ELECTROMECHANICAL DEVICE WITH VARIABLE RESONANCE FREQUENCY AND ACOUSTIC DEVICE THEREOF

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of microsystems, in particular microsystems for acoustic applications.

[0002] The present invention relates to an electromechanical device and, in particular, to an electromechanical device whose resonant frequency can be modulated. The present invention also relates to an acoustic device making use of an electromechanical device according to the invention. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] A loudspeaker is used to convert an electrical signal into sound pressure. For many years, loudspeakers have been miniaturized for integration into computers, cell phones, and wireless headphones. More specifically, the loudspeaker is an electro-mechanical-acoustic transducer. In its linear principle, the loudspeaker operates by actuating a membrane, coupled to the ambient air. The electrical signal passes through a first electromechanical transducer, which converts the voltage into displacement. A mechanical-acoustic transducer, very often a membrane, then converts this displacement into sound pressure.

[0004] A good loudspeaker is one that reproduces all frequencies in the audio band (20 Hz to 20 kHz) at the same amplitude, with low distortion. In practice, the lowest frequency at which a loudspeaker effectively produces sound is determined by the resonant frequency of the mechanical-acoustic transducer. In the context of miniaturization, the diaphragm guide system is more rigid and the mass of the diaphragm is lower, which increases the resonant frequency of the system and therefore reduces its bandwidth. In addition, to avoid destructive interference between the front and rear acoustic waves of the loudspeaker, a hermetic cavity is required. This hermetic cavity increases the apparent stiffness of the system and therefore its resonant frequency, thus reducing its bandwidth.

[0005] Furthermore, the pressure level radiated by a loudspeaker depends on the volume of air accelerated by the loudspeaker. This accelerated air volume depends on the product of the surface area and the maximum displacement of the membrane. In a context of miniaturization, the surface area of ​​the membrane is greatly reduced, and a large displacement is therefore necessary to obtain a satisfactory pressure level.

[0006] To achieve large displacements, electromagnetic transduction remains a solution of choice, and it is this one that equips the vast majority of loudspeakers. Although this type of loudspeaker shows good performance, their dimensions do not allow integration into portable systems. In addition, the use of a magnet makes the manufacture of these loudspeakers incompatible with micro-manufacturing processes.

[0007] Another means of transduction showing notable performances is piezoelectric transduction. Although not providing displacements as large as electromagnetic transduction, piezoelectric transduction has the advantage of being compatible with micro-fabrication processes. For example, it is possible to use the bimetallic effect and an actuator positioned on a membrane in order to obtain relatively large displacements. However, this is not the only possible configuration. For example, in another configuration, the piezoelectric actuators are offset from the membrane, this solution making it possible to produce a "pistonic" movement of the latter (see for example patent US9980051B2). Document FR3000354 relates to a piezoelectric type loudspeaker, comprising a membrane whose displacement is controlled.

[0008] However, state-of-the-art solutions have limitations, particularly in terms of frequency response. The [ Fig. 1 ] shows the frequency response of a MEMS loudspeaker, with and without a 100 mm 3< rear cavity. Increasing the resonant frequency removes much of the radiated pressure in the low frequencies.

[0009] There is therefore a need for an electromechanical device whose frequency can be varied, and in particular lowered, as required. There is also a need for an acoustic device equipped with such an electromechanical device so as to have a loudspeaker whose resonant frequency can vary, thus making it possible to reproduce all the frequencies of the audio band with substantially the same amplitude, with a low distortion rate, the loudspeaker also being compatible with micro-fabrication techniques. SUMMARY OF THE INVENTION

[0010] The invention provides a solution to the problems mentioned above by proposing an electromechanical device whose frequency can be varied, and in particular lowered, as required. The invention also provides an acoustic device using such an electromechanical device and in which the resonant frequency of the loudspeaker(s) adapts to the frequency or frequencies of the acoustic signal emitted by said loudspeaker(s).

