EP3213531A1

EP3213531A1 - Electroacoustic transducer, and associated assembly and system

Info

Publication number: EP3213531A1
Application number: EP15791719.6A
Authority: EP
Inventors: Stéphane Durand; Petr HONZIK; Alexey PODKOVSKIY; Nicolas JOLY; Michel Bruneau
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite du Maine
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite du Maine
Priority date: 2014-10-27
Filing date: 2015-10-23
Publication date: 2017-09-06
Anticipated expiration: 2035-10-23
Also published as: FR3027762A1; FR3027762B1; WO2016066938A9; WO2016066938A1; US20170245059A1; US10567885B2; EP3213531B1

Abstract

The present invention concerns an acoustic transducer capable of converting a sound signal into an electric signal. The transducer comprises a mobile element that is movable under the effect of the sound signal, a fixed element opposite the mobile element, a recess (4), and a dissipative element (3) interposed between the mobile element and the fixed element. The coupled system formed in this way has a natural frequency corresponding to a resonance frequency of the transducer at which the sensitivity of same is at a maximum. The mobile element, the fixed element, the dissipative element and the recess are configured such that the quality factor of the acoustic transducer is greater than two. The recess (4) has a straight prismatic or cylindrical or frustoconical general shape, the mobile element forming a first base of the prism, cylinder or frustum, the fixed element being disposed inside said prism, cylinder or frustum, over the second base of the prism, cylinder or frustum. Such a transducer also incorporates an analogue filtering function for filtering the signal around the natural frequency of same.

Description

Electroacoustic transducer, assembly and associated system

The present invention relates to the field of electroacoustic transducers. It relates more particularly to an electroacoustic transducer capable of converting an aerial acoustic signal into an electrical signal. Such an electroacoustic sensor can implement various transduction technologies. It may in particular be of the capacitive, piezoresistive, piezoelectric, electrodynamic or optical type. In general, such an electroacoustic transducer comprises a movable element (such as a deformable membrane, or a suspended or deformable plate, or a flexible blade) whose movement, caused by an acoustic wave, is transformed into an electrical quantity, image of the acoustic pressure, by a transduction element.

A piezoresistive electroacoustic sensor implements piezoresistive gauges placed in the zones of maximum stress of a membrane constituting the movable element.

A piezoelectric electroacoustic sensor implements a piezoelectric coating carried on a membrane constituting the mobile element and electrodes configured to characterize the stresses in the membrane.

An electrodynamic electroacoustic sensor uses a coil and magnets to perform a current measurement when the coil carried by the movable member moves in a fixed magnetic field.

An optical electroacoustic sensor implements an optical measurement of the displacement of the movable element.

Capacitive sensing is the one that offers the greatest sensitivity to the small displacements of the moving element, and thus constitutes the technology preferentially, but not necessarily, implemented in the invention. An electroacoustic capacitive effect sensor, also called electrostatic transducer, comprises a movable electrode positioned facing a fixed rear electrode. The mobile electrode is generally made of a deformable membrane covered with a conductive layer. The movable electrode may also consist of a conductive plate, or according to other known configurations.

The moving electrode and the fixed electrode thus form the armatures of a capacitor, charged by a DC voltage. An acoustic pressure exerted on the moving electrode causes the displacement vis-à-vis the rear electrode, usually by deformation of the membrane that constitutes it. This causes a variation in the capacitance formed between the moving electrode and the fixed back electrode.

Since the electrical charge of the capacitor thus formed is kept constant and equal to the product of the voltage and the capacitance, the variation of the capacitance produces an inverse variation of voltage.

In the known state of the art, this type of sensor corresponds to a microphone technology. The microphones are configured to present on their bandwidth sensitivity as constant as possible. Their bandwidth extends as widely as possible in a band between about 20 Hz and about 20 kHz, which corresponds to the entire audible spectrum.

