GB2489700A

GB2489700A - Controlling the vibration modes of a vibrating support

Info

Publication number: GB2489700A
Application number: GB1105681.9A
Authority: GB
Inventors: Ga Rald Kergourlay; Olivier Doare
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2011-04-04
Filing date: 2011-04-04
Publication date: 2012-10-10
Anticipated expiration: 2031-04-04
Also published as: GB201105681D0; GB2489700B

Abstract

The method concerns determining the position and the dimensions of at least one vibration controlling member 20 placed on a vibrating support 22 , e.g. a loudspeaker diaphragm, that is liable to vibrate according to a plurality of vibration modes when excited. The method includes (i) providing a plurality of pairs of position and dimensions of at least one vibration controlling member placed on the vibrating support; (ii) obtaining for each pair at least one value of a physical magnitude produced by the vibrating support, the vibration controlling member, or the vibrating support when the vibrating support is subjected to an excitation; and (iii) from the at least one value of physical magnitude obtained for each pair, determining at least one pair for which a first vibration mode is distinguishable from at least one second vibration mode. The vibration controlling member 20 may be a piezoelectric material, PZT or PVDF, operating in either an active or passive manner.

Description

METHOD AND DEVICE FOR CONTROLLING THE VIBRATION MODES OF A

VIBRATING SUPPORT

The invention relates to a method of controlling the vibration modes of a vibrating support.

In particular, the invention applies to loudspeaker drivers.

Figures 1 and 2 schematically represent two types of a loudspeaker driver.

As illustrated in Figure 1, the loudspeaker driver comprises a membrane 1 acting as a vibrating support that is able to vibrate when excited according to several vibration modes. The membrane 1 has a plane shape.

The loudspeaker driver further comprises a motor 2 which is typically formed of a voice coil being immersed in a magnetic field created by a permanent magnet. The voice-coil is connected to the membrane.

is The membrane is fixed at its opposite extremities or at its periphery 3, e.g. by clamping.

The loudspeaker driver in Figure 2 also comprises a membrane I and a voice-coil 2 connected thereto.

Contrary to Figure 1, the Figure 2 membrane is suspended at its opposite extremities or at its periphery by using passive suspending members such as surrounds or foams.

The membrane has a frustoconical shape.

Both loudspeaker drivers operate in the same way.

In particular, a periodic motion is imparted to the voice-coil and therefore transmitted to the membrane.

When thus excited the membrane vibrates and generates a sound.

Figure 3 illustrates the frequency response of a loudspeaker driver of one of the above types which is expressed by the Sound Pressure Level (denoted SPL and usually in dB) as a function of the frequency (in Hz).

As represented in Figure 3 the upper curve corresponds to the frequency response while the lower curve corresponds to the distortion curve.

The upper curve has a relatively flat portion between the two vertical dotted lines. The flat portion corresponds to a frequency range which is called the bandwidth of the loudspeaker: this is the portion of the curve that is used for audio reproduction.

The wider the bandwidth, the higher the loudspeaker performances.

Most of the time, the bandwidth is limited in its upper part by undesired vibration modes which create accidents in the response.

In a general manner, the loudspeaker drivers in Figures 1 and 2 vibrate according to two main kinds of vibration modes when subject to an excitation: -a main mode also called equivalent piston mode: this mode has the lowest frequency and brings the greatest part of the acoustic energy to the loudspeaker bandwidth; -break-up modes which correspond to the higher-order flexion modes appearing at frequencies above the main mode frequency.

Known prior art methods have already been envisaged to provide active vibration control of a loudspeaker driver.

GB 2,256,111 discloses a transducer for use in the active control of vibration in or sound radiation from a vibrating body on which the transducer is mounted.

The active portion of the transducer has dimensions such that it extends over substantially the whole vibrating body along at least one dimension thereof.

The transducer response is so weighted that the signal output by the transducer (or the drive signal applied to it) is proportional to the net volume velocity of the vibrating body.

According to this document a combination of vibrating modes is measured with a view to controlling the whole vibration of the vibrating body.

WO 02/078099 discloses a method of manufacturing a laminar piezoelectric transducer that is to be connected to a beam with a view to measuring and controlling strain modes.

The transducer comprises a continuous piezoelectric sheet.

According to this method, in-plane piezoelectric expansion coefficients of the sheet transducer are modulated by specifically patterning the electrode covering at least one of the faces of the sheet.