[0011] For this, a first aspect of the invention relates to an electromechanical device comprising: At least one mobile mechanical structure comprising at least one mobile mechanical element; For each mobile mechanical element of each mobile mechanical structure: a first piezoelectric layer, said first layer being arranged on a first part of the mobile mechanical element so as to be able to actuate said mobile mechanical element; a second piezoelectric layer, said second layer being arranged on a second part of the mobile mechanical element, distinct from the first part, so as to be able to convert the mechanical energy associated with the movement of the mobile mechanical element into electrical energy, said second layer forming a capacitance;

[0012] The electromechanical device according to the invention is remarkable in that it comprises an electrical circuit connected to the second piezoelectric layer in parallel with the capacitance formed by said second layer and comprising an adjustable capacitance which can take a negative value.

[0013] Thanks to the invention, it is possible to vary the resonance frequency of the moving element(s) (and therefore of the moving mechanical structure) by varying the value of the adjustable capacitance.

[0014] In addition to the characteristics which have just been mentioned in the preceding paragraph, the electromechanical device according to a first aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations.

[0015] In one embodiment, the adjustable capacitance capable of taking a negative value is achieved using an operational amplifier.

[0016] In one embodiment, the adjustable capacitance capable of taking a negative value is produced by micro-fabrication.

[0017] In one embodiment, the movable mechanical structure consists of a movable mechanical element in the form of a disc-shaped membrane, the first piezoelectric layer and the second piezoelectric layer being arranged on the surface of said membrane.

[0018] In one embodiment, the mobile mechanical structure comprises a rigid surface configured to be able to perform a translational movement perpendicular to its surface, the mobile mechanical element(s) of the mobile mechanical structure being configured to actuate the rigid surface according to said movement.

[0019] In one embodiment, each movable mechanical element is made using a beam embedded at one of its ends and guided, the first piezoelectric layer being disposed on a first portion of an upper surface of the beam and the second piezoelectric layer being disposed on a second portion of the upper surface of the beam.

[0020] In one embodiment, each movable element is made using a beam embedded at its two ends.

[0021] A second aspect of the invention relates to an acoustic device comprising: an input port for receiving an audio electrical signal; a loudspeaker comprising an electromechanical device according to a first aspect of the invention, the loudspeaker being associated with a resonant frequency at rest and comprising a measuring means configured to measure the movement of the loudspeaker; and a digital signal processing means configured to determine the instantaneous frequency of the audio electrical signal received on the input port.

[0022] Furthermore, in the acoustic device according to the invention, the resonant frequency of the loudspeaker is adjustable around the resting frequency of said loudspeaker and the digital signal processing means is configured to send the instantaneous frequency of the signal to the loudspeaker, the latter being configured to adjust its resonant frequency to the instantaneous frequency of the audio electrical signal received on the input port.

[0023] “Audio electrical signal” means an electrical signal corresponding to an electrical signal intended to be converted into an audio signal by one or more loudspeakers.

[0024] In addition to the characteristics which have just been mentioned in the preceding paragraph, the acoustic device according to a second aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations.

[0025] In one embodiment, the acoustic device comprises a bandpass filter configured to isolate, in the audio electrical signal received on the input port, the audio electrical signal at a predetermined frequency, the electrical signal at the output of the bandpass filter being sent to the loudspeaker, the bandpass filter being configured to adjust its predetermined frequency to the instantaneous frequency of the signal received on the input port and determined by the digital signal processing means.

[0026] A third aspect of the invention relates to an acoustic device comprising: an input port for receiving an electrical audio signal; a plurality of loudspeakers, each loudspeaker of the plurality of loudspeakers comprising an electromechanical device according to a first aspect of the invention, each loudspeaker being associated with a resting resonance frequency, different from the resting resonance frequency of the other loudspeakers, and comprising a measuring means configured to measure the movement of the loudspeaker in question; a means for processing a digital signal configured to determine the frequency content of the electrical audio signal received on the input port; and for each loudspeaker of the plurality of loudspeakers, a bandpass filter configured to isolate, in the electrical audio signal received on the input port, the electrical audio signal at a predetermined frequency, the electrical signal at the output of the bandpass filter being sent to the loudspeaker in question.

[0027] Furthermore, in the device according to a third aspect of the invention, the resonant frequency of each loudspeaker of the plurality of loudspeakers is adjustable around the resting resonant frequency of the loudspeaker considered in a predetermined frequency range, and the means for processing a digital signal is configured to send, to each loudspeaker of the plurality of loudspeakers, the instantaneous frequency of the signal received on the input port having the highest amplitude in the predetermined frequency range associated with the loudspeaker considered, the latter being configured to adjust its resonant frequency to this instantaneous frequency, the bandpass filter associated with the loudspeaker considered being configured to adjust its predetermined frequency to this same instantaneous frequency.

