EP2461219B1 - Acoustic membrane for a musical box or an alarm watch - Google Patents

Description

The invention relates to an acoustic radiation membrane for a music box, such as a musical watch, or a striking watch.

The invention also relates to a watch, which comprises an acoustic radiation membrane. The watch comprises a watch case consisting essentially of a middle part and a case-back fixed in a sealed and removable manner on the middle part. A mirror is placed on a side opposite the bottom to seal said box. A watch movement is kept inside the watch case and provided with a striking mechanism capable of being activated in determined periods to produce sound or music. The acoustic radiation membrane is connected to the box to radiate the sound produced by the striking mechanism to the outside of the box.

In the field of watchmaking, a watch movement of traditional architecture can also include a striking mechanism for generating sound or music. The bell of the striking watch or the keypad of the musical watch are placed inside the watch case. Thus the vibrations of the stamp or the blades of the keyboard are transmitted to the trim parts. These covering parts are, for example, the middle part, the bezel, the crystal and the back of the watch case. These large rooms begin to radiate sound in the air under the effect of the vibrations transmitted. When a sound is produced either by a timbre struck by a hammer, or by one or more vibrating keys of the keyboard, these trim pieces are able to radiate the sound produced in the air.

In a traditional ringing or musical watch, the acoustic efficiency, based on the complex vibro-acoustic transduction of the trim parts, is low. To improve and increase the acoustic level perceived by the user of the striking or musical watch, account must be taken of the material, geometry and boundary conditions of the trim pieces. The configurations of these trim parts are also dependent on the aesthetics of the watch and the operating constraints, which can limit the possibilities of adaptation.

It is known in the watchmaking technique to use, in a particularly electronic watch, an acoustic type membrane, which is dedicated to vibro-acoustic transduction. To activate such a membrane in an electronic watch, a piezoelectric element is placed for example on the membrane to make it vibrate as mentioned in the patent. CH 581 860 . So that the sound radiation from the membrane is not lost in the watch, which must be waterproof, a double bottom of the watch case can be provided, which is open to the outside. In this case, the back of the watch case has one or more openings for the transmission of sound from the vibrating membrane.

Generally with the use of a traditional acoustic radiation membrane, there remains a problem of frequency bandwidth. In the case of a minute repeater striking watch, an alarm clock or even a quartz alarm, excellent results can be obtained by amplifying a single dominant frequency, tuned with the exciter. On the other hand, in the case where the acoustic membrane is to be fitted to a music box, the frequencies to be radiated effectively must typically extend between 1 kHz and 4 kHz. The acoustic response of the membrane must therefore be relatively uniform in this frequency range. However, standard uniform membranes never succeed in fulfilling this condition, the response level being therefore generally very inhomogeneous in this frequency interval.

In a standard striking watch, which is provided for example with an acoustic membrane, this membrane is sandwiched between part of the middle part and the back of the watch case. In the case of a luxury watch, the back can be made of a precious material, such as gold. On contact with the membrane, which is generally made of steel, with the gold bottom, an electro-chemical potential difference can occur, especially in a humid environment. This is liable to promote corrosion of said membrane at the level of contact with the gold background, which is a drawback. It must therefore also be found a stainless material without a potential difference with gold and whose internal damping is low.

The object of the invention is therefore to overcome the drawbacks of the above-mentioned state of the art by providing an acoustic radiation membrane for a music box or a striking watch, produced in such a way as to obtain the most uniform acoustic performance possible. on the audible frequency band, mainly in the frequency interval between 1 kHz and 4 kHz.

To this end, the invention relates to an acoustic radiation membrane comprising the characteristics defined in independent claim 1.

Particular forms of the acoustic radiation membrane are defined in dependent claims 2 to 11.

An advantage of the acoustic radiation membrane according to the present invention lies in the fact that it is made with at least two regions of circularly asymmetric shape formed in the material of the membrane or with at least two regions of circularly asymmetric shape having a different thickness from the general thickness of the membrane. It may include several regions of circularly asymmetric shape which are excavated in the material of the membrane. Preferably, two excavated regions of dimension different are planned. A first region is machined for example by etching or hollowing in the membrane to have a first constant thickness, and a second region is machined in the membrane to have a second constant thickness less than the first thickness. The two regions of circularly asymmetric shape are machined to define, for example, first and second ellipses as a circularly asymmetric shape. These ellipses are offset from each other with respect to the center of the membrane and partially overlap.