US 4,868,447 discloses an integrated distributed piezoelectric sensor/actuator for attachment to a mechanical structure with a view to sensing and controlling complex motions in the structure (with bending, stretching and twisting components).

The integrated distributed piezoelectric sensor/actuator includes a laminate comprising at least four stacked laminae of piezoelectric material (PVDF) with the principal axes of the adjacent laminae being skewed with respect to one another.

Both bending and torsional motions of the attached mechanical structure can be determined without signal processing from signals generated by the first pair of laminae.

Complex motions can be generated in the mechanical structure by supplying electrical signals to the second pair of lam mae.

According to this document, sensing and generating a specific vibration mode require not only that the surface be shaped according to a particular function, but also that the polarization profile be varied in the PVDF as well.

None of the above prior art methods turns out to be satisfactory.

An object of the invention is to easily control at least one vibration mode of a vibrating body among a plurality of vibration modes.

Another object of the invention is to extend the frequency bandwidth of a loudspeaker driver.

The present invention aims at attaining at least one of these objects by providing a method of determining the position and the dimensions of at least one vibration controlling member placed on a vibrating support that is liable to vibrate according to a plurality of vibration modes when excited, the method aimed at controlling the vibration modes of the vibrating support including: -providing a plurality of pairs of position and dimensions of at least one vibration controlling member placed on the vibrating support; -obtaining for each pair among the plurality of pairs, at least one value of a physical magnitude produced by the vibrating support, the at least one vibration controlling member, or the vibrating support provided with the vibration controlling member when the vibrating support is subjected to an excitation.

-from the at least one value of physical magnitude obtained for each pair among the plurality of pairs, determining at least one pair for which a first vibration mode is distinguishable from at least one second vibration mode.

This method makes it possible to determine a pair of position and dimensions of one or several vibration controlling members for which a first vibration mode can be differentiated from at least one second vibration mode.

This method is particularly easy to implement.

is In particular, no polarization of a PVDF film is required.

Further, the method does not incur high costs.

The determined position and dimensions of the vibration controlling member are thus optimized in order to subsequently be able to focus on one mode of interest.

According to a possible feature, the method further includes: -determining, among the plurality of pairs, a set of pairs for which, in each pair, minimum value(s) of the physical magnitude or a minimum variation in the value(s) of the physical magnitude is obtained in the first vibration mode; -selecting among the set of pairs thus determined, a pair for which maximum value(s) of the physical magnitude or a maximum variation in the value(s) of the physical magnitude is obtained in the at least one second vibration mode.

The advantage is to control the mode of interest (the at least one second vibration mode) without acting/sensing the first vibration mode, thereby without running the risk of reducing its vibration amplitude.

According to other possible features: -the first vibration mode is the piston mode and the at least one second vibration mode is a break-up mode; the functional mode being the piston mode which defines the loudspeaker sensitivity, controlling the perturbing break-up mode should not decrease its vibration amplitude; in addition, choosing the piston mode as the first vibration mode and the break-up mode as the second vibration mode enables widening of the "loudspeaker bandwidth"; -the physical magnitude is selected from a voltage, an intensity of an electrical current, a mechanical force, a displacement, an acoustic pressure; such a choice makes the control process particularly flexible; thus, voltage/current may be used when a PVDF/PZT film is used, pressure when a microphone is used, and force/displacement when a passive film is used.

According to a possible feature of the method, the following: -providing a plurality of pairs of position and dimensions of the at least one vibration controlling member placed on the vibrating support, -obtaining for each pair at least one physical magnitude value and, -determining at least one pair for which a first vibration mode is distinguishable from at least one second vibration mode, are performed for several vibration controlling members placed on the vibrating support so as to distinguish a first vibration mode from several vibration modes.

Thus, the repetition of the method steps for each among several different vibration controlling members makes it possible to differentiate a first mode from other modes.

The vibration controlling members are thus optimized in terms of position and dimensions on the vibrating support.

According to another aspect, the invention concerns a device comprising: -a vibrating support that is liable to vibrate according to a plurality of vibration modes when excited, -at least one vibration controlling member placed on the vibrating support and whose position on the vibrating support and dimensions are adjusted so as to control the vibration modes of the vibrating support by distinguishing a first vibration mode from at least one second vibration mode.

This device may be devised thanks to the above-mentioned method according to any one of the above possible features.

