitor charged under the effect of the electric field created by said electret sheet and delivering a voltage signal representative of the vibration movements of the second membrane. 4. electro-acoustic transducer according to claim 3, characterized in that said front electrode comprises a layer of electrically conductive material (25) carried by the front face of said electret sheet. 5. electro-acoustic transducer according to claim 3 or 4, characterized in that said rear electrode comprises a rigid plate (16), electrically conductive placed inside the closed chamber (19), near said sheet d electret (17), said plate separating said closed chamber in two and being pierced with at least one hole (16c). 6. electro-acoustic transducer according to claim 1, characterized in that said coupling means comprise a rigid mechanical connection (28) between said first and second membranes (respectively 26 and 17). 7. electro-acoustic transducer according to claim 6, characterized in that said mechanical connection is constituted by a spacer (28) fixed to said first and second membranes, substantially at the centers thereof. 8. electro-acoustic transducer according to claim 6 or 7, characterized in that said second membrane is constituted by an electret sheet (17) having a front face facing the opening of the receptacle (1) and a face rear and that the means for producing said output signal comprise a front electrode comprising a layer of electrically conductive material (25) carried by the front face of said electret sheet and a rear electrode (26) arranged opposite and close to the rear face of the electret sheet, said front and rear electrodes forming a variable capacitor charged under the effect of the electric field created by said electret sheet and delivering a voltage signal representative of the vibration movements of the second membrane. 9. electro-acoustic transducer according to claim 8, characterized in that the first membrane (26) is metallic and constitutes said rear electrode and that said first membrane separated from said electret sheet (17) by an intermediate space, is pierced with orifices (26a) putting said intermediate space in communication with a closed chamber situated at the rear of said first membrane in order to constitute a rear acoustic cavity (27) of sufficient volume for said second membrane (17) 10. electro-acoustic transducer according to one of the preceding claims, characterized in that the means for vibrating the first membrane (12; 26) comprise an electromagnet (4) formed by a fixed part placed between said first membrane and a bottom (3) which has said receptacle (1) and comprising a core (5b) of soft magnetic material surrounded by a coil (6) to which is applied said electrical excitation signal and a movable armature (13 ), also made of soft magnetic material fixed on said first membrane, opposite said fixed part. 11. electro-acoustic transducer according to one of the preceding claims, characterized in that it further comprises a cover (20) covering the opening of the receptacle (1) and pierced with at least one orifice (20c) for the passage of acoustic waves.

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