Thanks to the production of ellipses in the membrane, this makes it possible to have a double number of eigenmodes of vibration for each ellipse with respect to a region of circular shape. A maximization of the number of natural modes in the audible frequency band is thus obtained, in particular between 1 kHz and 4 kHz. The overall response of the membrane to vibration is thus flattened by eliminating circular symmetry and using such a circularly asymmetric region in the form of an ellipse seen in plan.

Advantageously, the membrane can be made of amorphous metal or of metallic glass, or also of gold, or even of brass or of another material having a density, a Young's modulus and an elastic limit, which are similar. The arrangement of the circularly asymmetric regions can thus make it possible to increase the number of natural frequencies in the useful acoustic frequency band, namely between 1 kHz and 4 kHz, in order also to increase the overall acoustic level. With such a diaphragm, the widening of the soundtrack can be combined with a very low internal damping, which makes it possible to obtain a very good sound efficiency.

To this end, the invention also relates to a watch provided with an acoustic radiation membrane comprising the characteristics defined in independent claim 12.

Particular forms of the watch are defined in dependent claims 13-15.

• la figure 1 représente de manière simplifiée une vue de dessus de la membrane de rayonnement acoustique selon l'invention,
• la figure 2 représente de manière simplifiée une coupe diamétrale selon A-A de la figure 1 de la membrane de rayonnement acoustique selon l'invention,
• la figure 3 représente un graphe de la force totale appliquée sur l'air par la membrane selon l'invention en comparaison d'une membrane circulaire, en fonction de la fréquence d'excitation de la membrane, et
• la figure 4 représente de manière simplifiée une coupe partielle d'une montre à sonnerie ou musicale, qui est munie d'une membrane acoustique selon l'invention.

The aims, advantages and characteristics of the acoustic radiation membrane for a music box or a striking watch will appear better in the following description on the basis of at least one non-limiting embodiment illustrated by the drawings in which:

• the figure 1 shows in a simplified manner a top view of the acoustic radiation membrane according to the invention,
• the figure 2 shows in a simplified manner a diametral section along AA of the figure 1 of the acoustic radiation membrane according to the invention,
• the figure 3 represents a graph of the total force applied to the air by the membrane according to the invention in comparison with a circular membrane, as a function of the excitation frequency of the membrane, and
• the figure 4 shows in a simplified manner a partial section of a striking or musical watch, which is provided with an acoustic membrane according to the invention.

In the following description, reference will be made mainly to the configuration of an acoustic radiation membrane, in particular to equip a music box, such as a musical watch, or a striking watch.

The figure 1 shows a top view of the acoustic radiation membrane 1 for a music box, such as a musical watch, or a striking watch. In this embodiment, the membrane 1 is produced with a spatially inhomogeneous thickness, that is to say it comprises regions, which have been machined in the total thickness of the membrane. The machined regions each have a different uniform thickness. The excavated regions of different thickness have circular asymmetric shapes in top view. The circularly asymmetrical shapes are preferably ellipses 2, 3 excavated in a bottom part 4 of a circular membrane 1, which may be shaped like a cuvette as explained below with reference to figures 2 and 4 . These ellipses 2, 3 are partly superimposed. The presence of several circularly asymmetric regions makes it possible to considerably increase the number of eigen modes or eigenfrequencies of vibration. This therefore makes it possible to increase the bandwidth, preferably between 1 kHz and 4 kHz, and the uniformity of the amplification in this band of audible frequencies.

