Acoustic device with variable resonance frequency

Abstract

According to one aspect, the invention relates to an acoustic device (DA) comprising: an input port (IN) for receiving an audio electrical signal; a loudspeaker (HP) associated with an idle resonance frequency and comprising a measuring means (CP2) configured to measure the movement of the loudspeaker (HP); a means for processing a digital signal (MT) configured to determine the instantaneous frequency of the audio electrical signal received on the input port (IN); the device being characterised in that it comprises a controller (CTR), the controller being associated with a gain, the resonance frequency of the loudspeaker being a function of the gain of the controller (CTR), the controller (CTR) being configured to control, on the basis of the measurement of the movement of the loudspeaker (HP) performed by the measuring means (CP2), in a closed loop, the resonance frequency of the loudspeaker (HP) on the instantaneous frequency of the audio electrical signal received on the input port and determined by the means for processing a digital signal (MT).

Description

Acoustic device with variable resonant frequency 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 acoustic device and, in particular, to an acoustic device comprising one or more loudspeakers whose resonant frequency can be modulated. 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 membrane (see for example patent US9980051B2).

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

[0009] There is therefore a need for an acoustic device equipped with one or more loudspeakers whose resonance 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.

[0010] The invention provides a solution to the problems mentioned above, by proposing an acoustic device in which the resonance 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 acoustic device comprising:An input port intended to receive an audio electrical signal;A loudspeaker associated with a resonant frequency at rest and comprising a measuring means configured to measure the movement of the loudspeaker;A means for processing a digital signal configured to determine the instantaneous frequency of the audio electrical signal received on the input port;

[0012] In addition, the device comprises a controller, the controller being associated with a gain, the resonant frequency of the loudspeaker being a function of the gain of the controller, the controller being configured to control, from the measurement of the movement of the loudspeaker carried out by the measuring means, in a closed loop, the resonant frequency of the loudspeaker on the instantaneous frequency of the audio electrical signal received on the input port and determined by the digital signal processing means.

[0013] Thanks to the device according to the first aspect of the invention, it is possible to reproduce all the frequencies of the audio band with substantially the same amplitude, with a low distortion rate, the resonance frequency of the loudspeaker adapting to the frequency of the sound to be reproduced.

[0014] The term "electrical audio signal" means an electrical signal corresponding to an electrical signal intended to be converted into an audio signal by one or more loudspeakers. The electrical audio signal comprises, for example, an instantaneous frequency, that is to say it includes a predominant frequency component, that is to say one whose amplitude is significantly greater (for example at least twice, or at least five times, or even ten times greater, or even more) than the other frequencies of this signal. The electrical audio signal can thus be a single-frequency signal (then comprising a single frequency which is the predominant frequency), or a multi-frequency signal comprising the predominant frequency.The audio electrical signal may also be a multi-frequency signal comprising several predominant frequencies, which are sufficiently spaced from each other, in the frequency spectrum of this signal, to allow the application of a band-pass filter around each of the predominant frequencies in order to retain, in the filtered frequency band, only one of the predominant frequencies.

[0015] 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 audio electrical signal received on the input port and determined by the digital signal processing means.

[0016] A second aspect of the invention relates to an acoustic device comprising:An input port intended to receive an electrical audio signal;A plurality of loudspeakers, each loudspeaker of the plurality of loudspeakers 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;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.

[0017] Furthermore, in the device according to a second 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 acoustic device comprises, for each loudspeaker of the plurality of loudspeakers, a controller associated with a gain, the resonant frequency of the loudspeaker being a function of the gain of the controller, the controller being configured to control, from the measurement of the movement of the loudspeaker carried out by the measuring means associated with the loudspeaker considered, in a closed loop, the resonant frequency of the loudspeaker considered as a function of 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 and determined by the means for processing a digital signal,the bandpass filter being configured to adjust its predetermined frequency to this same instantaneous frequency.,

[0018] Thanks to the device according to the second aspect of the invention, it is possible to benefit from the advantages of the device according to a first aspect of the invention even when the audio electrical signal is not monotonous.

