WO2012073262A2

WO2012073262A2 - "a device for sound reproduction with coaxial acoustic actuators"

Info

Publication number: WO2012073262A2
Application number: PCT/IT2010/000478
Authority: WO
Inventors: Ferruccio Resta; Francesco Castelli Dezza; Simone Cinquemani; Francesco Braghin; Matteo Cibelli
Original assignee: Politecnico Di Milano; Esarc Hi-Tech S.R.L.
Priority date: 2010-12-01
Filing date: 2010-12-01
Publication date: 2012-06-07
Also published as: WO2012073262A3

Abstract

A device for sound reproduction (1) comprising: a first acoustic actuator (2) a second acoustic actuator (4), wherein the first and the second acoustic actuators (2, 4) are arranged coaxially to each other. Furthermore: the second acoustic actuator (4) is arranged substantially within the first acoustic actuator (2), the device comprises a connection structure (6) through which the first acoustic actuator (2) and the second acoustic actuator (4) are connected to each other, the connection structure (6) comprises elastic elements (68, 98) which connect and support one with respect to the other the first and the second acoustic actuators (2, 4).

Description

"A device for sound reproduction with coaxial acoustic actuators" Field of the invention

The present invention refers to a device for sound reproduction of the type couplable to a support means. More precisely, the device comprises different types of acoustic actuators and it is adapted to cause vibrations in the support means to which it is coupled.

Description of the prior art

In the field of devices for sound reproduction, sound reproduction quality plays a fundamental role given that the user's satisfaction or dissatisfaction generally depends on it.

Considering the range of high performance products, the previously described need is greatly felt in that the users for whom the product is intended often have higher requirements than those of an average user.

Besides, there also arises the aesthetic requirement, i.e. that of having the possibility of installing a device for sound reproduction hiding it within, for example, a structural and or furniture element, a motor vehicle, a boat.

Regarding this, devices for sound reproduction of the type adapted for coupling with support means are particularly important. Such devices are arranged for converting the audio signal input therein into a mechanical action on the support means with frequency bound to that of the audio signal and capable of causing vibrations in the support means.

In such type of devices the use of acoustic actuators of the magnetostrictive type, i.e. actuators employing an element made of magnetostrictive material is widely spread.

As known, magnetostriction is, concisely, a property of some materials, called magnetostrictive materials, to vary their dimensions when submerged in a magnetic field variable over time. Magnetostrictive acoustic actuators exploit dimensional variations of an element made of magnetostrictive material to impart displacements and high frequency forces which result into vibrations of the support means to which they are coupled characterized by the same frequency.

It is also known that acoustic actuators of the magnetostrictive type guarantee an ideal sound reproduction quality, though limited to the high and medium-high frequencies range. In the low and medium-low frequency range, the deterioration of performance thereof generally dictates to rely upon other soluiions, because a possible adaptation of the characteristics^' thereof to the abovementioned frequency range causes considerable technical complications and a loss of flexibility regarding installation.

Solutions proposed in the prior art, also including various solutions proposed by the Applicant, provide for combining a magneto strictive acoustic actuator with a magnetodynamic acoustic actuator. The latter is generally optimised for offering the best performance in the low and medium-low frequency range and generally comprises a permanent magnet and a coil movable one with respect to the other. The relative movement is obtained by feeding the coil with a current variable over time.

However, the Applicant observed that the simple combination between magnetostrictive acoustic actuators and magnetodynamic acoustic actuators is not per se enough to guarantee correct sound reproduction in the audible frequency range. Besides the mere combination of two different types of actuators, the way the abovementioned actuators are coupled to each other is also particularly important.

The Applicant previously proposed in such field several solutions characterised by the use of different types of actuators, just like there are several publications addressing this subject in the prior art.

However, the inventors observed that within the prior art solutions the need for attaining high performance is often set above that of obtaining high compactness, hence making the installation of the devices addressed herein difficult at times. This can constitute a considerable hindrance to the application of the prior art solutions for example in structures having a sophisticated design and highly irregular shapes, in which the available spaces are often very limited.

Field of the invention

The object of the present invention is that of overcoming the technical drawbacks described previously, in particular that of providing a device for sound reproduction capable of guaranteeing, the performance with respect to prior art devices being equal, high compactness and small overall dimensions.