As could be observed during the acoustic radiation test of such a membrane, it is optimally planned to produce two circularly asymmetrical regions 2, 3 in the thickness of the membrane 1. These two regions in the form of an ellipse with different uniform thickness, are of different size or surface without being directly dependent on the size of the membrane. The membrane 1 can be a circular membrane with a diameter of about 40 mm at the edge and a diameter of about 31 mm at its bottom 4 by way of non-limiting example. These two regions in the form of an ellipse occupy a large part of the surface of the membrane seen in plan so as to increase the number of natural modes of vibration in the band of audible frequencies desired between 1 kHz and 4 kHz.

où E est le module d'Young, h est l'épaisseur de la membrane, a et b sont les demi-axes de l'ellipse, p est la masse volumique du matériau de la membrane, v est le coefficient de Poisson (à peu près de l'ordre de 0.3), n et m sont des nombres entiers, qui numérotent les modes de vibration et représentent le nombre de nœuds spatialement de la vibration correspondante de la membrane. Le nombre de nœuds dans la direction du demi-axe a et du demi-axe b est m-1, respectivement n-1. Dans le cas d'un mode indiqué par n=2 et m=3, cela correspond à une vibration, qui a deux nœuds dans la direction du demi-axe a, et un nœud dans la direction du demi-axe b. Dans le cas d'un mode indiqué par n=1 et m=1, il n'y a pas de nœuds dans l'une ou l'autre direction des demi-axes.The regions in the form of an ellipse are normally determined taking into account the following simplified formula for the frequency ω of the vibration modes of an elliptical membrane: $${{\mathrm{<figure-callout\; id="2"\; label="\omega "\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">\omega </figure-callout>}}^2}_{\mathrm{not},\mathrm{m}}\approx \mathrm{E}\cdot \mathrm{h}\cdot \left(\mathrm{not}/{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">b</figure-callout>}}^2+\mathrm{m}/{\mathrm{at}}^2\right)/\left(\mathrm{\rho }\cdot \left(1-{\mathrm{<figure-callout\; id="2"\; label="v"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">v</figure-callout>}}^2\right)\right)$$

where E is the Young's modulus, h is the thickness of the membrane, a and b are the half-axes of the ellipse, p is the density of the material of the membrane, v is the Poisson's ratio (at roughly of the order of 0.3), n and m are integers, which number the modes of vibration and represent the number of nodes spatially of the corresponding vibration of the membrane. The number of nodes in the direction of half-axis a and half-axis b is m-1, respectively n-1. In the case of a mode indicated by n = 2 and m = 3, this corresponds to a vibration, which has two nodes in the direction of semi-axis a, and a node in the direction of semi-axis b. In the case of a mode indicated by n = 1 and m = 1, there are no nodes in either direction of the semi-axes.

According to the above-mentioned vibration frequency formula, the frequency increases with the square root of the Young's modulus E and the thickness h, but on the other hand decreases by increasing the semi-axes a and b, that is to say the area of the ellipse. By way of comparison, for the same surface and the same thickness, and in the desired frequency range, an elliptical membrane has a double number of vibration modes with respect to a circular membrane. It is therefore possible to flatten the overall frequency response by eliminating the circular symmetry of the membrane. The circularly asymmetric regions, preferably in the form of an ellipse 2, 3 of the acoustic membrane 1 according to the invention, are configured so that the first natural modes of vibration are located in the audible frequency band between 1 kHz and 4 kHz. With such ellipses, this makes it possible to perform better geometric optimization, than for other circularly asymmetric shapes.

ayant ce nombre de nœuds. Ces modes diffèrent entre eux par leur forme spatiale et/ou leur orientation dans le plan de la membrane. La différence d'énergie entre ces modes dépend de l'épaisseur et de la forme des régions S_j et peut donc être réduite à discrétion. Cette multiplication des modes dans chaque intervalle d'énergie permet d'élargir la bande de réponse de la membrane. Par simplicité de calcul et de réalisation pratique, il est considéré le cas spécifique où les régions d'épaisseur différente sont deux ellipses.It should be noted that a circular membrane of uniform thickness supports several eigen modes of vibration, which are defined by k _N. Each eigenmode is characterized by a defined number of nodes N. Thanks to the realization of regions of different thickness, which are called S _j with j ranging from 1 to n, and for each defined number of nodes N, it is counted several vibration modes $k_{\mathrm{NOT}}^l$

having this number of nodes. These modes differ from each other by their spatial form and / or their orientation in the plane of the membrane. The energy difference between these modes depends on the thickness and the shape of the regions S _j and can therefore be reduced at will. This multiplication of modes in each energy interval makes it possible to widen the response band of the membrane. For simplicity of calculation and of practical realization, it is considered the specific case where the regions of different thickness are two ellipses.