However, other methods enabling adjustment of the position and dimensions of the at least one vibration controlling member so as to distinguish a first vibration mode from at least one second vibration mode may be envisaged..

The device as described above has therefore improved performances, in particular as regards its bandwidth: the first functional mode, e.g. the piston mode, is favoured while the higher order vibration modes are avoided.

According to possible features: -the at least one vibration controlling member is fixed to the vibrating support, -the at least one vibration controlling member is a film; -the film is an active film; such a film proves to be very efficient when used as an actuator; in particular, it has good sensing capacities; -the film is of the PZT or PVDF type; -the film is a passive film; such a film proves to be easy to implement and cheap; -the film is selected so as to add stiffness, mass and/or damping to the vibrating support; -the film is annular in shape; -a voice-coil is connected to the vibrating support in an area which is located inside the annular-shaped film; -the vibrating support is not free at its periphery or at one or both of its opposed ends; -the vibrating support is a membrane or a beam; -two vibration controlling members are placed on either side of the vibrating support; -the first vibration mode is the piston mode and the at least one second vibration mode is a break-up mode; -the physical magnitude is selected from a voltage, an intensity of an electrical current, a mechanical force, a displacement, an acoustic pressure.

According to still another aspect, the invention concerns a method of using a device for producing sound waves, the device comprising: a vibrating support that is liable to vibrate according to a plurality of vibration modes when excited, at least one vibration controlling member placed on the vibrating support and whose position on the vibrating support and dimensions are adjusted so as to control the vibration modes of the vibrating support by distinguishing a first vibration mode from at least one second vibration mode, the method including applying an excitation to the device to cause said device to vibrate.

is When in use the device according to the invention radiates sound over a wider bandwidth than in the prior art. This is because for instance the break-up modes have been shifted toward higher frequencies. In another embodiment a wider bandwidth may be obtained by decreasing the amplitude of the break-up mode(s).

According to a possible feature, the excitation is applied to the vibrating support to cause said vibrating support to vibrate.

According to another possible feature which may be combined with the previous one, the method includes applying an electrical excitation signal to the at least one vibration controlling member which is able to be deformed when electrically excited.

Thus, by exciting the at least one vibration controlling member it is possible to control vibration modes of the vibrating support.

According to another possible feature, two vibration controlling members are respectively placed on either side of the vibrating support (membrane), the method including applying an electrical excitation signal to each vibration controlling member, the phases of the two signals being opposite to each other.

Thanks to the use of opposite-phased signals in the symmetrically-arranged device, the bending modes are well controlled through perfect shear of the membrane (no elongation of the neutral surface or fiber) in both directions (x and y).

The neutral surface or fiber only bends when thus excited.

From a practical point of view, when thus excited one side of the device is elongated while the other is shortened with the same amplitude.

Also, an electrical excitation signal is applied to the vibrating support.

Such a combined excitation provides satisfactory results in terms of speaker sensitivity and bandwidth: the sensitivity -related to the piston mode amplitude -is not decreased and the bandwidth is extended thanks to the control of the break-up mode.

According to still another feature, the phases of the two signals are different from the phase of the vibrating support electrical excitation signal.

is Other features and advantages will emerge from the following detailed description, which is merely given as a non-limiting example with reference to the drawings in which: -Figures 1 and 2 schematically represent two types of prior art loudspeaker driver; -Figure 3 schematically represents the frequency response curve of

a prior art loudspeaker driver;

-Figure 4 schematically represents a loudspeaker driver according to one embodiment of the invention; -Figure 5 represents an algorithm of the method according to the invention; -Figure 6 schematically represents a loudspeaker driver according to another embodiment of the invention; -Figure 7 illustrates another embodiment of a loudspeaker driver according to the invention; -Figure 8 schematically represents the contour levels of the modal force of the first vibration mode exerted by the films 38 and 40 in Figure 7 in their plane; -Figure 9 schematically represents the contour lines of the piezoelectric effort on the second vibration mode as well as zeros of the first mode in the plane of films 38 and 40 of Figure 7; -Figure 10 schematically represents several frequency response curves of the Figure 7 device when operated differently; -Figure 11 schematically represents the directivity index of the Figure 6 device as a function of the frequency.

Figure 4 illustrates a device 10, also called loudspeaker driver, which comprises a vibrating support 12 that is liable to vibrate according to a plurality of vibration modes when submitted to an excitation.