Summary of the invention

The object of the present invention is attained by a device for sound reproduction having the features forming the subject of the following claims, which form an integral part of the technical disclosure herein provided in relation to the invention. Furthermore, the invention is configured as an improvement of a device previously proposed by the Applicant.

Detailed description of the attached drawings

Now, the present invention shall be described with reference to the attached drawings, provided purely by way of non-limiting example, wherein:

- figure 1 is a perspective view of a device for sound reproduction according to an embodiment of the present invention,

- figure 2 is a perspective view according to the arrow II of figure 1 ,

- figure 3 is a sectional view according to the line III-III of figure 1, and - figures 4A-B illustrate a comparison between a detail indicated by the arrow IV of figure 3 and carried out according to an advantageous aspect of the present invention and the same detail carried out in a known manner.

Detailed description of the invention

A device for sound reproduction according to an embodiment of the present invention is indicated with 1 in figure 1. Furthermore, with reference to figures 2, 3, the device 1 comprises, all coaxial with respect to a main axis X, a first acoustic actuator 2, a second acoustic actuator 4 arranged substantially within the first acoustic actuator 2, a connection structure 6 and a transmission foot 8.

The first acoustic actuator 2 is of the magnetodynamic type (reason why, in the following description, it will at times be indicated with the term "magnetodynamic actuator 2") and comprises, in this embodiment, an inner annular plate 10, an outer annular plate 12, a permanent magnet 14, a tubular element 16, a counterplate 18 and a coil 20 carried by a support ring 22. All components of the magnetodynamic actuator 2 indicated previously are coaxial to each other and with respect to the axis X.

The inner annular plate 10 comprises an inner diameter Dl and an outer diameter D2. Analogously, the outer annular plate 12 comprises an inner diameter D3 larger than the outer diameter D2 of the inner annular plate 10 and an outer diameter D4. The difference between the. diameters D3 and D2 defines an annular shaped free volume with thickness G (evidently equivalent to the difference between the diameters D3 and D2).

The support ring 22 and the coil 20 fixed thereto are inserted into the abovementioned free volume and they have an overall radial thickness smaller than the dimension G.

The permanent magnet 14 is constrained to the outer annular plate 12 by means of structural adhesives, while the tubular element 16 is fixed to the inner annular plate 10 by means of a plurality of screws 26. Furthermore, the tubular element 16 preferably has an inner diameter equivalent to the diameter Dl of the inner annular plate 10.

The counterplate 18 is annular shaped and it is fixed both to the permanent magnet 14 by means of structural adhesives and to the tubular element 16 by means of a plurality of screws 30 identical to the screws 26. Furthermore, the counterplate 18 is positioned at an opposite side of the tubular element 16 (in the axial direction) with respect to the annular plates 10, 12, and it has, preferably, an inner diameter equivalent to the diameter Dl and outer diameter equivalent to the diameter D4.

Preferably, the annular plates 10, 12 and the counterplate 18, like the tubular element 16, are made using ferromagnetic material. The permanent magnet 14 is preferably a ferrite-based magnet.

The group of elements comprising the inner and outer annular plates 10,

12, the tubular element 16, the counterplate 18 and the permanent magnet 14 shall be as a whole indicated in the following, for reasons that will appear clear over the description, by the term "moving assembly of the magnetodynamic actuator" or at times, more briefly as, "moving assembly".

It should be observed that the sequence of the inner annular plate 10, the tubular element 16 and the counterplate 18 substantially defines a cylindrical- shaped central cavity C wherein the second acoustic actuator 4 is housed.

The second acoustic actuator 4 is of the magnetostrictive type (reason why, in the present description, it will at times be indicated by the term "magnetostrictive actuator 4") and comprising a casing 32, a coil 34 and a magnetostrictive core indicated in its entirety with reference number 36.

The casing 32 is substantially bell- shaped and comprises, at a first end thereof, a connection flange 38 including a plurality of holes 39 having axis parallel to the axis X and arranged in a crown along a circumference having diameter D5. At a second end of the casing 32, opposite with respect to the flange 38, there are provided threaded bosses 40, preferably three. The casing 32 is made using diamagnetic material.