[0028] In addition to the characteristics which have just been mentioned in the preceding paragraph, the acoustic device according to a third aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations.

[0029] In one embodiment, the quiescent resonant frequencies of the plurality of speakers are distributed based on the harmonics of a string instrument.

[0030] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0031] The figures are presented for information purposes only and in no way limit the invention. There [ Fig. 1 ] illustrates the change in acoustic pressure associated with the presence of a cavity. The [ Fig. 2] illustrates an equivalent diagram of an electromechanical device according to the invention. The [ Fig. 3 ] illustrates the evolution of the resonance frequency as a function of the value of the adjustable capacitance of an electromechanical device according to the invention. The [ Fig. 4 ] illustrates that the adjustable capacitance is realized using an operational amplifier in one embodiment of an electromechanical device according to the invention. The [ Fig. 5A] and [Fig. 5B ] illustrate an embodiment in which the mobile structure is made using a circular membrane. The [ Fig. 6 ] illustrates an embodiment in which the mechanical structure comprises a rigid membrane actuated by cantilevers. The [ Fig. 7 ] illustrates an embodiment of a cantilever comprising, on its upper surface, a first piezoelectric layer and a second piezoelectric layer. The [ Fig. 8] schematically illustrates the operating principle of an acoustic device according to the invention without a bandpass filter. The [ Fig. 9 ] schematically illustrates the operating principle of an acoustic device according to the invention with a bandpass filter. The [ Fig. 10 ] schematically illustrates the operating principle of an acoustic device according to the invention with a plurality of loudspeakers. The [ Fig. 11 ] illustrates a possible distribution of resonant frequencies and frequency ranges associated with a plurality of loudspeakers (8 loudspeakers) of an acoustic device according to the invention. The [ Fig. 12 ] represents the amplitude of the signal in time / frequency of an acoustic device according to the invention. DETAILED DESCRIPTION

[0032] Unless otherwise specified, the same element appearing in different figures has a single reference. Electromechanical device à variable resonant frequency

[0033] A first aspect of the invention illustrated in [ Fig. 2 ] has [ Fig. 7] relates to an electromechanical device comprising at least one mobile mechanical structure SM (hereinafter mobile structure), said mobile structure SM comprising at least one mobile mechanical element EM (hereinafter mobile element) and, for each of these mobile elements EM, a first piezoelectric layer CP1, said first layer CP1 being arranged on a first part of the mobile element EM so as to be able to actuate said mobile element EM, and a second piezoelectric layer CP2, said second layer CP2 being arranged on a second part of the mobile element EM, distinct from the first part, so as to be able to convert the mechanical energy associated with the movement of the mobile element EM into electrical energy (thus the first layer CP1 and the second layer CP2 are separated from each other), the second layer CP2 forming a capacitance.The electromechanical device according to the invention is remarkable in that it comprises an electrical circuit CEL connected to the second piezoelectric layer CP2, said electrical circuit comprising an adjustable capacitance which can take a negative value. Furthermore, this adjustable capacitance is in parallel with the capacitance formed by the second piezoelectric layer CP2 so that the equivalent capacitance can take a positive value or a negative value as required.

[0034] In order to understand the advantages resulting from an electromechanical device according to the invention, it is interesting to note that a piezoelectric layer CP1, CP2 acts as a transducer. Also, the electrical components connected to such a layer have an influence on the mechanical behavior of the mobile element EM on which said piezoelectric layer CP1, CP2 is deposited.