[0019] In one embodiment, the quiescent resonant frequencies of the plurality of speakers are distributed based on the harmonics of a string instrument.

[0020] In addition to the characteristics which have just been mentioned in the preceding paragraphs, the acoustic device according to a first or 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.

[0021] In one embodiment, each bandpass filter is a digital bandpass filter whose predetermined frequency can be digitally varied.

[0022] In one embodiment, each loudspeaker comprises at least one movable mechanical structure, said movable mechanical structure comprising at least one movable mechanical element and, for each of these movable mechanical elements, a first piezoelectric layer, said first layer being arranged on a first part of the movable mechanical element so as to be able to actuate said movable mechanical element, and a second piezoelectric layer, said second layer being arranged on a second part of the movable mechanical element, distinct from the first part, so as to be able to convert the mechanical energy associated with the movement of the movable mechanical element into electrical energy and thus measure the displacement of the movable mechanical element.

[0023] In one embodiment, the movable mechanical structure consists of a movable 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.

[0024] In one embodiment, the first layer forms a first ring and the second layer forms a second ring surrounding the first ring.

[0025] In one embodiment: The diameter of the disc-shaped membrane is between 1 mm and 25 mm;The first piezoelectric layer has a width between And , and has an internal radius between 0 mm and ;The second piezoelectric layer has a width between And and has an internal radius between And

[0026] 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.

[0027] In one embodiment, the closed loop is implemented by series-connecting the controller with a subtractor and the speaker.

[0028] 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

[0029] The figures are presented for information purposes only and in no way limit the invention.

[0030] Illustrates the change in acoustic pressure associated with the presence of a cavity.

[0031] Shows a schematic representation of a device according to a first aspect of the invention without a bandpass filter.

[0032] Illustrates the evolution of the resonance frequency as a function of the gain of the controller of an electromechanical device according to the invention.

[0033] Shows a schematic representation of a device according to a first aspect of the invention comprising a bandpass filter.

[0034] Shows a schematic representation of a device according to a second aspect of the invention comprising a bandpass filter.

[0035] Illustrates a possible distribution of resonance frequencies and frequency ranges associated with a plurality of loudspeakers (8 loudspeakers) of an acoustic device according to the invention.

[0036] La represents the amplitude of the signal in time / frequency of an acoustic device according to the invention.

[0037] Figure 8B illustrates an embodiment in which the mobile structure is made using a circular membrane.

[0038] Figure 9 illustrates an embodiment in which the mechanical structure comprises a rigid membrane actuated by cantilevers.

[0039] Figure 10 illustrates an embodiment of a cantilever comprising, on its upper surface, a first piezoelectric layer and a second piezoelectric layer.

[0040] Figures 11A and 11B illustrate one embodiment of the mobile structure made using a circular membrane.

[0041] Figure 12 shows a schematic representation of a device according to a first aspect of the invention. DETAILED DESCRIPTION

[0042] Unless otherwise specified, the same element appearing in different figures has a single reference.

[0043] Device according to a first aspect of the invention

[0044] A first aspect of the invention illustrated herein concerns an acoustic device DA comprising: an input port IN intended to receive an audio electrical signal; a loudspeaker HP associated with a resonant frequency at rest and comprising a measuring means CP2 configured to measure the movement of the loudspeaker HP; a means for processing a digital signal MT configured to determine the instantaneous frequency of the audio electrical signal received on the input port IN.

[0045] The resting resonant frequency is the resonant frequency of the HP loudspeaker in the absence of control as described in the following paragraph.