Thanks to the realization of two ellipses in the membrane, for each defined number of nodes, there are four modes of vibration, including two vibration modes per ellipse and not a single vibration mode for a circular shape of a traditional membrane. A maximization of the number of modes in the audible frequency band is thus obtained. The overall response of the membrane to vibration is thus flattened by eliminating circular symmetry and using such a circularly asymmetric region in the form of an ellipse seen in plan.

For the typical dimensions of a watch, with the two regions having an elliptical shape, it is possible to obtain a better geometric optimization than with any other circularly asymmetric shape. Depending on the sufficiently large size of each ellipse relative to the size of the membrane, the first modes of vibration are found with uniform amplification in the desired frequency band, for example between 1 kHz and 4 kHz. The overall acoustic level is also increased for the perception by a user of the notes radiated by the membrane of the music box or the striking watch.

As described above, the circular membrane 1 can have a diameter equal to 40 mm at the edge and a diameter equal to 31 mm at its bottom 4. It can be made with a material, such as metallic glass based on zirconia, which has a density or density equal to 5,100 kg / m ³ . The material used for the membrane can have a Young's modulus, which can vary between 97 and 110 GPa, while its elastic limit can vary between 1.5 and 2.2 GPa. The maximum thickness of the membrane can be of the order of 0.3 mm, while the minimum thickness can vary between 0.1 mm and 0.2 mm, depending on the sound effects to be obtained. If the density is larger, while the Young's modulus is smaller, a membrane thickness greater than 0.3 mm may be allowed, but under these conditions the membrane is less acoustically efficient.

The size of the first ellipse 2, which is excavated in the bottom 4 of the membrane, is 12 mm for the semi-major axis and 6 mm for the semi-minor axis with a thickness of 0.15 mm. The size of the second ellipse 3, which is excavated in the bottom 4 of the membrane partly superimposed on the first ellipse and crossed, is 11 mm for the semi major axis and 7 mm for the semi minor axis with a thickness of 0.2 mm. The centers c, c 'of the first and second ellipses 2, 3 can be offset from each other for example by 13.5 mm, and the angle between the major axes of the two ellipses can be of the order of 60 ° . With a relatively similar size of the two ellipses, the density of vibration modes of the membrane is maximized in the desired audible frequency band. It can also be envisaged to adapt the thicknesses and the surface of the ellipses as a function of the desired tightness, of the indeformability or the deformability of the desired membrane.

In general, the ratio between the half-axes of the ellipses excavated in the membrane and the radius of the circular membrane should in principle be in the range 2/3 to 1. The ratio between the two thicknesses of the ellipses should be in the interval 1/2 to 4/5. The minimum thickness should not be more than 2/3 of the total thickness of the circular membrane.

The figure 2 represents a diametral section along AA of the figure 1 of the acoustic radiation membrane 1. This membrane can take the form of a cup with a bottom 4 and a peripheral edge for mounting in particular in a watch case as explained below with reference to figure 4 . The circularly asymmetric regions in the shape of an ellipse 2 and 3 are made in the bottom 4 of the membrane 1. Each region is excavated in the membrane each with a different uniform thickness. It should also be noted that the excavated regions can be found indifferently on the movement side or on the external side of the membrane, not shown.

It should also be noted that instead of making circularly asymmetrical regions 2, 3 by etching, milling or hollowing in the total thickness of the membrane 1, it can be envisaged to produce on a membrane of minimum thickness two regions in the form of ellipse in overthickness and which intersect. A first region has a first thickness greater than the minimum thickness of the membrane, and a second region has a second thickness greater than the first thickness of the first region. These regions in the form of an ellipse thus form protruding portions on the membrane, but the circularly asymmetrical shape of which provides advantages equivalent to the advantages of the ellipses excavated in the membrane explained above. These regions can be obtained by selective deposition of the same material as the base material of the membrane. The material may be metallic glass based on zirconia or based on platinum, or also gold.