[0035] The behavior of the mobile element EM can then be modeled by an equivalent electrical diagram such as that illustrated in [ Fig. 2 ]. In this diagram, the electromechanical structure according to the invention is represented by an equivalent electrical circuit comprising a series resistance R g representing the output resistance of the amplifier (responsible for actuating the mobile element EM), a capacitor It's not representing the capacitive effect of the first piezoelectric layer CP1, a transformer γ a representing the electromechanical transduction of the first piezoelectric layer CP1 (in other words, representing the conversion of the electrical energy supplied by the amplifier into mechanical energy of the mobile element EM), a resistance R ms representing the viscous losses in the mechanical domain undergone by the moving element EM, an inductance M ms representing the moving mass of the moving element EM, a capacitance C msrepresenting the apparent stiffness of the mobile element EM relative to the fixing point or points of said mobile element EM, a transformer S d representing the mechano-acoustic transduction, an impedance Z rad representing the radiation impedance of the EM moving element, a capacitance C ps representing the capacitive effect of the second piezoelectric layer CP2 and a capacitance C n in parallel with the capacity created by the second piezoelectric layer CP2 and which can take negative values ​​(this aspect will be detailed later).

[0036] Knowing that the second piezoelectric layer CP2 has an influence on the behavior of the mobile element EM, connect this second layer CP2 to a capacitor C nhaving a negative value makes it possible to reduce the apparent stiffness of the EM mobile element and therefore to modify its resonance frequency. From the previous equivalent diagram, it is possible to express the resonance frequency of the EM mobile element using the following relationship: f s = 1 2 π M ms C eq Or M ms is the equivalent mass of the moving element EM and C eq is the equivalent capacitance of the circuit given by the following relation: C eq = 1 1 C ms + 1 C ps + C n γ s 2

[0037] Thus, it is possible to increase or decrease the resonance frequency of the EM mobile element by playing on the negative value of the capacitance C n as shown in the [ Fig. 3 ]. In this figure, the frequency response of the EM moving element is represented for different values ​​of C ps + C nlocated between -1nF and -10nF. Preferably, the values ​​of the capacitances are chosen so that the resonant frequency can be varied within a range of + / - 20% around the resonant frequency at rest (corresponding to the situation in which the CEL circuit in parallel with the capacitance formed by the second layer is open).

[0038] In one embodiment illustrated in [ Fig. 4 ], the adjustable capacity is achieved using an operational amplifier. The impedance thus obtained is given by the following relationship: C n = − R 2 R 1 C n ′

[0039] Or R 1 , R 2 and C n ′ take positive values, at least one of which may vary. Preferably, the value of C n ′ can be varied.

[0040] The mobile structure SM according to the invention can adopt different configurations depending on the intended use.

[0041] In one embodiment illustrated in [ Fig. 5A ] and the [ Fig. 5B ], the mobile structure SM consists of a mobile element EM in the form of a disc-shaped membrane, the first piezoelectric layer CP1 and the second piezoelectric layer CP2 being arranged on the surface of said membrane. In one embodiment, the diameter of the disc-shaped membrane is between 1 mm and 25 mm. Preferably, the first piezoelectric layer CP1 is located in a part of the surface of the membrane separated from the part of the surface of the membrane where the second piezoelectric layer CP2 is located, the separation between the two parts being at the level of the inflection of the membrane (in a section along the diameter of the latter), generally at 2 / 3 of the radius from the center of the membrane. In one embodiment illustrated in [ Fig. 5A], the first piezoelectric layer CP1 forms a disc in the center of the membrane and the second piezoelectric layer CP2 forms a ring around this disc. In an alternative embodiment illustrated in [ Fig. 5B ], the first layer CP1 forms a first ring and the second layer CP2 forms a second ring surrounding the first ring.

[0042] In one embodiment illustrated in [ Fig. 6], the mobile structure SM comprises a rigid surface SR configured to be able to perform a translational movement perpendicular to its surface (movement represented by the dotted arrow in the figure), the mobile element(s) EM of the structure SM being configured to actuate the rigid surface SR according to said movement. Of course, the equivalent electrical diagram and the operating principle remain the same as in the case introduced previously: the influence of the second piezoelectric layer CP2 present on each mobile element EM makes it possible to modify the resonance frequency of the mobile structure SM as a whole.

[0043] In one embodiment illustrated in [ Fig. 7], each mobile element EM is produced using a beam embedded at one of its ends at a frame CR and guided (taking into account the symmetry of the structure), the first piezoelectric layer CP1 being arranged on a first part of an upper surface of the beam and the second piezoelectric layer CP2 being arranged on a second part of the upper surface of the beam, distinct from the first part. In one embodiment, the boundary between the first part and the second part is located at an inflection point of the beam (when the latter is set in motion). In the example of the [ Fig. 7 ], taking into account the symmetry of the beam, this boundary is located at mid-length of the beam, that is to say at a distance L / 2 from one end of the beam, L being the length of the beam.