Vibrating support 12 may be a membrane which has for instance a planar shape. Alternatively, vibrating support 12 may be a beam.

In an embodiment vibrating support 12 may assume the shape of a disc.

is As illustrated in Figure 4, vibrating support 12 is fixed at its opposite extremities or, in the case of a disc-shaped vibrating support, it is fixed at its periphery 14, e.g. by clamping.

Other fixing means may alternatively be envisaged.

Although not represented in this figure, device 1 0 may further include a motor which is typically formed of a voice coil immersed in a magnetic field created by a permanent magnet.

The voice-coil is an excitation mean which is able to excite vibrating support 12 when powered, thereby causing said vibrating support to vibrate according to a plurality of vibration modes.

Furthermore, device 10 comprises a vibration controlling member 16 placed on vibrating support 12.

A method will now be described with reference to Figure 5, the aim of which is to control the vibration modes of the vibrating support.

More particularly, performance of the method will make it possible to distinguish a first vibration mode from one or several second vibration modes.

In this respect, different configurations of vibration controlling members are tested with a view to determining the position of a vibration controlling member on the vibrating support and its dimensions which make it possible to distinguish at least two vibration modes from each other.

Figure 5 illustrates a method of determining the position and the dimensions of at least one vibration controlling member placed on a vibrating support with a view to controlling the vibration modes of the vibrating support.

Although this method will be described with reference to Figure 4 configuration, it may nevertheless be applied to any other configuration of a device comprising at least one vibration controlling member placed on a vibrating support that is liable to vibrate according to a plurality of vibration modes when excited.

The Figure 5 method comprises a first step Si of initialization.

Next, in step S2 a plurality of pairs of positions and dimensions of a vibration controlling member placed on vibrating support 12 is provided.

For instance, Figure 4 schematically illustrates in dotted lines several possible positions and dimensions of a vibration controlling member placed on vibrating support 12.

The vibrating support may be placed on one side or the other (upper or lower face) of vibrating support 12.

Alternatively, several vibration controlling members may be placed on vibrating support 12, e.g. on either face of the vibrating support.

By way of example, each vibration controlling member may be annular in shape.

This shape provides the advantage of leaving a clear space in the middle of vibrating plate 12, thereby enabling local arrangement of excitation means such as a voice-coil.

The different pairs of positions and dimensions of a vibration controlling member placed on the vibrating support may comprise a vibration controlling member having the form of a patch placed in the middle of vibrating support 12 and which extends radially along axis X over different possible dimensions.

The thickness or height (taken along axis Z) may also vary.

Thus, a pair of position and dimensions of a vibration controlling member may comprise a position defined with respect to the centre of vibrating support 12 (the position may additionally be defined with respect to the face of the vibrating support on which is placed the vibration controlling member), and its dimensions defined by its radial extension along axis X and its dimensions along axis Z (thickness).

Each pair will therefore be tested in accordance with the Figure 5 method.

Next step S3 provides for selecting a first pair of position and dimensions of a vibration controlling member placed on the vibrating support among a plurality of pairs of positions and dimensions.

Then, step S4 is carried out to apply an excitation to the Figure 4 device configured in accordance with the above-mentioned selected first pair.

Appropriate excitation is obtained for instance through using a motor including a voice coil and a magnet of the above-described type.

The excitation signal is a wide band signal which enables amplitude values at several frequencies to be obtained: white noise, pink noise.

Next, in step S5 one or more values of a physical magnitude provided by the excited above-mentioned configured device are obtained.

The physical magnitude may be selected from the following physical magnitudes: a voltage, the intensity of an electrical current, a mechanical force, a displacement, an acoustic pressure radiated by the device or an element thereof.

It is to be noted that the value or values of the selected physical magnitude may be produced by the vibrating support alone, by the vibration controlling member of the first pair alone or by the whole configured device comprising the vibrating support provided with the vibration controlling member.

It is also to be noted that obtaining the value or values of the physical magnitude may result from a measuring step or a simulating step.

Furthermore, the value or values of several physical magnitudes may be obtained for the first pair.

The vibration controlling member may be a film that is an active film, e.g. of the PZT or PVDF type.

A PVDF film is more flexible than a PZT film and has less influence on mechanical impedance when not powered.

A PZT film provides better actuation capacities but proves to be stiffer. Thus, additional stiffness in the vibrating support (e.g. membrane) mechanical impedance has to be taken into account.