In a capacitive electroacoustic transducer, the coupled system consists of the moving electrode, a dissipative element (that is to say able to cause energy dissipation), and can typically be an air gap located between the moving electrode and the fixed electrode, and a cavity, has a natural frequency, which corresponds to a resonance frequency of the electroacoustic transducer according to a resonance mode of its own. This is the case for any capacitive electroacoustic transducer. In the case of a capacitive electroacoustic transducer whose moving electrode is a deformable membrane, the resonant frequency can be defined - to a certain extent - by adjusting the voltage of the membrane.

As constant a sensitivity as possible over the bandwidth for which the microphone is configured is obtained on the one hand by configuring the transducer to deviate its first resonant frequency beyond the bandwidth used, and damping this resonance.

Typically, for a microphone, it is conventional to shift this resonance frequency towards the high frequencies, for example beyond 9KHz or more depending on the application: up to 140 kHz for microphones intended for measurements on models (in order to to keep a wavelength at the scale of the model, it is necessary to use a frequency shifted accordingly towards the high frequencies), even up to 0,5 MHz for the study of the waves of shock or the animals emitting in the ultrasounds such as bats. With regard to obtaining a damping allowing the attenuation of the resonance peak according to the main mode or according to other modes that can be located in the bandwidth used, the presence of a film of air between the membrane and the electrode induces damping caused by viscous losses in the air. This acoustic resistance will condition the quality factor, which is a preponderant parameter for characterizing the behavior of the electroacoustic transducer. The quality factor is a dimensionless parameter that characterizes the damping rate of an oscillating system.

The quality factor can be measured or calculated in various known ways. It is defined as the ratio of the natural frequency, at which the gain is maximum, to the width of the bandwidth of the system at -3 dB of the level of the resonance.

The higher the quality factor, the smaller the bandwidth, and the greater the resonance is reflected in a large, high and narrow gain peak.

Thus, an electroacoustic transducer must have a low quality factor, reflecting the absence of a significant peak of resonance. The applications of an electroacoustic transducer operating as a sensor are multiple. In some applications, it is necessary to determine whether the electroacoustic transducer is exposed or not at a given frequency.

For this purpose, it is known to employ an electronic filter that is selective for the frequency in question, or the frequency range considered. Such a filter nevertheless entails a certain complexity of implementation, requires a certain computing power (in the case of a digital filter), and requires a power supply which, too, complicates the implementation of the system. This complexity is detrimental, particularly in systems employing electroacoustic transducers of small size and / or in large numbers. A large number of electroacoustic transducers used as sensors may indeed be necessary to discriminate several frequencies or frequency ranges, and / or to determine the exposure to certain frequencies in an extended space, which requires the dissemination of sensors.

The present invention aims to solve at least one of the aforementioned drawbacks.

In particular, the invention relates to an acoustic transducer adapted to convert an acoustic signal into an electrical signal, comprising a mobile element, under the effect of said acoustic signal, a fixed element arranged facing the mobile element, a cavity, and a dissipative element interposed between the movable element and the fixed element, the coupled system consisting of the movable element, the dissipative element and the cavity having a natural frequency corresponding to a resonant frequency of the transducer at which its sensitivity is maximum wherein the movable element, the fixed element, the dissipative element and the cavity are configured so that the quality factor of the acoustic transducer is greater than two. The cavity is of generally straight or cylindrical or frustoconical prismatic shape, the mobile element forming a first base of the prism, cylinder or truncated cone, the fixed element being disposed inside said prism, cylinder or truncated cone in elevation from the second base of the prism, cylinder or truncated cone. A quality factor greater than two characterizes a selective filter around the eigenfrequency of the moving electrode. Thus, unlike a device such as those known in the state of the art, the filtration of the acoustic signal received is performed directly at the electroacoustic transducer employed as a sensor, analogically, without requiring the use of additional digital means. This results in a great ease of implementation, a low power consumption of the whole, and the lack of complementary calculation means.