[0044] In an alternative embodiment, each movable element is made using a beam embedded at its two ends.

[0045] An electromechanical device according to the invention can advantageously be used to produce a HP loudspeaker whose resonant frequency is variable, the resonant frequency of the loudspeaker being given by the frequency of the electromechanical device of said loudspeaker. Similarly, it is possible to produce a plurality of HP loudspeakers, each HP loudspeaker being produced with an electromechanical device according to the invention and associated with a resonant frequency at rest, preferably different for each HP loudspeaker. Preferably, each HP loudspeaker comprises a mobile structure SM produced using a membrane as shown in [ Fig. 5A ] or to the [ Fig. 5B]. Preferably, each loudspeaker HP comprises a control circuit configured to determine, from a set frequency, the value of the negative capacitance making it possible to obtain a resonance frequency equal to the set frequency, the control circuit being further configured to control the electrical circuit CEL of the electromechanical device according to the invention in order to obtain the value of the capacitance (and therefore the resonance frequency) desired. In addition, the control circuit is configured to determine the movement of the loudspeaker using the second piezoelectric layer CP2 and, from this movement, the vibration frequency of the loudspeaker. Acoustic device comprising an electromechanical device with variable resonant frequency

[0046] The electromechanical device according to the invention can advantageously be used in an acoustic device in order to obtain an acoustic device comprising a loudspeaker whose resonance frequency can be modulated (i.e. a loudspeaker as presented previously). For this, a second aspect of the invention illustrated in [ Fig. 8 ] relates to an acoustic device DA comprising: an input port IN intended to receive an electrical audio signal; a loudspeaker HP comprising an electromechanical device according to the invention, the loudspeaker HP being associated with a resonant frequency at rest and comprising a measuring means CP2 configured to measure the movement of the loudspeaker HP; and a means for processing a digital signal MT configured to determine the instantaneous frequency of the electrical audio signal received on the input port IN.

[0047] Furthermore, in the device according to a second aspect of the invention, the resonant frequency of the HP loudspeaker is adjustable around the resting frequency of said HP loudspeaker and the digital signal processing means MT is configured to send the instantaneous frequency of the signal to the HP loudspeaker, the latter being configured to, from this instantaneous frequency, adjust its resonant frequency.

[0048] The DA acoustic device according to the invention can, for example, be integrated into a mobile phone, a tablet or any other equipment requiring small speakers and / or requiring only a single frequency to be reproduced, although it varies over time (e.g. a buzzer in an electronic device).

[0049] In the device according to a second aspect of the invention, the loudspeaker being produced using an electromechanical device DE according to a first aspect of the invention, it is possible to control the frequency of the latter via the electrical circuit CEL of the electromechanical device DE and measure the movement of the loudspeaker HP (and possibly, from this movement, deduce the vibration frequency) and to control the variable capacitance in order to adjust the resonance frequency of the loudspeaker HP.

[0050] In one embodiment illustrated in [ Fig. 9], the acoustic device DA comprises a bandpass filter FPB configured to isolate, in the audio electrical signal received on the input port IN, the audio electrical signal at a predetermined frequency, the filtered electrical signal at the output of the filter being sent to the loudspeaker HP. In addition, the predetermined frequency of the bandpass filter FPB is adjusted according to the instantaneous frequency of the audio electrical signal received on the input port IN. Thus, when the input signal is not monotonous, the bandpass filter FPB makes it possible to filter out the parasitic signals in order to send to the loudspeaker HP only the signal at the instantaneous frequency of the input signal, the latter being moreover the resonance frequency of the loudspeaker HP. In other words, the predefined frequency of the bandpass filter FPB and the resonance frequency of the loudspeaker HP are slaved to the instantaneous frequency of the input signal.

[0051] In one embodiment, the instantaneous frequency of the input electrical audio signal is determined using a Short-Time Fourier Transform (STFT). Of course, this is only an example. Other methods well known to those skilled in the art may be used, such as a Hilbert Transform or a derivative of the phase as a function of time.