Alternatively, the vibration controlling member may be a passive film, the material of which has to be appropriately selected so as to modify stiffness mass and br damping of the vibrating support.

The value or values of the physical magnitude or physical magnitudes are obtained for a plurality of excitation modes applied to the device. The frequencies of these excitation modes lie in the range of frequencies of the excitation signal.

The same three steps S3, S4 and S5 are reiterated for each pair of position and dimensions of a vibration controlling member placed on the vibrating support until obtaining, for each pair among a plurality of pairs, one or more values of the physical magnitude or physical magnitudes.

Step S6 is then performed to determine, among the plurality of pairs, the pairs which have provided a minimum value or minimum values of the physical magnitude in a first vibration mode.

Put it another way, the pairs which are thus determined are those having no or minimal influence on the first mode.

Alternatively, the pairs which are thus determined may be those for which a minimum variation in the value or values of the physical magnitude is obtained in the first variation mode.

By way of example, a minimum variation may be detected by obtaining values of the physical magnitude with and without the vibration controlling member.

Searching for a minimum variation in the value or values of the physical magnitude may be obtained with acoustic pressure.

For instance, the sound pressure level (SPL) is obtained either through a measuring step or a simulating step in response to an excitation of the vibrating support.

The SPL may therefore be obtained and represented in a graph as a function of the frequency of the excitation.

Once step S6 has been carried out following step S7 may be performed.

In step S7 the pairs which have been determined at previous step S6 are collected and a set of pairs is therefore determined.

Next step S8 makes provision for comparing the different values of the physical magnitude obtained for each pair of the determined set of pairs in at least one second vibration mode.

It is to be noted that the first vibration mode mentioned above is for example the piston mode, whereas the second vibration mode is a break-up mode.

It is to be noted that several second vibration modes of this type may be taken into account in the described method.

Comparing the different values to each other leads to selecting (step S9) a pair for which a maximum value or maximum values of the physical magnitude have been obtained.

Alternatively, it is also to be noted that when a minimum variation in the value or values of the physical magnitude is taken into account at step S6 then a maximum variation in the value or values of the physical magnitude is taken into account at step S9.

The pair which has thus been selected enables the first vibration mode to be distinguished from the at least one second vibration mode.

During the steps S8 and S9 a search is made of the pairs of position and dimensions of a vibration controlling member placed on a vibrating support providing the greatest value or variation in the value among the pairs which have the least influence on the first vibration mode.

This method makes it possible to control the vibration modes of the vibrating support and, more particularly, to control/sense at least one second vibration mode without influencing the first vibration mode.

As has already been mentioned above, the method illustrated in Figure 5 may be applied to several vibration controlling members of different types which are placed on the vibrating support and tested in accordance with the method so as to distinguish a vibration mode from one or several second vibration modes.

Thus, one type of vibration controlling member may be selected as the best candidate for being placed on the vibrating support so as to differentiate the first vibration mode from one or several second vibration modes.

For example, one type of vibration controlling member may be characterized by an annular shape, whereas another type may be characterised byadiscshape.

Other types of vibration controlling members may be characterized by the active or passive feature of a film placed on the support (e.g. by gluing).

Also, for an active film, a PZT and a PVDF film may constitute two distinct types of vibration controlling member.

Several positions of the vibration controlling member on the vibrating support and dimensions are chosen so as to constitute a plurality of pairs of position and dimensions for each vibration controlling member placed on the vibrating support.

Figure 6 schematically illustrates a type of vibration controlling member placed on a vibrating support and which may have been determined through the execution of the Figure 4 method steps.

Thus, an annular-shaped piezoelectric patch 20 having an inner radius a and an outer radius b is fixed to a vibrating support 22, e.g. a membrane.

Although not represented in Figure 6, a voice coil has a radius rO.

As a variant embodiment (not depicted in the drawings), the above method enables a device or loudspeaker driver having only one disc-shaped vibration controlling member (e.g. a film) on the vibrating support and no voice-coil to be obtained. Using the method makes it possible to suitably locate/design the disc so that the first mode (piston mode) is excited without exciting second modes (break-up modes). The disc with its appropriately determined radius may be placed in the middle of the vibrating support (on one side). This variant may advantageously be used with a vibrating support (e.g. membrane) of small dimensions, which needs little energy for being excited.

The above method also makes it possible to obtain the device or loudspeaker driver configured as represented in Figure 7.