[0046] The acoustic device DA according to the first aspect of the invention is original in that it comprises a CTR controller, the controller being associated with a gain, the resonant frequency of the loudspeaker being a function of the gain of the CTR controller. In addition, the CTR controller is configured to control, in a closed loop, the resonant frequency of the loudspeaker HP on the instantaneous frequency of the audio electrical signal received on the input port and determined by the means for processing a digital signal MT. This closed loop control corresponds to a transfer function equal to 1 in the frequency domain. Also, the device being a high-pass filter, the resonant frequency adapts (for example decreases) to tend towards a frequency response equal to 1. Such control is illustrated in which represents the evolution of the resonant frequency of the loudspeaker HP as a function of the gain K of the CTR controller.In the (or leset), the CTR controller is present in the lower branch of the servo loop, but this is only an illustration and it could also be positioned in the upper branch of the loop.

[0047] The instantaneous frequency is, for example, determined in quasi-real time by the digital signal processing means MT, during the reception of the audio electrical signal. That is to say, the instantaneous frequency is not determined prior to the use of the acoustic device to reproduce the audio electrical signal, but on the fly, by the processing means MT, during the reception of the audio electrical signal on the input port IN.

[0048] In an embodiment illustrated in the, the device according to a first aspect of the invention comprises a bandpass filter FPB, preferably a digital bandpass filter, configured to isolate, in the audio electrical signal received on the input port IN, the audio electrical 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 audio electrical signal received on the input port IN and determined by the digital signal processing means MT. 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 (taking into account the control already described).

[0049] 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).

[0050] Device according to a second aspect of the invention

[0051] The acoustic device DA according to a first aspect of the invention 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.

[0052] For this, a second aspect of the invention illustrated herein concerns an acoustic device DA comprising: An input port IN intended to receive an audio electrical signal; A plurality of HP loudspeakers, each HP loudspeaker of the plurality of HP loudspeakers 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 in question; A means for processing a digital signal MT configured to determine the frequency content of the audio electrical signal received on the input port IN; For each HP loudspeaker of the plurality of HP loudspeakers, 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 electrical signal at the output of the bandpass filter FPB being sent to the HP loudspeaker in question.

[0053] The device according to a second aspect of the invention is original 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 acoustic device DA comprises, for each HP loudspeaker of the plurality of HP loudspeakers, a CTR controller associated with a gain, the resonance frequency of the loudspeaker being a function of the gain of the CTR controller, the controller being configured to control, in a closed loop, the resonance frequency of the HP loudspeaker considered as a function of 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 and determined by the digital signal processing means MT,the FPB bandpass filter being configured to adjust its predetermined frequency to this same instantaneous frequency.,

[0054] As already mentioned, in the device according to a second aspect of the invention, 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). More particularly, the represents the frequency response of eight HP loudspeakers, each curve being associated with an HP loudspeaker, each peak corresponding to the resting resonance frequency of the loudspeaker, the gray band surrounding each of the peaks representing the frequency band of the loudspeaker associated with said peak and in which the resonance frequency can be adjusted.

[0055] In one embodiment, the quiescent (or intrinsic) resonant frequencies of the plurality of HP speakers are distributed according to the harmonics of a string instrument, for example a guitar, a piano, etc. As illustrated in which represents the amplitude of the signal in time / frequency, such a DA device makes it possible to reproduce (without attack) guitar notes with a vibrato.

[0056] Common features

[0057] In addition to the common general principle, the devices according to the first aspect and the second aspect of the invention have many common optional features.

[0058] 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.

[0059] In an embodiment illustrated in Figure 10, each loudspeaker comprises 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 and thus measure the displacement of the mobile element (the first layer CP1 and the second layer CP2 being separated from each other).

[0060] The mobile structure SM according to the invention can adopt different configurations depending on the intended use.

[0061] In one embodiment shown in Figure 8B, 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 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 shown in , the first piezoelectric layer CP1 forms a disk in the center of the membrane and the second piezoelectric layer CP2 forms a ring around this disk. In an alternative embodiment shown in Figure 8B, the first layer CP1 forms a first ring and the second layer CP2 forms a second ring surrounding the first ring.