It should also be noted that instead of making circularly asymmetric regions 2, 3 by etching, milling or hollowing in the total thickness of the membrane 1, it can be envisaged to make an asymmetric membrane circularly, by modifying its physicochemical properties locally and deterministically during manufacturing or post-processing. This procedure makes it possible to produce uniform regions having circularly asymmetric shapes and therefore to multiply the modes of vibration and to flatten the frequency response, according to the same physical principle explained previously.

To the figure 3 , there is shown a graph of the frequency response of the proposed membrane compared with the response of an ordinary circular membrane made from the same material. The total force Fz applied by the membrane to the air is represented as a function of the excitation frequency of the membrane. The circular membrane can be flat in this example and of a diameter of the order of 31 mm. The same excitation force was applied in all the cases considered.

The MA curve represents the response of an ordinary circular membrane (force of the membrane on air) in the frequency range of 1 kHz to 4 kHz. Note that the force on the air of this ordinary membrane only has a peak of greater amplitude between 2.5 kHz and 3 kHz with a relatively low overall amplitude. Curve A represents the response of a circular membrane, in which is made a centered ellipse with a semi-major axis equal to 15 mm and a semi-minor axis equal to 9 mm with a thickness of 0.07 mm and an ellipse not centered with a semi-major axis equal to 13.5 mm and a semi-minor axis equal to 10 mm with a thickness of 0.09 mm. Curve B represents the response of a circular membrane, in which a centered ellipse with a semi-major axis equal to 14 mm and a semi-minor axis equal to 10 mm is produced with a thickness of 0.08 mm and a non-centered ellipse of semi-large axis equal to 12 mm and semi-minor axis equal to 11 mm with a thickness of 0.1 mm. Finally, the curve C represents the response of a circular membrane, in which a centered ellipse with a semi-major axis equal to 15 mm and a semi-minor axis equal to 9 mm is made with a thickness of 0.09 mm and a centered ellipse of semi-large axis equal to 13.5 mm and semi-minor axis equal to 10 mm with a thickness of 0.11 mm. The amplitude of the force applied to the air by the membrane, in which ellipses are produced, is maximized and relatively flattened for natural frequencies of vibration between 1 kHz and 4 kHz, which is sought by the invention.

The figure 4 shows a partial cross section of a striking or musical watch 10. Watch 10 essentially comprises an acoustic radiation membrane 1 according to the invention to improve the acoustic performance of a note or of notes produced by a striking mechanism. This acoustic membrane 1 can comprise two regions in the shape of an ellipse 2 and 3, excavated in the bottom 4 of the membrane. This acoustic membrane can be made, for example, in an amorphous metal or in metallic glass, which is a stainless material. The total thickness of this membrane 1 may be less than or equal to 1 mm, but preferably close to 0.3 mm.

The striking or musical watch 10 also comprises a horological movement 20, which is generally mounted on a plate 24. An edge piece 22 is fixed to the plate 24, which defines a watch cage. Usually both the plate 24 and the edge piece 22 are made of a metallic material.

The watch movement 20 comprises a striking mechanism, not shown. This striking mechanism may include at least one stamp mounted on a stamp holder integral with the plate 24, and at least one hammer rotatably mounted on the plate to strike said stamp in determined periods. The generally circular gong surrounds the various parts of the clockwork movement of the striking watch. Such a striking mechanism is provided to signal a programmed alarm or minute repeaters.

In a more elaborate embodiment of a musical watch, the striking mechanism can comprise a keyboard with a set of blades connected to a heel, which is fixed on the plate 24. A note or a succession of musical notes are produced by the vibrations of the keyboard blades. Each reed is normally configured to produce a particular note, but it may be provided to have certain groups of two reeds so that each group produces the same particular note. To produce music for example in programmed periods, the keys of the keyboard are lifted and released by pins integral with a cylinder or a rotating disk on the plate 24. Each activated blade oscillates mainly at its first natural frequency. The vibrations generated by the activated blades are transmitted to the watch trim parts, which must make it possible to acoustically radiate the sound produced by each vibrating blade.