Such a device 13 comprises a vibrating support 32, e. g. a membrane 32, that is fixed at its periphery or its extremities 34.

For example, the membrane is clamped at its extremities or periphery.

Membrane 32 is connected to a voice-coil 36 in its central part.

is Two vibration controlling members 38 and 40 are placed on the vibrating support 32 and, more particularly, on either side thereof.

Thus, vibration controlling member 38 is placed on the upper face of the vibrating support 32 whereas vibration controlling member 40 is placed on the lower face thereof.

The two vibration controlling members (ex: films) are identical and have each an annular shape such as represented in Figure 6 (with inner radius a and outer radius b).

More particularly, these vibration controlling members are piezoelectric patches which are fixed to vibrating support 32, e.g. by gluing.

The annular shape of these vibration controlling members makes it possible to dispose voice-coil 36 in the middle thereof.

In particular, voice-coil 36 is connected to the lower face of vibrating support 32 in the middle of vibration controlling member 40.

Thus, an annular-shaped vibration controlling member makes it possible both to correct the second vibration mode while leaving clear the central part of the vibrating support for the voice-coil.

The positions and dimensions of vibration controlling members 38 and 40 have been adjusted thanks to the above-mentioned method so as to control the vibration modes of vibrating support 32.

Such control enables distinction of a first vibration mode from one or several second vibration modes.

The loudspeaker driver membrane of Figure 9 is modeled as a plate of radius R0, that is fully clamped at its periphery. The Young's modulus of the plate's material is E0, its surface density is p, and its Poisson's coefficient v0.

The thickness of the plate is h0. The same parameters, for each film, are denoted E, p, v, and h. To take into account the presence of the two films (here modeled as films of internal radius a and external radius b), the surface density is written as follows, I'o + 2p.Ffr). (1) where F(r) is a function that equals one where there is a piezoelectric material, and zero elsewhere, F(r) II(r -(.1) -1(i -5). (2) Similarly, the flexural rigidity of the plate is expressed as follows, D(r) = Do + 2D])Fft). (3) D0 being the flexural rigidity of the main plate, = 12(1) (4) and D being the equivalent flexural rigidity of one piezoelectric patch, 2 + hh0 + 1-vu 4 2 Oj (5) The equation governing the vertical displacement w(r,t) of the plate reads then, Dfr)V'w + jt(r) 3w F((r, t) ± )(T t). (6) where F is the force generated by the voice coil, and F is the force generated by the films when the latter are considered as active piezoelectric patches (i.e. being able to produce a secondary force). When considering an axisymmetrical configuration, their expression in polar coordinates can be deduced from the article entitled "Laminated piezopolymer plates for torsion and bending sensors and actuators", from C.K. Lee and F.C. Moon, J. Acoust. Soc. Am., 85 (1989) 2432-2439, where it is expressed in cartesian coordinates, (J2F 13F F3(r. t) = -h(i0 ± h13)f31 ± n9(t) = (7) u(t) being the tension applied at the piezoelectric patches that are mounted in series. The boundary conditions, which the displacement of the plate meet are those of a clamped plate at r =R0: w(R0) = = 0.

Or r (8) The first step is to compute the axisymetric eigenmodes of the free plate problem, (equation (6) with boundary conditions (8) with F=F=O), either analytically or numerically using finite elements or Galerkin projection methods.

These eigen modes are noted cP (r), n C N. The force exerted on the plate due to the piezoelectric patches is projected over each eigenmode, r2 pRo f;11, =< f(r), ç&(r) >= j J fth(r)rdrdO 0=0 v=O = h(h0 + h)Pc31 L'QH() -bç(b)].

where (.) denotes derivation with respect to r.

Contour plots of forces f, and f,2 are iJlustrated respectiveJy in Jeft-hand and right-hand sides of Figure 8.

More particularly, Figure 8 schematically represents the contour levels of the modal force of the first vibration mode (for example, the piston or global mode) exerted by the films (such as 38 and 40 in Figure 6) in their plane.

The different concentric curves partially illustrated in Figure 8 represent the values of the force produced by each film (selected physical magnitude at the output of the vibration controlling member).

A maximum value for the first mode is located in the middle of these curves.

For this application, the optimal values of a and b are those that maximize f,2, the modal projection of the force on the first break-up mode and minimize f, for i-i!= 2, especially f, , the projection on the first mode, coming from the primary load (referred to as global mode). Put it another way, maximization of f,2 over (a, b) and minimization of f,,i are desired.