Preferably the fixed element has a lower surface to the surface of the movable element. In a variant, the ratio between the surface of the movable element and that of the fixed element is less than or equal to 1/6. In particular, the ratio between the surface of the movable element and that of the fixed element is less than or equal to 1/12.

The dissipative element may consist of a gas or a mixture of gases. For example, the dissipative element may consist of air.

In one embodiment, the movable member and the fixed member are circular.

The cavity may in particular be of cylindrical general shape of revolution.

The coupled system consisting of the moving electrode, the dissipative element and the cavity can be configured so that its natural frequency is between 20Hz and 20KHz. This is the case in the preferred applications of the invention, but a transducer according to some embodiments of the invention may have a resonance frequency in certain ranges of ultrasound or infrasound. The transducer may for example be configured to have a maximum sensitivity frequency between 20kHz and 140KHz. The transducer may for example be configured to have a maximum sensitivity frequency of the order of 500KHz.

The transducer is, in a preferred embodiment, capacitive. In this case, the movable element is a moving electrode, the fixed element is a fixed electrode. The movable electrode may comprise a deformable membrane. The capacitive transducer according to one embodiment of the invention can be configured so that exposure of the moving electrode to an acoustic wave of frequency corresponding to the resonant frequency of the transducer causes contact between the moving electrode and the fixed electrode, so that the transducer forms a switch.

The transducer may further comprise a device for balancing the static pressure prevailing on both sides of the movable element. Such a device for balancing the static pressure may comprise a capillary tube. The static pressure balancing device may comprise a plurality of capillary tubes.

Other features and advantages of the invention will become apparent in the description below.

In the accompanying drawings, given as non-limiting examples:

- Figure 1 shows schematically an electroacoustic transducer according to one embodiment of the invention;

FIG. 2 diagrammatically represents on a graph the behavior of an electroacoustic transducer according to a first configuration of the embodiment of FIG. 1;

FIG. 3 shows schematically on a graph similar to FIG. 2 the behavior of an electroacoustic transducer according to a second configuration of the embodiment of FIG. 1;

FIG. 4 diagrammatically shows on a graph similar to FIGS. 2 and 3 the behavior of an electroacoustic transducer according to a third configuration of the embodiment of FIG.

Figure 1 shows schematically, in a sectional view of a transducer according to one embodiment of the invention. In the embodiment shown here by way of example, the electroacoustic transducer is of revolution about a main axis z.

A capacitive electroacoustic transducer as shown comprises a movable electrode 1 constituting a movable element. In the mode embodiment shown here, the movable electrode 1 is a deformable membrane constituting an electrical conductor (or having an electrically conductive coating). A fixed electrode 2, constituting a fixed element, is arranged facing the moving electrode 1. An air gap between the moving electrode 1 and the fixed electrode 2 constitutes a dissipative element 3. The dissipative element is also resistive. The dissipative element 3 causes a viscous damping effect of the movement of the movable element. In other embodiments not shown, other dissipative fluids may be employed, such as another gas or gas mixture, or a polymer layer. The electroacoustic transducer further comprises a cavity 4, having in the example shown an annular shape because of the particular configuration of the transducer. The cavity 4 thus has, in the example shown here, a general shape of a cylinder of revolution. The moving electrode 1 and the fixed electrode 2 form the armatures of a capacitor, biased by a DC voltage. When an acoustic pressure is exerted on the moving electrode 1, the latter moves towards the fixed electrode. In the example shown here, this movement corresponds to a deformation of the mobile electrode membrane 1. This causes a variation in the capacitance formed between the membrane and the fixed electrode, which produces an inverse voltage variation.

In an electroacoustic transducer according to the invention, the moving electrode 1, the fixed electrode 2, the dissipative element 3 and the cavity 4 are configured so that the quality factor of the acoustic transducer is greater than two.