The magnetostrictive core 36 comprises a spool 42 including a first and a second end collar 44, 46 and a central through hole 48. Within the central through hole 48 there is housed, coaxial to the axis X, a rod 50 made of magnetostrictive material whose ends are at direct contact with a first and a second disc made of amagnetic material 52, 54, preferably aluminium. The discs 52, 54 are in turn at direct contact with a first and a second discoidal magnet 56, 58 preferably made of neodymium. The rod 50 also has a length greater than that of the central through hole 48, so that the previously described pairs of discs 52, 56 and 54, .58 are housed at least partly in the collars 44, 46 and they are never both in contact with a bottom surface, fiat and orthogonal to the axis X, of the collars 44, 46.

The coil 34 is wound on the spool 42 between the end collars 44, 46, so as to surround the rod 50.

Once again with reference to figure 3, the connection structure 6 comprises a first and a second diaphragm element 60, 62. With reference to figures 2, 3, the first diaphragm element 60 is substantially circular-shaped and comprises a first crown 64, a first hub 66 and first elastic elements 68.

The crown 64 comprises a plurality of fixing brackets 70 (here in number of six) and a circular seat 72 having a maximum diameter equivalent to D4. The circular seat 72 and the fixing brackets 70 define a single abutment surface Al orthogonal to the axis X. The hub 66 is substantially annular- shaped and it develops axially. In the hub 66 there is provided a plurality of axial holes (thus with axis parallel to the axis X) indicated with reference number 74.

The axes of the holes 74 are arranged along a circumference having diameter D5, same as the axes of the holes 39. The hub 66 further comprises a collar 75 radially comprised between the holes 74 and having a central through hole 75A coaxial to the axis X. In this embodiment, three radial supports, each generally indicated with reference number 76, are integral with the hub 66 and they are arranged alternating with respect to the elastic elements 68.

Each radial support 76 comprises, in this embodiment, three brackets 78 integral with a track 80 in which there is provided a circumferential groove 82 in which there is received the support ring 22 of the coil 20. The elastic elements 68 are arranged in a bridge-like fashion between the hub 66 and the crown 64, connecting the two latter. In the embodiment illustrated herein, the diaphragm element 60 comprises three elastic elements 68 integral with the crown 64 and the hub 66, angularly equally spaced (thus staggered by 120° one with respect to the other) and being substantially S-shaped.

In particular, with reference to figure 2, each elastic element 68 comprises a first and a second end stretch 84, 86 converging, respectively, in the hub 66 and in the crown 64 and having with respect thereto and to the diaphragm element 60 a substantially radial orientation. Each connection elastic element 68 further comprises a central stretch 88 developing circumferentially which is joined to the stretches 84, 86 by means of a first and a second curved joint 90, 92 thus defining the abovementioned "S" shape.

With reference to figures 1, 3, the second diaphragm element 62 comprises a second circular crown 94, a second hub 96 and a plurality of second elastic elements 98.

The second crown 94 comprises a plurality of fixing brackets 100 angularly equally spaced (here in number of six and identical to the brackets 70) and a circular seat 102. The circular seat 102 has a maximum diameter equivalent to D4 and defines, with the fixing brackets 100, a second abutment surface A2.

The hub 96 is circular-shaped and it comprises, in this embodiment, three through holes 104 having a homologous arrangement with respect to the bosses 40. The hub 96 further comprises an axial tubular protrusion 106 projecting with respect thereto on the side whereon the fixing brackets 100 are provided.

The elastic elements 98 are substantially S-shaped in this embodiment in a manner identical to that of the elastic elements 68 and they extend in a bridge-like fashion between the hub 96 and the crown 94 connecting them. In this embodiment the elastic elements 98 are integral with the hub 96 and the crown 94, they are three and angularly equally spaced (thus they are staggered by 120° one with respect to the other).

Each elastic element 98 comprises a first and a second end-stretch 108, 110 converging, respectively, in the hub 96 and in the crown 94, and an intermediate stretch 1 12 having a circumferential development and joined with respect to the end stretches 108, 1 10 by means of a first and a second curved joint 114, 116, which define the abovementioned "S" shape.