The contour lines of the influence on the first break-up mode along with the lines where the influence on the first and third modes equals zero are given in Figure 9.

Figure 9 schematically illustrates the contour levels of the piezoelectric effort on the second mode as well as the zeros of the first mode (which correspond to the nodal line [NI of the first mode) in the plane of the film 38 of Figure 7.

Considering that it is more important to have no or minimal influence on the global mode (mode 1) that get the maximum influence on the strain modes (modes 2 and 3), the values of a and b taken for the final projection are determined thanks to Figure 5 method and are as follows.

a = 0.030 m b = 0.075 m.

The cross designated by OP in Figure 9 identifies this optimized set of parameters (a, b) determined by virtue of the method.

This cross is proximate to the maximum value for the second mode that is located in the middle of the set of curves C. Figure 10 schematically represents, for comparison purpose, several frequency response curves expressed by the Sound Pressure [evel in the axial direction (denoted SP[ and in dB) as a function of the frequency (in Hz).

More particularly, curve I represents the frequency response of a loudspeaker driver as illustrated in Figure 7 when an electrical excitation signal is applied to the voice-coil, thereby causing the vibrating support (ex: membrane 32) to vibrate. For example, a voltage of 0.45V is applied as an excitation signal.

No electrical excitation signal is applied to the two vibration controlling members (ex: piezoelectric films 38 and 40).

As represented in Figure 10, a peak p1 appears in the curve around 1300 Hz, which corresponds to the frequency of a second vibration mode (first break-up mode).

This curve shows that the two vibration controlling members enable correct driving of the first break-up mode.

Curve denoted 2 represents the frequency response of a device according to the invention (e.g. the Figure 7 device) when an electrical excitation signal is applied to the two vibration controlling members (ex: piezoelectric films 38 and 40) only, thereby causing the vibrating support (ex: membrane 32) to vibrate.

It is to be noted here that the vibrating support is excited at the frequency of a second vibration mode (mode 2) which is a first break-up mode.

For example, two voltages of 11 6V and -11 6V are respectively applied to the two members.

Curve denoted 3 represents the frequency response of Figure 7 device when excitation is applied to both the two vibration controlling members and the vibrating support.

More particularly, an electrical excitation signal is applied to each vibration controlling member with the phases of the two excitation signals opposite to each other. This provides better effect on the neutral fiber of the vibrating support in flexion mode than only one vibration controlling member (e.g. film) on one side of the vibrating support.

Furthermore, one of the two excitation signals is in phase with the electrical excitation signal applied to the vibrating support whereas the phase of the other excitation signal is opposed to that of the vibrating support excitation signal.

For instance, in Figure 7 device the phase of the excitation signal applied to upper vibration controlling member is opposed to the phase of the vibrating support excitation signal.

Thanks to a combined excitation (not necessarily with the above-described phase relationship), the peak denoted p2 on curve 3 and which corresponds to the first break-up mode has been shifted toward higher frequencies and its amplitude has been reduced (attenuation of the break-up mode).

This therefore provides a wider useful frequency range.

Thus, a combined excitation of both the two vibration controlling members and the vibrating support makes it possible to control the first break-up mode.

Figure 11 represents the directivity index of the Figure 7 device as a function of the frequency.

Two curves denoted 4 and 5 are illustrated.

Curve 4 represents the directivity index that is obtained when only the voice-coil of the device is excited.

At low frequencies (e.g. between 100 and 200 Hz) the device radiates in every direction.

The directivity then too rapidly increases with the frequency, particularly above 1000 Hz.

Curve 5 represents the directivity index that is obtained when combined excitation is applied to Figure 7 device.

Contrary to curve 4, curve 5 has a slower increase with frequency above 1000 Hz, up to 2000 Hz. This slower increase is very close to that of curve denoted 6 (in dotted lines) which stands for ideal increase in directivity.

Thus, the directivity index curves show that the device according to the invention provides second vibration mode(s) (break-up mode(s)) control in every direction. Control is therefore better than with prior art active signal control.