Such a transducer, used as a sensor, allows, in one application of the invention, the detection of a localized acoustic field having as frequency that defined by the resonance frequency of the coupled system constituted by the moving electrode, the blade of air between the latter and the back electrode and the cavity. An electroacoustic transducer having a high quality factor, typically greater than two, is said to be resonant.

Obtaining a high quality factor is allowed by the adequate choice of the numerous parameters defining the aforementioned elements constituting the transducer (the mobile electrode 1, the fixed electrode 2, the dissipative element 3 and the cavity 4). The choice of these parameters influencing the behavior of the transducer aims in particular to limit the damping of the system, without decreasing the acoustic sensitivity of the transducer. The damping in such a device is caused by the viscous shear of the air knife (or adequate dissipative element 3) located under the membrane (or other mobile electrode 1).

Among the parameters influencing the quality factor, the ratio between the surface of the moving electrode 1, that is to say the surface exposed to the acoustic waves, for example the surface of the membrane adapted to deform under the received waves effect, and the surface of the fixed electrode 2, is preponderant. A resonant capacitive electroacoustic transducer, typically characterized by a quality factor greater than 1.5, and preferably greater than 2, is generally obtained by employing a fixed electrode 2 having an area less than 1/6 of the surface of the moving electrode. 1. For a circular membrane and a fixed electrode 2 also circular, as in the exemplary embodiment shown here, the ratio of the radius of the fixed electrode 2 to the radius of the membrane is thus preferably chosen less than 2/5, corresponding to a surface ratio of 4/25, ie about 1/6.

Reduction of the surface of the fixed electrode 2 relative to that of the moving electrode 1 results in a reduction of the viscous friction damping within the dissipative element 3 (for example the air gap) situated between these two electrodes, when the movement of the moving electrode 1 flushes the dissipative element to the cavity 4, which may in particular be a rear or peripheral cavity. The decrease in the space between the moving electrode 1 and the fixed electrode 2 (also called inter-electrode space) makes it possible to maintain the sensitivity of the transducer without appreciably increasing the viscous damping. For electrodes (fixed and mobile) of the same surface, the spacing of the electrodes to significantly reduce the viscous damping induces a decrease in the static capacity of the transducer, which results in a significant decrease in sensitivity. The surface ratio mentioned above is generalizable to many forms of membranes and fixed electrode 2. Alternatively to a circular membrane, a square, polygonal, oval membrane, etc., can be used as a mobile electrode 1 . Alternatively to a circular electrode, a square, polygonal, oval electrode, etc., may be employed as a fixed electrode 2. All combinations of the aforementioned forms of membrane (or more generally mobile electrode 1) and fixed electrode 2 may be employed in the invention, especially with a surface ratio as above expressed. Indeed, by way of example, the deformation of a square membrane is expressed by a cosine product, and the deformation of a circular membrane according to a Bessel function. For the first mode, and outside the corners of the square membrane, the series developments of their deformed functions are identical up to the second order.

Of course, many other parameters determining the transducer configuration affecting the damping of the system, this maximum surface ratio can be substantially exceeded if the other parameters give the system a very low damping, and this surface ratio must in any state This should be adopted in combination with other parameters that give the system adequate low damping.

The damping of the system is directly related to the dissipative element interposed between the mobile membrane and the fixed membrane and to its viscosity. In particular, the thickness of the viscous boundary layer of the dissipative element 3 (generally air) is important, so that the distance between the moving electrode 1 and the fixed electrode 2 constitutes an important parameter of configuration electroacoustic transducer. Thus, in order to limit the damping to maximize the quality factor, the thickness of the dissipative element (for example the air layer) between the moving electrode 1 and the fixed electrode 2 can be increased.