The transmission foot 8 comprises, with reference to figure 3, a first and a second axisymmetric element 118, 120 comprising, respectively, a first and a second flange 122, 124. The first and the second element 1 18, 120 are joined to each other by means of a screw 126.

The modalities of assembly of the components of the device 1 will now be described.

With reference to figures 2, 3, the first diaphragm element 60 receives the outer annular plate 12 in abutment against the first support surface Al, thus it receives it within the circular seat 72 and on the fixing brackets 70. A plurality of fastening screws 128 fix the outer annular plate 12 to the fixing brackets 70 (preferably a single screw 128 is provided for each bracket 70) thus connecting the moving assembly of the magnetodynamic device to the crown 64. Alternatively, a structural adhesive can be used for fixing the plate 12 to the crown 64.

The coil 20 carried by the support ring 22 is instead connected to the hub 66, and in particular to the support formations 76, so that the ring 22 is inserted into the circumferential groove 82.

The transmission foot 8 is positioned, traversing the hole 75A, within the hub 66 so that the second flange 124 is axially supported by the collar 75 of the hub. In this embodiment, as illustrated in figure 3, a disc spring 130 is interposed between the flange 124 and the collar 75.

The magnetostrictive actuator 4 is arranged within the cavity C and it is fixed to the hub 66 by means of screws traversing the holes 74 and 39 (not illustrated). Thus, the magnet 56 is in direct contact with the flange 124 and the entire magnetostrictive core 36, which is thus comprised between the transmission foot 8 and the casing 32, is preloaded along the axis X by the disc spring 130. Such disc spring will result compressed when tightening the screws that fix the casing 32 to the hub 66.

The second diaphragm element 62 is fitted both on the magnetodynamic actuator 2 and on the magnetostrictive actuator 4 and it is coupled thereto. In particular, the crown 94 is coupled to the counterplate 18 so that the latter is received within the circular seat 102 and it abuts against the abutment surface A2, thus also abutting on the fixing brackets 100. A plurality of fastening screws 132, preferably one for each fixing bracket 100, fix the counterplate 18 to the crown 94.

Alternatively, the counterplate 18 can be fixed to the crown 94 by means of structural adhesives.

Thus, the magnetodynamic actuator 2, due to the connection existing between the counterplate 18 and the permanent magnet 14, the tubular element 16 and the inner annular plate 10, is fixed to the crown 94 (obviously except for the support ring 22 and the coil 20). The axial tubular protrusion 106 is instead fitted on the casing 32 of the magnetostrictive actuator 4 and screws 134 traverse the holes 104 and are engaged in threaded bosses 40, thus fixing the hub 96 to the casing 32 of the actuator 4. Alternatively, fixing can be obtained in another manner, for example by gluing. The device for sound reproduction 1 operates as follows.

The device 1 is arranged for connection to an apparatus for reading media on which the audio signal is stored. The connection can be of the direct type (for example a connection to a digital reader apparatus - such as, hypothetically, a digital reader -) or of indirect type, for example by interposing an amplifier between the reader apparatus and the device 1. In such case, the device 1 receives an input signal from the abovementioned amplifier.

In any case, the signal entering the device 1 is the result, of a processing of the original audio signal by a reader apparatus and possibly, if present, of the amplifier. More precisely, the processing of the original audio signal generates a signal characterised by an electric current variable over time which feeds the coils 20, 34. The distribution of the frequency components of the abovementioned signal between the coils 20, 34 can be carried out by means of a so-called "crossover" device, per se known.

With reference to figure 3, each of the two actuators 2, 4 is capable of causing, in a support means to which the device 1 is connected by means of the foot 8, vibrations with frequencies belonging to range whose extremes are determined by the dynamic characteristics of the device 1. For example, extremely low frequency components - in the order of 1 Hz - generally cannot be transmitted to the support means.

Generally, such interval is a subgroup of the frequency range of the signal entering the device 1.

In any case, the modalities through which the previously described components are coupled allow identifying substantially two distinct subgroups within the device 1.

In particular, there can be identified a substantially rigid core and a periphery with a certain degree of axial deformability and capable of relative movement with respect to the rigid core.

More in detail, the abovementioned rigid core substantially comprises the hubs 66, 96, the first coil 20 together with the support ring 22 (as a logical consequence of the coupling to the hub 66) and the second actuator 4.