[0062] In some embodiments, as illustrated in Figures 11A and 11B: The diameter of the disc-shaped membrane is between 1 mm and 25 mm;The first piezoelectric layer CP1 has a width between And , and has an internal radius between 0 mm and ;The second piezoelectric layer CP2 has a width between And and has an internal radius between And .

[0063] In an embodiment illustrated in Figure 9, 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.

[0064] In an embodiment illustrated in Figure 10, 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 Figure 10, taking into account the symmetry of the beam, this boundary is located at mid-length of the beam, i.e. at a distance L / 2 from one end of the beam, L being the length of the beam.

[0065] In an alternative embodiment, each movable element is made using a beam embedded at its two ends.

[0066] In one embodiment, compatible with the previous embodiments, the closed loop is induced by series connection of the CTR controller with a subtractor and the loudspeaker HP, in particular the mobile element EM. The subtractor, as shown in Figures 2, 4 and 5, is used to compare, in particular by subtraction, the audio electrical signal, received on the input port IN, with the measurement of the displacement of the mobile element EM.

[0067] The CTR controller is, for example, a voltage amplifier, for example based on an inverting or non-inverting circuit, including an operational amplifier, or any other analog circuit amplifier based on transistors. In the example in Figure 12, detailing only the closed loop, it is a non-inverting circuit.

[0068] In the embodiment of Figure 12, showing a block diagram of the acoustic device, the subtracter D is a circuit based on an operational amplifier AO, typically comprising the operational amplifier AO and four resistors R. This subtracter has:Its first input IN1 (the positive input of the subtracter) connected to the input port IN; andIts second input IN2 (the negative input of the subtracter) connected to the output of the controller CTR;Its output OUT connected to the mobile element EM, and more particularly to the first piezoelectric layer CP1.

[0069] The controller CTR is therefore responsible for receiving the measurement of the displacement of the mobile element EM, in particular of the second piezoelectric layer CP2, of amplifying this measured signal, in particular by multiplication by the gain, and of transmitting the signal thus amplified to the subtracter D. The subtracter D, by subtracting the audio electrical signal with the amplified signal, modifies, or even adjusts, the resonance frequency of the loudspeaker HP, via the actuation of the first piezoelectric layer CP1.

[0070] The gain of the electronic circuit of the controller can be controlled by a digital command issued by the digital signal processing means MT to the CTR controller, for example using a DSP.

[0071] In some embodiments, compatible with the previous embodiments, the output OUT of the subtractor D is connected to the second piezoelectric layer CP2, and the first piezoelectric layer CP1 is connected to the input of the controller CTR. The second piezoelectric layer CP2 is then used to generate the displacement of the mobile element EM in order to reproduce the audio electrical signal, while the first piezoelectric layer CP1 is used to measure the displacement of the mobile element EM. In other words, the connection of the loudspeaker HP, via the first and second piezoelectric layers CP1 and CP2, to the controller CTR and to the subtractor D, in the closed loop, can indifferently be implemented to adjust the resonant frequency of the loudspeaker HP.