[0052] In the preceding embodiments, the acoustic device DA is configured to emit only a single frequency, the instantaneous frequency of the input signal (this frequency can however vary over time). It may however be interesting to be able to emit in a plurality of frequencies, for example to reproduce the sound emitted by a string instrument or to vary the tones of a buzzer.

[0053] For this, a third aspect of the invention illustrated in [ Fig. 10] (for an example in which three loudspeakers are present - the device can of course include a greater number of loudspeakers) relates to a DA acoustic device comprising: an input port IN intended to receive an electrical audio signal; a plurality of HP loudspeakers, each HP loudspeaker of the plurality of HP loudspeakers comprising an electromechanical device according to the invention, the HP loudspeaker being associated with a resting resonance frequency, different from the resting resonance frequency of the other HP loudspeakers and comprising a measuring means CP2 configured to measure the movement of the HP loudspeaker considered; A means for processing a digital signal MT configured to determine the frequency content of the electrical audio signal received on the input port IN; and For each HP loudspeaker of the plurality of HP loudspeakers, a bandpass filter FPB configured to isolate, in the electrical audio signal received on the input port IN, the electrical audio signal at a predetermined frequency, the filtered electrical signal at the output of the bandpass filter FPB being sent to the HP loudspeaker considered.

[0054] In addition, the acoustic device DA according to the invention is remarkable in that the resonance frequency of each HP loudspeaker of the plurality of HP loudspeakers is adjustable around the resting resonance frequency of the HP loudspeaker considered in a predetermined frequency range, and in that the digital signal processing means MT is configured to send, to each HP loudspeaker of the plurality of HP loudspeakers, the instantaneous frequency of the signal received on the input port IN having the highest amplitude in the predetermined frequency range associated with the HP loudspeaker considered, said HP loudspeaker being configured to adjust its resonance frequency to this instantaneous frequency. In addition, the bandpass filter FPB associated with the HP loudspeaker considered is configured to adjust its predetermined frequency to this same instantaneous frequency.In other words, among all the frequencies present in the signal which are in the frequency band compatible with the HP loudspeaker considered, the frequency associated with the greatest signal amplitude is that on which the control of the resonance frequency of the HP loudspeaker and the predefined frequency of the FPB bandpass filter will take place.

[0055] As already mentioned, each HP loudspeaker of the plurality of HP loudspeakers is associated with a resting resonance frequency, different from the resting resonance frequency of the other HP loudspeakers. In addition, each HP loudspeaker of the plurality of HP loudspeakers is adjustable around the resting resonance frequency of the HP loudspeaker considered in a predetermined frequency range (this frequency range therefore defining a frequency band). The [ Fig. 11] represents the frequency response of eight HP loudspeakers, each curve being associated with one HP loudspeaker, each peak corresponding to the speaker's resting resonant frequency, the gray band surrounding each of the peaks representing the frequency band of the loudspeaker in which the resonant frequency can be adjusted.

[0056] In one embodiment, the quiescent resonant frequencies of the plurality of HP speakers are distributed according to the harmonics of a string instrument, e.g., a guitar, a piano, etc. As illustrated in [ Fig. 12 ] which represents the amplitude of the signal in time / frequency, such a DA device makes it possible to reproduce (excluding attack) guitar notes with a vibrato.

Claims

1. An electromechanical device comprising: - at least one movable mechanical structure comprising at least one movable mechanical element (EM); - for each movable mechanical element (EM) of each movable mechanical structure (SM): ∘ a first piezoelectric layer (CP1), said first layer (CP1) being disposed on a first part of the movable element (EM) so as to be able to actuate said movable mechanical element (EM); o a second piezoelectric layer (CP2), said second layer (CP2) being disposed on a second part of the movable element (EM), distinct from the first part, so as to be able to convert mechanical energy associated with the movement of the movable mechanical element (EM) into electric energy, said layer forming a capacitance; the device being characterised in that it includes an electrical circuit connected to the second piezoelectric layer (CP2) in parallel with the capacitance formed by said layer (CP2) and including an adjustable capacitance which can assume a negative value.

2. The device according to the preceding claim, wherein the adjustable capacitance which can assume a negative value is made using an operational amplifier.