In this embodiment, the acoustic membrane 1 is in the form of a cup, the upper edge of which is mounted in a sealed manner by an annular gasket 18 on an internal annular rim of the bottom 15 of the box. The diameter of this cup, which may be equivalent to the diameter of the watch crystal 12, may be between 20 and 40 mm. A support 21 of annular shape supports on one side the plate 24 with the edge piece 22 and rests on the upper edge of the acoustic membrane 1. When the case 14 is fastened to the back 15 of the watch case, the support 21 and the peripheral edge of the acoustic radiation membrane 1 are clamped between the case 14 and the rim of the back 15.

It should be noted that provision may be made to fix the acoustic membrane 1 by its edge in a manner other than that presented above. It can be envisaged that the membrane is fixed in a point-wise manner at 2, 3, 4 or more points by its border, or that it is fixed elastically or with a simple support condition.

The back 15 is mounted by known means and removably on the middle part 14 with a sealing gasket 19. A watch crystal 12 is fixed in particular to the bezel 13 to seal the watch case. A dial 23 is kept at the edge of the caseband and disposed below the watch crystal 12. For a mechanical striking watch 10, time indication hands, not shown, are provided on the dial, which also carries hourly indexes on the periphery.

The central part of the acoustic membrane is not in contact with the support 21 and the inner surface of the bottom 15. As a result, sufficient space 17 in the box is provided so that the acoustic membrane can freely vibrate or radiate acoustically. The whole of the acoustic membrane 1 and the bottom 15 thus constitutes a double bottom. One or more openings 16 are also provided laterally through the bottom 15 to allow the acoustic membrane to radiate the sound produced by the striking mechanism to the outside.

During operation of the striking mechanism, the note or notes generated by said striking mechanism are transmitted directly to the acoustic radiation membrane 1 to make it vibrate. A transmission of vibration to the acoustic membrane 1 is also provided at the edge of the acoustic membrane by the connecting pieces 21, 22 and 24. As the acoustic membrane comprises regions in the form of an ellipse 2, 3 excavated in the bottom 4 of the membrane, it is likely to vibrate at several first natural frequencies depending on the number of notes expected to radiate. These first natural frequencies are preferably located in the useful acoustic band between 1 kHz and 4 kHz. The second natural frequencies of vibration notes are on the other hand located above 4 kHz. This is advantageous because the second vibration frequencies are often sound destructive.

These desired natural frequencies of sound vibration of the membrane, which can be made of amorphous metal, are dependent on physical properties, such as density and Young's modulus. In addition, with such an acoustic radiation membrane 1, very low damping is observed, which provides very good sound efficiency for this acoustic membrane. In addition, the destructive interference effect of the second natural frequencies is mixed, given that a second natural frequency mode is generally found close in frequency to a first natural frequency mode having an orthogonal orientation. In other words, the membrane never vibrates in a pure mode of the second natural frequency.

Thanks to the fact that this membrane is composed of stainless materials, it can be mounted on a base, for example made of precious metal, such as gold. No difference in electrochemical potential is thus observed even in a humid environment, which means that no corrosion occurs in contact with the membrane 1 and the base 15.

The metallic glass or amorphous metal used for example for producing the membrane can also be a metal alloy based on titanium, zirconium and beryllium. Thus, by way of a more specific example, the amorphous metal alloy can comprise 41% of zirconium, 14% of titanium, 12% of copper, 10% of nickel and 23% of beryllium. The Young's modulus of this alloy is 105 GPa and the elastic limit is 1.5 GPa. The amorphous metal alloy can also be made of 57.5% platinum, 14.7% copper, 5.3% nickel and 22.5% phosphorus. The Young's modulus of this alloy is worth in this case 98 GPa and the elastic limit is worth 1.4 GPa.

From the description which has just been given, several variant embodiments of the acoustic radiation membrane for a music box or a striking watch can be designed by those skilled in the art. trade without departing from the scope of the invention defined by the claims. The acoustic membrane may be located in the watch case at the level of the middle part with an opening through the middle part for the sound radiation of the vibrating acoustic membrane. The acoustic membrane can be located on an external part of the watch case, but placed at least on an opening of the case so that the note or notes generated by the striking mechanism can make the membrane vibrate. Several acoustic membranes can be placed in several places inside the watch case or superimposed. The membrane can have a shape other than circular, for example rectangular and be planar. The membrane may include an ellipse shaped region on a first face and another ellipse shaped region on an opposite second face of the membrane.