The abovementioned periphery instead comprises the crown 64, 94, the annular plates 10, 12, the tubular element 16, the permanent magnet 14 and the counterplate 18, alongside the respective connection elements.

As mentioned previously, the annular plates 10, 12, the permanent magnet 14, the tubular element 16 and the counterplate 18 (connected as described) functionally define a moving assembly (dynamically an inertial mass) of the magnetodynamic actuator 2 which is elastically connected with respect to the substantially, rigid core due to the elastic connection elements 68 and 98, which substantially have the function of axial springs.

Thus, the elastic elements 68, 98 connect and support the actuator 2 with respect to the actuator 4 and enable the moving assembly of the magnetodynamic actuator 2 to move relatively to the magnetostrictive actuator 4 however discharging the constraint reactions on the transmission foot 8.

The device for sound reproduction 1 is arranged for coupling to a support means such as, for example, a panel or a part of a vehicle body, a furnishing and/or structural element or generally any structure capable of vibrating in a wide frequency spectrum without jeopardizing the structural integrity thereof.

The transmission foot 8 is fixed to the abovementioned support means through, for example, a gel or a double sided adhesive tape (both having high mechanical transmissibility) and transmits, to the abovementioned support means, the forces that are discharged on the transmission foot 8 by the acoustic actuators 2, 4.

The low and medium-low frequency signals arrive, downstream of the processing carried out by the abovementioned crossover device, in form of electric current in the coil 20.

The direct consequence is the generation of an electromagnetic force directed parallel to the axis X which causes a translation in the axial direction (parallel to the axis X) of the moving assembly of the actuator 2. Such electromagnetic force is generated as an effect of the interaction between the coil

20 traversed by a current and the permanent magnet 14.

The high and medium-high signals reach, downstream of the processing carried out by the abovementioned crossover device, the coil 34 in form of electric current. Upon variation of the current circulating in the coil 34 the induced magnetic field in the rod 50 made of magnetostrictive material is modified, causing several contractions and dilatations in the axial direction with frequencies corresponding to the variations of the current circulating in the coil 34.

This results in a transmission of forces to the transmission foot 8 having the same frequency as the current circulating in the coil 34. As known to a man skilled in the art, the forces (not stationary) transmitted to the foot 8 through the actuators 2, 4 have different frequencies which overlap (generally without interfering) and globally transmit to the support means to which the device 1 is coupled a force whose frequency components, as mentioned, are a subgroup of those of the signal entering the device 1.

When operating, the transmission foot 8 remains substantially fixed with respect to the support means to which it is constrained, while the rigid core moves with respect to the abovementioned transmission foot due to the action of the rod 50 made of magnetostrictive material.

The movement of the moving assembly of the magnetodynamic actuator 2 is, due to the connection structure 6, substantially decoupled by the movement of the rod 50 made of magnetostrictive material (in particular by the contractions and dilatations thereof).

The methods through which the actuators 2, 4 are connected to each other, in particular due to the connection structure 6, allows obtaining an extremely compact device for sound reproduction, hence enabling the installation even in narrow spaces.

Generally, the configuration of the diaphragm elements 60, 62, and in particular of the elastic elements 68, 98, can vary depending on the rigidity values intended to be assigned thereto.

In the case of substantially S-shaped elastic elements, such as those described previously, the inventors observed that such geometry represents a possible good compromise between radial overall dimension and rigidity.

In other words, the "S" shape of the connection elements 68, 98 enables having sufficiently low rigidity values (hence influencing the lower frequency limit reproducible by the device 1) due to the length- wise development of each of them, but keeping the radial overall dimension thereof (and thus the radial distance between the corresponding hubs 66, 96 and the corresponding crowns 64, 94) low given that each of the elastic elements 68, 98 is substantially folded on itself to define an "S" shape. This enables achieving high compactness, such to allow the installation of the device 1 even on complex structures.

The rigidity of the elastic elements 68, 98 can obviously be modulated according to the needs by intervening on one or more factors: besides the shape of the elastic elements, the desired rigidity value can be obtained through a different choice of material or of the shape or of the path along which each elastic element develops. For example, in a preferred embodiment, the material of the diaphragm elements 60, 62 is a polymeric material. Metal materials, for example harmonic steel, can however be used for making elastic elements 68, 89.