Among the set of configuration parameters influencing the quality factor of the system are:

- For the definition of the moving element, if it is a membrane: its dimensions - typically its surface or its radius if it is circular - its thickness, the material constituting its substrate and the conductive coating it carries, its voltage (mechanical) at rest;

- For the definition of the fixed electrode: its dimensions - typically its surface or radius if it is circular -, its position in the transducer;

- For the definition of the dissipative element: its constituent material, its thickness, its pressure and its temperature if it is a gas;

- For the definition of the cavity: its general shape and its dimensions,

- For all: the general configuration, including the position and orientation of the aforementioned elements, the polarization voltage of the capacitor thus formed (in the case of a capacitive transducer).

Of course, according to various embodiments of the invention, the parameters to be defined may vary. In particular, the movable element may for example be a bending plate, a rigid suspended plate or a flexible blade embedded at one of its ends (the movement of the other end being characterized). In this case, the parameters defined are in particular those which define the mechanical properties of the movable element.

In order to obtain an adequate configuration for obtaining a maximum gain at the desired frequency, a preferred geometry of the invention, as shown in FIG. 1, comprises an annular cavity defined by a cylinder. of revolution within which the fixed electrode is positioned in elevation with respect to one of the bases of the cylinder. The height of elevation of the fixed electrode 2 makes it possible to adjust the distance between said fixed electrode 2 and the membrane in order to obtain the desired low damping without, however, limiting the sensitivity too much.

Although described according to a preferred embodiment illustrated in FIG. 1, numerous variants can be envisaged without departing from the scope. of the invention. In particular, the cavity 4, previously described in an embodiment in which it is cylindrical of revolution, may, however, have another general shape, including straight prismatic square or rectangular base. The membrane may have a corresponding shape (square, rectangular, ...) especially in the case where it is machined in a silicon wafer by anisotropic chemical etching.

The membrane may alternatively and non-exhaustively be constituted by a bending plate, a flexible blade or a rigid plate suspended by bending arms, provided that cuts allowing the movement of arms do not constitute a short one. acoustic circuit between the front face of the sensor and the air knife or other dissipative element at the rear of the plate.

The electroacoustic transducer may advantageously comprise a device that balances the static pressure, that is to say the atmospheric pressure on either side of the mobile element. This device may comprise one or more capillary tubes.

This static pressure balancing device allows the acoustic sensor to function as a differential sensor. Indeed, the static pressure (atmospheric) being balanced on both sides of the movable element (typically the membrane), the static mechanical tension of the membrane, which influences the sensitivity of the sensor, remains constant. The sensor thus obtained is therefore sensitive only to a dynamic pressure differential, the dimensioning of the capillary tube or tubes so that they behave as an infinite impedance preventing the dynamic pressure from entering the rear of the moving element. typically in the transducer cavity.

In addition, the previously described geometry, in which the fixed element is disposed inside the raised cavity of one of its bases is advantageously applicable to any transducer technology. Typically, for a piezoelectric, piezoresistive, electrodynamic or optical sensor, the movable element (for example a membrane) is positioned facing a stud having an elevation arrangement in the cavity, as previously described for the fixed electrode of a capacitive transducer.

Many alternative embodiments are possible without departing from the scope defined in the invention. Typically, depending on the embodiment considered, the material constituting the mobile part may be silicon, a polymer, metal, or any other suitable material. The paramount parameter which determines the behavior of the moving element is its surface mass, which results from the product density by its thickness (if it is constant).

In the case of the use of silicon, it is possible by deep reactive ion etching ("Deep Reactive Ion Etching" or "DRIE") to produce many forms of membrane and fixed electrode (if the sensor is capacitive ). However, if an anisotropic chemical etching method is used in an aqueous bath, the shape obtained depends on the crystallographic orientation of the silicon wafer and the orientation of the patterns on this wafer with respect to the crystallographic reference. The shapes obtained for initially square or round masks may be as varied as cylinders or cones with square, octagonal or hexagonal bases, or other shapes depending on the orientation of the pattern on the silicon wafer and the orientation of this pattern. last in relation to the crystallographic reference.