Claims

CLAIMS1. Method of determining the position and the dimensions of at least one vibration controlling member placed on a vibrating support that is liable to vibrate according to a plurality of vibration modes when excited, the method aimed at controlling the vibration modes of the vibrating support including: -providing a plurality of pairs of position and dimensions of at least one vibration controlling member placed on the vibrating support; -obtaining for each pair among the plurality of pairs, at least one value of a physical magnitude produced by the vibrating support, the at least one vibration controlling member, or the vibrating support provided with the vibration controlling member when the vibrating support is subjected to an excitation; -from the at least one value of physical magnitude obtained for each pair among the plurality of pairs, determining at least one pair for which a first vibration mode is distinguishable from at least one second vibration mode.
2. The method according to claim 1, characterized in that it further includes: -determining, among the plurality of pairs, a set of pairs for which, in each pair, minimum value(s) of the physical magnitude or a minimum variation in the value(s) of the physical magnitude is obtained in the first vibration mode; -selecting among the set of pairs thus determined, a pair for which maximum value(s) of the physical magnitude or a maximum variation in the value(s) of the physical magnitude is obtained in the at least one second vibration mode.
3. The method according to any one of claims 1 to 2, characterized in that the first vibration mode is the piston mode and the at least one second vibration mode is a break-up mode.
4. The method according to any one of claims 1 to 3, characterized in that the physical magnitude is selected from a voltage, an intensity of an electrical current, a mechanical force, a displacement, an acoustic pressure.
5. The method according to any one of claims 1 to 4, characterized in that the following: -providing a plurality of pairs of position and dimensions of the at least one vibration controlling member placed on the vibrating support, -obtaining for each pair at least one physical magnitude value and, -determining at least one pair for which a first vibration mode is distinguishable from at least one second vibration mode, are performed for several vibration controlling members placed on the vibrating support so as to distinguish a first vibration mode from several second vibration modes.
6. Device comprising: -a vibrating support that is liable to vibrate according to a plurality of vibration modes when excited, -at least one vibration controlling member placed on the vibrating support and whose position on the vibrating support and dimensions are adjusted so as to control the vibration modes of the vibrating support by distinguishing a first vibration mode from at least one second vibration mode.
7. The device according to claim 6, characterized in that the at least one vibration controlling member is fixed to the vibrating support.
8. The device according to any one of claims 6 to 7, characterized in that the at least one vibration controlling member is a film.
9. The device according to claim 8, characterized in that the film is an active film.
10. The device according to claim 9, characterized in that the film is of the PZT or PVDF type.
11. The device according to claim 8, characterized in that the film a passive film.
12. The device according to claim 11, characterized in that the film is selected so as to add stiffness, mass and/or damping to the vibrating support.
13. The device according to any one of claims 8 to 12, characterized in that the film is annular in shape.
14. The device according to claim 13, characterized in that a voice-coil is connected to the vibrating support in an area which is located inside the annular-shaped film.
15. The device according to any one of claims 6 to 14, characterized in that the vibrating support is not free at its periphery or at one or both of its opposed ends.
16. The device according to any one of claims 6 to 15, characterized in that the vibrating support is a membrane or a beam.
17. The device according to any one of claims 6 to 16, characterized in that two vibration controlling members are placed on either side of the vibrating support.
18. The device according to any one of claims 6 to 17, characterized in that the first vibration mode is the piston mode and the at least one second vibration mode is a break-up mode.
19. The device according to any one of claims 6 to 18, characterized in that the physical magnitude is selected from a voltage, an intensity of an electrical current, a mechanical force, a displacement, an acoustic pressure.
20. Method of using a device for producing sound waves, the device comprising: a vibrating support that is liable to vibrate according to a plurality of vibration modes when excited, at least one vibration controlling member placed on the vibrating support and whose position on the vibrating support and dimensions are adjusted so as to control the vibration modes of the vibrating support by distinguishing a first vibration mode from at least one second vibration mode, the method including applying an excitation to the device to cause said device to vibrate.
21. The method according to claim 20 characterized in that the excitation is applied to the vibrating support to cause said vibrating support to vibrate.
22. The method according to claim 20 or 21, characterized in that the method includes applying an electrical excitation signal to the at least one vibration controlling member which is able to be deformed when electrically excited.
23. The method according to any one of claims 20 to 22, characterized in that two vibration controlling members are respectively placed on either side of the vibrating support, the method including applying an electrical excitation signal to each vibration controlling member, the phases of the two signals being opposite to each other.
24. The method according to claim 23, characterized in that at least one of the phases of the two signals is different from the phase of the vibrating support electrical excitation signal.