Claims

Acoustic device (DA) comprising:An input port (IN) intended to receive an electrical audio signal;A loudspeaker (HP) associated with a resonant frequency at rest and comprising a measuring means (CP2) configured to measure the movement of the loudspeaker (HP);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);the device being characterized in that it comprises a controller (CTR), the controller being associated with a gain, the resonant frequency of the loudspeaker being a function of the gain of the controller (CTR), the controller (CTR) being configured to slave, from the measurement of the movement of the loudspeaker (HP) carried out by the measuring means (CP2), in a closed loop, the resonant frequency of the loudspeaker (HP) to the instantaneous frequency of the electrical audio signal received on the input port and determined by the means for processing a digital signal (MT). Acoustic device (DA) according to the preceding claim comprising a band-pass 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 electrical signal at the output of the band-pass filter (FPB) being sent to the loudspeaker (HP), the band-pass filter (FPB) being configured to adjust its predetermined frequency to the instantaneous frequency of the audio electrical signal received on the input port (IN) and determined by the digital signal processing means (MT). Acoustic device (DA) comprising:An input port (IN) intended to receive an electrical audio signal;A plurality of loudspeakers (HP), each loudspeaker (HP) of the plurality of loudspeakers (HP) being associated with a resting resonance frequency, different from the resting resonance frequency of the other loudspeakers (HP) and comprising a measuring means (CP2) configured to measure the movement of the loudspeaker (HP) in question;A digital signal processing means (MT) configured to determine the frequency content of the electrical audio signal received on 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 on 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) in question; the device (DA) being characterized in that the resonance frequency of each loudspeaker (HP) of the plurality of loudspeakers (HP) is adjustable around the resonance frequency at rest of the loudspeaker (HP) in question in a predetermined frequency range, and in that the acoustic device (DA) comprises, for each loudspeaker (HP) of the plurality of loudspeakers (HP), a controller (CTR) associated with a gain, the resonance frequency of the loudspeaker being a function of the gain of the controller (CTR), the controller being configured to control from the measurement of the movement of the loudspeaker (HP) carried out by the measuring means (CP2) associated with the loudspeaker (HP) in question, in a closed loop,the resonance frequency of the loudspeaker (HP) considered as a function of the instantaneous frequency of the signal received on the input port (IN) having the highest amplitude in the predetermined frequency range associated with the loudspeaker (HP) considered and determined by the digital signal processing means (MT), the bandpass filter (FPB) being configured to adjust its predetermined frequency to this same instantaneous frequency., Acoustic device (DA) according to the preceding claim in which the resonant frequencies at rest of the loudspeakers (HP) of the plurality of loudspeakers (HP) are distributed according to the harmonics of a string instrument. Acoustic device (DA) according to one of claims 2 to 4 wherein the bandpass filter (FPB) or each bandpass filter (FPB) is a digital bandpass filter whose predetermined frequency can be modified digitally. Acoustic device (DA) according to one of the preceding claims in which each loudspeaker (HP) comprises at least one mobile mechanical structure (SM), said mobile mechanical structure (SM) comprising at least one mobile mechanical element (EM) and, for each of these mobile mechanical elements (EM), a first piezoelectric layer (CP1), said first layer (CP1) being arranged on a first part of the mobile mechanical element (EM) so as to be able to actuate said mobile mechanical element (EM), and a second piezoelectric layer (CP2), said second layer (CP2) being arranged on a second part of the mobile mechanical element (EM), distinct from the first part, so as to be able to convert the mechanical energy associated with the movement of the mobile mechanical element (EM) into electrical energy and thus measure the displacement of the mobile mechanical element (EM). Acoustic device (DA) according to the preceding claim, in which the mobile mechanical 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. Acoustic device (DA) according to the preceding claim in which the first layer (CP1) forms a first ring and the second layer (CP2) forms a second ring surrounding the first ring. Acoustic device (AD) according to claim 8, wherein:The diameter of the disc-shaped membrane is between 1 mm and 25 mm;The first piezoelectric layer (CP1) has a width between And , and has an internal radius between 0 mm and ;The second piezoelectric layer (CP2) has a width between And and has an internal radius between And . Acoustic device (DA) according to claim 6 wherein the mobile mechanical structure (SM) comprises a rigid surface (SR) configured to be able to perform a translational movement perpendicular to its surface, the mobile mechanical element(s) (EM) of the mobile mechanical structure (SM) being configured to actuate the rigid surface (SR) according to said movement. Acoustic device (DA) according to one of claims 6 to 9, in which the closed loop is implemented by series connection of the controller (CTR) with a subtractor (D) and the loudspeaker (HP).

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