In the case of machining a metal or a polymer, it is generally possible to achieve the desired shape, under the conditions and tolerances of machining. FIG. 2 shows a first example of behavior of an electrostatic transducer according to an embodiment of the invention, and configured according to a first configuration.

On the ordinate is carried the sensitivity of the transducer in decibels (dB) referenced to 1 V / Pa, in abscissa the frequency in hertz (Hz).

The general geometry of the transducer corresponds to the embodiment shown in FIG. The main parameters of this first configuration example are as follows. The membrane is circular of a 2.5mm radius, 50 microns thick and 4000 kg density. m ^"3. The tension of the membrane is 213 ^{Nm" 1.} The fixed electrode has a radius of 0.6mm. The distance between the membrane and the fixed electrode is 13 microns. The annular cavity has a depth of 4mm. The bias voltage is 5 V.

The resonant frequency of the transducer for which the sensitivity is maximum is about 5300Hz. The quality factor of the transducer is about 7, which guarantees good selectivity as a sensor. This results in a high and narrow peak of sensitivity around the resonant frequency of the transducer.

FIG. 3 shows schematically on a graph similar to FIG. 2 the behavior of an electroacoustic transducer according to a second configuration of the embodiment of FIG. 1. The scales used are similar to those in Figure 2.

The general geometry of the transducer corresponds to the embodiment shown in FIG. The main parameters of this second configuration example are as follows. The membrane is circular with a radius of 2.5 mm, a thickness of 50 microns and a density of 4000 kg. m ^"3. The tension of the membrane is ^{213N.m" 1.} The fixed electrode has a radius of 0.6mm. The distance between the membrane and the fixed electrode is 15 microns. The annular cavity has a depth of 4mm. The bias voltage is 5 V.

The resonant frequency of the transducer for which the sensitivity is maximum is about 7000Hz. The quality factor of the transducer is about 10, which guarantees good selectivity as a sensor. This results in a high and narrow peak of sensitivity around the resonant frequency of the transducer. This figure nevertheless illustrates the fact that resonance modes may be close to the main mode. In this case, in order to obtain a selective filter around the main mode only, the damping of the system should not be too limited. Thus, for a given surface ratio between the membrane and the fixed electrode, there can be a maximum distance between said membrane and said electrode beyond which other modes of resonance will not be sufficiently attenuated. Thus, while the maximization of the gap between the membrane and the fixed electrode is a general rule of design, certain configurations limit this spacing because of the need to maintain sufficient damping necessary because of the proximity between the main mode of resonance and other modes.

The scales used are similar to those of Figure 1 and Figure 2.

The general geometry of the transducer corresponds to the embodiment shown in FIG. The main parameters of this third configuration example are as follows. The membrane is circular with a radius of 6.5mm, a thickness of 25 microns and a density of 1420 kg. m ^"3. The tension of the membrane is 2621 ^{Nm" 1.} The fixed electrode has a radius of 0.1 mm. The distance between the membrane and the fixed electrode is 5 microns. The annular cavity has a depth of 2mm. The bias voltage is 2 V.

The resonant frequency of the transducer for which the sensitivity is maximum is about 16800Hz. The quality factor of the transducer is about 805. This ensures extreme selectivity as a target frequency sensor. This results in a high and extremely narrow sensitivity peak around the resonant frequency of the transducer.

Moreover, it can be seen that the significant decrease in the radius of the fixed electrode implies a decrease in the inter-electrode space (distance between the membrane and the fixed electrode), which makes it possible to maintain the value of the static capacitance of the transducer . This can nevertheless, in the case of a capacitive transducer, cause diaphragm collapse phenomena (also referred to as "pull-in") which correspond to a maximum deflection of the diaphragm which reaches the contact with the back electrode. It is then necessary to reduce the polarization voltage of the transducer to a much lower level (often referred to as "V_pull_out") in order to release the latter. This phenomenon, which should generally be avoided, can however be exploited in the case where the transducer would no longer be used as a variable capacitor but as a switch activated by an acoustic wave of a given frequency (in this case the resonance frequency of the transducer).