Obviously, without prejudice to what has been described above, the elastic elements 68, 98 can be provided with a shape that can vary even significantly from the previously described substantially S-shaped element. Maintaining other parameters (for example the shape and the material) constant, allows obtaining higher rigidity values by shortening the elastic elements 68, 98, i.e. passing from the abovementioned substantially "S" shape to progressively more rectilinear geometries (i.e. reducing the entity of the folding on itself - so to speak - of each elastic element 68, 98). Rectilinear elastic elements., possibly more than three (actually, it should be observed that with rectilinear elastic elements the circumferential overall dimension, considering the same radial overall dimension, is smaller with respect to the configuration with substantially S-shaped elastic elements), can be used.

Even in this case, difficulties related to installation in complex structures can be overcome due to the resulting compactness.

It should be observed that the described solutions provide the same mechanical connection scheme between the acoustic actuators 2, 4 substantially reproducing a structure comprising a rigid core and an axially deformable peripheral axially movable with respect to the aforementioned rigid core.

Furthermore, according to an advantageous aspect of the present invention, the performance of the device 1 can be maintained constant for a considerable longer time interval with respect to devices of the known type due to the interposition of the discs 52, 54 between the discoidal magnets 56, 58 and the rod 50. With reference to figures 4A, 4B, the difference as regards with the behaviour and the property of the magnetic field in case of use of a magnetostrictive core such as the core 36 or a magnetostrictive core of the known type can be observed.

With reference to figure 4A, which corresponds to the magnetostrictive core 36, the magnetic field generated by the coil 34 comprises field lines FL which, as known, have a toroidal development around the axis X (generally around the axis of the coil). The path of the field lines FL is a closed path which, on a radial plane, diverges in proximity of the ends of the rod 50, thus intersecting the surfaces thereof orthogonal to the axis X.

Thus, there is a substantial uniformity of the characteristics of the material constituting the rod 50 along the entire length thereof, neglecting the slight non- uniformity in the divergence area. However, it is important that the characteristics of the rod 50 be uniform up to the ends given that it is at the latter that the forces are exchanged with the surrounding components.

In the case of devices of the known type, in which the magnetostrictive core is provided as illustrated in figure 4B, the magnetic field generated by the coil 34 and traversing the rod 50 always has a toroidal development with field lines FL which, however, diverge at a point of the axis X considerably distant with respect to the end of the rod 50, hence causing the lines FL not to intersect the surfaces of the rod 50 orthogonal to the axis X but the lateral surfaces, parallel to the latter.

This creates great non-uniformity between the end regions of the rod 50 and the central region, which is associated to a marked deterioration of the mechanical characteristics of the rod 50 at the ends. This leads to an early wear and disintegration of the material of the rod 50 right at the ends, hence . seriously jeopardizing the capacity of the rod 50 to transmit a force to the surrounding components and, lastly, considerably deteriorates the sound reproduction quality.

Concretely, the discs made of amagnetic material 52, 54 (preferably aluminium, ceramic or mylar) have the capacity of nearing the field lines FL towards the axis X, thus promoting the exit thereof from the rod 50 at the ends.

Obviously, the construction details and the embodiments may be widely varied with respect to what has been described and illustrated without departing from the scope of protection of the present invention, as defined by the attached claims.

Claims

1. A device for sound reproduction (1) comprising:

- a first acoustic actuator (2)

- a second acoustic actuator (4),

wherein said first and second acoustic actuators (2, 4) are arranged coaxially to each other,

the device (1) being characterised in that:

- said second acoustic actuator (4) is arranged substantially within said first acoustic actuator (2),

- it comprises a connection structure (6) through which said first acoustic actuator (2) and said second acoustic actuator (4) are connected to each other,

- said connection structure (6) comprises elastic elements (68, 98) which connect and support one with respect to the other said first and second acoustic actuators (2, 4).

2. The device (1) according to claim 1, characterised in that it further comprises a transmission foot (8) prearranged for the coupling to a support means.

3. The device (1) according to claim 2, characterised in that said connection structure comprises a first and a second diaphragm element (60, 62).