Claims (15)

1. Acoustic radiating membrane (1) for a music box or striking watch (10), characterized in that it is made with at least two areas of circularly asymmetrical shape formed in the material of the membrane or with at least two areas of circularly asymmetrical shape hollowed out of a part of the membrane or projecting from one part of the membrane to increase the number of natural frequencies of vibrations between 1 kHz and 4 kHz.
2. Membrane (1) according to any of the preceding claims, characterized in that, each area is hollowed out of the membrane with a different uniform thickness, to maximise the first natural vibration frequencies of the membrane within the frequency range between 1 kHz and 4 kHz.
3. Membrane (1) according to any of the preceding claims, characterized in that the asymmetrical area or areas (2, 3) have the shape of an ellipse.
4. Membrane (1) according to claim 3, characterized in that the areas of elliptical shape are hollowed out of the membrane with a different uniform thickness from each other and less than the thickness of one bottom part of the membrane.
5. Membrane (1) according to claim 3, characterized in that the areas of elliptical shape are projecting portions made on a minimum thickness of the membrane, each elliptical area having a different thickness from the other.
6. Membrane (1) according to any of claims 4 and 5, characterized in that the membrane has a general shape of a dome with a bottom (4) in which the areas of elliptical shape (2, 3) are made.
7. Membrane (1) according to any of claims 4 to 6, characterized in that a first area (2) of elliptical shape is centred on the circular membrane, in that a second area (3) of elliptical shape is off centre on the membrane, and in that the two areas are partly superposed.
8. Membrane (1) according to claim 7 characterized in that the ratio between the semi-axes of the two ellipses (2, 3) hollowed out of the membrane and the radius of the circular membrane must be comprised within the range of 2/3 to 1, in that the ratio between the two thicknesses of the ellipses must be within the range of 1/2 to 4/5, and in that the minimum thickness of each ellipse must not be greater than 2/3 of the total thickness of the circular membrane.
9. Membrane (1) according to any of claims 1 to 5, characterized in that the uniform thickness of the membrane is 0.3 mm or less, in that the thickness of a first area (2) of elliptical shape is on the order of 0.15 mm and in that the thickness of the second area (3) of elliptical shape is on the order of 0.2 mm.
10. Membrane (1) according to claim 1, characterized in that it is made of gold or titanium or amorphous metal or metallic glass.
11. Membrane (1) according to claim 1, characterized in that the areas of asymmetrical shape are formed in the basic material of the membrane by altering the physico-chemical properties of the material locally and in a deterministic manner.
12. Striking or musical watch (10), including a watch case, which has a middle part (14) and a back cover (15) having at least one lateral aperture (16), wherein the back cover is secured in a sealed and removable manner to the middle part, a crystal (12) closing the case in a sealed manner, a watch movement (20) held inside the watch case and provided with a striking mechanism capable of being actuated at determined times to produce one note or several notes, and at least one acoustic radiating membrane (1) according to any of the preceding claims, which is arranged in the watch case.
13. Watch (1) according to claim 12, characterized in that the acoustic membrane (1), is held on an inner edge of the back cover (15) of the case and one part of the middle part (14), and in that the periphery of the acoustic membrane (1) is clamped with the periphery of a support (21) of the movement between the middle part (14) and the inner edge of the back cover (15) of the case.
14. Watch (10) according to claim 13, characterized in that the acoustic membrane (1) has the shape of a dome, whose top edge is clamped with the annular support between the middle part (14) and an inner annular edge of the back cover (15) of the case, an annular sealing gasket (18) being placed between the edge of the back cover (15) and the annular edge of the membrane, and in that a central part of the acoustic membrane is not in contact with the support (21) and an inner surface of the back cover (15) of the case to define a space (17) allowing said membrane to oscillate freely.
15. Watch (10) according to claim 12, characterized in that several acoustic radiating membranes (1) are connected to the watch case and arranged separately from each other or superposed on each other.

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