3. The device according to claim 1, wherein the adjustable capacitance capable of assuming a negative value is made by micro-fabrication.

4. The device according to one of the three preceding claims, wherein the movable mechanical structure (SM) consists of a movable element (EM) in the form of a disc-shaped membrane, the first piezoelectric layer (CP1) and the second piezoelectric layer (CP2) being disposed on the surface of said membrane.

5. The device according to the preceding claim, wherein the first piezoelectric layer (CP1) forms a first ring and the second piezoelectric layer (CP2) forms a second ring surrounding the first ring.

6. The device according to claim 1, claim 2 or claim 3, wherein the movable mechanical structure (SM) comprises a rigid surface (SR) configured to be able to perform translational movement perpendicular to its surface, the movable mechanical element(s) (EM) of the movable mechanical structure (SM) being configured to actuate the rigid surface (SR) according to said movement.

7. The device according to the preceding claim, wherein each movable element (EM) is made by means of a beam embedded at one end thereof and guided, the first piezoelectric layer (CP1) being disposed on a first part of an upper surface of the beam and the second piezoelectric layer (CP2) being disposed on a second part of the upper surface of the beam, distinct from the first part.

8. The device according to claim 6, wherein each movable mechanical element (EM) is made by means of a beam embedded at both ends thereof.

9. A loudspeaker comprising at least one device according to one of the preceding claims.

10. An acoustic device (DA) comprising: - an input port (IN) for receiving an electrical audio signal; - a loudspeaker (HP) according to the preceding claim, said loudspeaker being associated with a rest resonant frequency and comprising a measurement means (CP2) configured to measure movement of the loudspeaker (HP); - a digital signal processing means (MT) configured to determine instantaneous frequency of the electrical audio signal received at the input port (IN); the device being characterised in that the resonant frequency of the loudspeaker (HP) is adjustable about the rest frequency of said loudspeaker (HP) and the digital signal processing means (MT) is configured to send the instantaneous frequency of the signal to the loudspeaker (HP), the same being configured to adjust its resonant frequency on the basis of this instantaneous frequency.

11. The acoustic device (DA) according to the preceding claim comprising a bandpass filter (FPB) configured to isolate, in the electrical audio signal received at the input port (IN), the electrical audio signal at a predetermined frequency, the electrical signal at the output of the bandpass filter (FPB) being sent to the loudspeaker (HP), the bandpass filter (FPB) being configured to adjust its predetermined frequency to the instantaneous frequency of the electrical audio signal received at the input port (IN).

12. An acoustic device (DA) comprising: - an input port (IN) for receiving an electrical audio signal; - a plurality of loudspeakers (HP) according to claim 9, each loudspeaker (HP) of the plurality of loudspeakers (HP) being associated with a rest resonant frequency, different from the rest resonant frequency of the other loudspeakers (HP) and comprising measurement means (CP2) configured to measure movement of the loudspeaker (HP) considered; - a digital signal processing means (MT) configured to determine the frequency content of the electrical audio signal received at the input port (IN); - for each loudspeaker (HP) of the plurality of loudspeakers (HP), a bandpass filter (FPB) configured to isolate, in the electrical audio signal received at the input port (IN), the electrical audio signal at a predetermined frequency, the electrical signal at the output of the bandpass filter (FPB) being sent to the loudspeaker (HP) considered; the device (DA) being characterised in that the resonant frequency of each loudspeaker (HP) of the plurality of loudspeakers (HP) is adjustable about the rest resonant frequency of the loudspeaker (HP) considered within a predetermined frequency range, and the digital signal processing means (MT) is configured to send, to each loudspeaker (HP) of the plurality of loudspeakers, the instantaneous frequency of the signal received at the input port (IN) having the highest amplitude in the predetermined frequency range associated with the loudspeaker (HP) considered, said loudspeaker (HP) being configured to adjust its resonant frequency to this instantaneous frequency, the bandpass filter (FPB) associated with the loudspeaker (HP) considered being configured to adjust its predetermined frequency to this same instantaneous frequency.

13. The device (DA) according to the preceding claim, wherein the rest resonant frequencies of the loudspeakers (HP) of the plurality of loudspeakers (HP) are distributed as a function of harmonics of a stringed instrument.