4. The device (1) according to claim 1, characterised in that said first acoustic actuator (2) is of the magnetodynamic type, and in that said second acoustic actuator (4) is of the magnetostrictive type.

5. The device (1) according to claim 3, characterised in that each of said first and second diaphragm elements (60, 62) comprises:

- a crown (64, 94),

- a hub (66, 96), and

- a plurality of elastic elements (68, 98) arranged between and connecting said crown (64, 94) and said hub (66, 96).

6. The device (1) according to claim 5, characterised in that each of said elastic elements (68, 98) is substantially S-shaped,

7. The device (1) according to claim 6, characterised in that each of said elastic elements (68, 98) comprises:

- a first (84, 108) and a second (84, 108) end stretch converging, respectively, in said hub (66, 96) and in said crown (64, 94),

- an intermediate stretch (88, 1 12) having a substantially circumferential development, and - a first - (90, 1 14) .and a second (92, 1 16) curved joint joining said intermediate stretch (88, 1 12) to said first (84, 108) and second (84, 108) end- stretch, respectively.

8. The device (1) according to claim 5 characterised in that the crown (64, 94) of each diaphragm element comprises a circular seat (72, 102) and a plurality of fixing brackets (70, 100), wherein said circular seat and said fixing brackets define a single abutment surface (Al, A2).

9. The device (1) according to claim 8, characterised in that the hub (66) of said first diaphragm element (60) comprises:

- a collar (75) having a central through hole (75A), and

- radial supports (76) including a respective circumferential groove (82).

10. The device (1) according to_. claim 9, characterised in that said transmission foot (8) is arranged within the hub (66) of said first diaphragm element (60), it traverses said central through hole (75A) and it is axially elastically supported (130) with respect to said collar (75).

11. The device (1) according to claim 9, characterised in that said first acoustic actuator (2) comprises a moving assembly (10, 12, 14, 16, 18) and a coil (20) carried by a support ring (22).

12. The device (1) according to claim 1 1 , characterised in that:

- said support ring (22) is received in the circumferential grooves (82) of said radial supports (76)

- said moving assembly (10, 12, 14, 16, 18) is received in the circular seats (60, 72) and on the fixing brackets (70, 100) of said first diaphragm element (60) and second diaphragm element (62) so that it abuts against each abutment surface (A1, A2).

13. The device (1) according to claim 12, characterised in that said moving assembly comprises:

- an inner annular plate (10), preferably made of ferromagnetic material

- an outer annular plate (12) coaxial to said inner annular plate (10), preferably made of ferromagnetic material,

- a permanent magnet (14) fixed to said outer annular plate (12) and coaxial thereto,

- a tubular element (16), preferably made of ferromagnetic material, fixed to said inner annular plate (10) and coaxial thereto, and - a counterplate (18), preferably made of ferromagnetic material, annular- shaped, fixed to said tubular element (16) and permanent magnet (14) and coaxial thereto.

14. The device (1) according to claim 13, characterised in that said inner annular plate (10), tubular element (16) and counterplate (18) have an equal inner diameter (Dl) and define a central cavity (C) wherein said second acoustic actuator (4) is arranged.

15. The device (1) according to claim 10 characterised in that said second acoustic actuator (4) comprises a casing (32) fixed to the hubs (66, 96) of said first and second diaphragm element (60, 62) and a magnetostnctive core (36) inserted into said casing (32) and comprised between said casing (32) and said transmission foot (8).

16. The device (1) according to claim 15, characterised in that said magnetostrictive core (36) comprises:

- a spool (42) having a first and a second end collar (44, 46) and a central through hole (48),

- a rod (50) made of magnetostrictive material inserted into said spool (42),

- a first and a second disc made of amagnetic material (52, 54) arranged at opposite ends of said rod (50) and at contact therewith, and

- a first and a second" discoidal magnet (56, 58) arranged in contact, respectively, with said first and second disc made of amagnetic material (52, 54), wherein:

- said rod (50) made of magnetostrictive material is longer than said central through hole (48),

- a coil (34) is wound on said spool (42),

- said first disc made of amagnetic material (52) and first discoidal magnet (56) are received substantially within said first end collar (44),

- said second disc made of amagnetic material (54) and second discoidal magnet (58) are received substantially within said second end collar (46).