The above examples illustrate resonant transducers whose resonance frequency is between about 5000Hz and 17000Hz. The invention is of preferential application in the audible frequency range, that is to say from about 20 Hz to about 20 kHz, and preferably above 1 kHz.

In addition, the invention can also be applied in the field of ultrasound. The invention can also be applied in certain ranges of infrasound, but a low voltage of the membrane is generally a parameter opposing the obtaining of a resonant capacitive transducer, so that a high quality factor is particularly difficult to reach in the low frequencies.

Thanks to the frequency filtering performed by a transducer having a high quality factor, the transducer can be used successfully even in a noisy environment. Due to the great simplicity of implementation of such a system, and the absence of electronic filtration and the associated power consumption for each transducer, it is possible to easily multiply the number of transducers used, especially in space important and / or to capture several predefined frequencies.

Thus, by adopting a design contrary to that sought for conventional microphones, whose bandwidth should be the largest possible in the field of audible frequencies, the invention developed here proposes the design of an electroacoustic transducer whose resonance is little attenuated so that it behaves like a frequency filter selective. This results in a high quality factor, unknown in the field of microphones, typically greater than two.

Claims

1. Acoustic transducer adapted to convert an acoustic signal into an electrical signal, comprising a mobile element under the effect of said acoustic signal, a fixed element arranged facing the mobile element, a cavity (4), and a dissipative element (3) interposed between the movable element and the fixed element,

the coupled system consisting of the movable element, the dissipative element (3) and the cavity (4) having a natural frequency corresponding to a resonant frequency of the transducer at which its sensitivity is maximum, the movable element, the element fixed, the dissipative element (3) and the cavity (4) being configured so that the quality factor of the acoustic transducer is greater than two, characterized in that the cavity (4) is of generally straight or cylindrical prismatic shape or frustoconical, the movable element forming a first base of the prism, cylinder or truncated cone, the fixed element being disposed inside said prism, cylinder or truncated cone elevation of the second base of the prism, cylinder or trunk of cone.

2. acoustic transducer according to claim 1, wherein the fixed element has a lower surface to the surface of the movable element.

3. Acoustic transducer according to claim 2, wherein the ratio between the surface of the movable element and that of the fixed element is less than or equal to 1/6, and preferably less than or equal to 1/12.

4. Acoustic transducer according to one of the preceding claims, wherein the dissipative element (3) consists of a gas or a mixture of gases.

5. acoustic transducer according to claim 4, wherein the dissipative element (3) consists of air.

6. Acoustic transducer according to one of the preceding claims, wherein the movable element and the fixed element are circular.

7. acoustic transducer according to any one of the preceding claims, wherein the cavity (4) is generally cylindrical in shape of revolution.

8. acoustic transducer according to any one of the preceding claims, wherein the coupled system consisting of the moving electrode, the dissipative element (3) and the cavity (4) is configured so that its natural frequency is between 20Hz and 20KHz.

9. acoustic transducer according to one of the preceding claims, the movable element being a movable electrode (1), the fixed element being a fixed electrode (2), the transducer being a capacitive transducer.

10. Transducer according to claim 9, wherein the movable electrode (1) comprises a deformable membrane.

1 1. Transducer according to claim 9 or claim 10, configured so that exposure of the moving electrode (1) to an acoustic wave of frequency corresponding to the resonant frequency of the transducer causes contact between the moving electrode (1) and the fixed electrode (2), so that the transducer forms a switch.

12. Transducer according to one of the preceding claims, further comprising a device for balancing the static pressure prevailing on either side of the movable member.

13. Transducer according to claim 12, wherein the static pressure balancing device comprises a capillary tube.