WO1996013960A1

WO1996013960A1 - Electrodynamic loudspeaker with fluid-supported moving system

Info

Publication number: WO1996013960A1
Application number: PCT/DK1995/000429
Authority: WO
Inventors: Dan Kristoffersen
Original assignee: Dan Kristoffersen
Priority date: 1994-11-01
Filing date: 1995-10-31
Publication date: 1996-05-09
Also published as: DK126494A; DK171729B1; AU3801695A

Abstract

In electrodynamic loudspeakers, the primary air gap (9), in which the voice coil (2) is positioned, may be filled with magnetic fluid in order to improve the cooling of the voice coil (2) and the acoustical properties of the loudspeaker. However, the known suspensions in such loudspeakers are subject to axial friction hysteresis which prevents small electrical signals from being correctly converted into movements of the diaphragm (7) of the loudspeaker. The improved properties of the loudspeaker are thus not fully utilized. According to the invention, the loudspeaker is provided with one or more secondary air gaps (16) in which magnetic fluid is placed, and the moving system (2, 6, 7) is provided with means (10) which displace the magnetic fluid when the moving system (2, 6, 7) is displaced in axial direction, in order to obtain resilience. The moving system is thus free from friction hysteresis, and at the same time the desired resilience and stability against capsizing of the moving system (2, 6, 7) are obtained.

Description

Electrodynamic loudspeaker with fluid-supported moving system

The invention relates to an electrodynamic loud¬ speaker of the type stated in the introductory part of claim 1.

The moving system of such a loudspeaker is ge¬ nerally suspended in one or more flexible, mechanical suspensions, of which one is positioned at the edge of the diaphragm. Such suspensions may be produced from impregnated fabric, metal or reinforced or unreinforced plastic or rubber materials.

It has appeared that loudspeakers with mechanical suspension are subject to a special kind of distortion due to the inability of these loudspeakers to reproduce signals with a very low volume. This inability is per se difficult to measure in practice, and its influence on the reproduction of mixed signals may generally speaking only be demonstrated by subjective listening tests which require a standard of reference. However, a certain test may illustrate the phenome¬ non, ie. the reproduction by the loudspeaker of a beat on the musical instrument of a triangle the sound of which abates evenly, as it is a well-known fact. When participating in listening tests with comparison of the signal from the very triangle and of the signal reprodu¬ ced from the loudspeaker, the test persons stated that the abatement lasts shorter when reproduced by the loudspeaker than when heard directly from the triangle. This distortion is presumed to be caused by a form of mechanical hysteresis in the suspensions which is due to the fact that the static coefficient of friction of the suspension material is different from the dynamic coefficient of friction and it is presumed that both fabric, plastic and rubber materials are subject to such frictional hysteresis.

Even though the described distortion is of a modest size, it results in a relatively large deterioration of the general impression of sound. With today's great requirements on high-grade loudspeakers, there will be a heavy demand for a loudspeaker without this form of distortion. DE-2740 661 discloses an electrodynamic loudspea¬ ker where the voice coil is guided in an air gap in the magnetic system, and where the air gap is filled with a magnetic fluid. There is thus obtained a damping of the voice coil movement and a significant improvement of the heat dissipation from the voice coil. The sides of the air gap and the sides of the voice coil are designed with smooth surfaces in order to prevent that the magnetic fluid is thrown out of the air gap by large diaphragm fluctuations in a low frequency loudspeaker, without thus reducing the desired acoustic damping.

US-4,017,694 discloses a method for damping an electrodynamic loudspeaker by filling the air gap with a non volatile magnetic liquid having a viscosity of 600 to 10,000 centipoises containing from about 4 to about 20% of colloidal magnetic particles. The objects obtained are an improved frequency response, a suppres¬ sion of undesired hiss from the loudspeaker and an improvement of the maximum output of a low frequency loudspeaker. It is known eg. from US-3,682,518 and US-4,717,266 to use magnetic fluids in radial and thrust bearings for rotatable shafts, the bearing being constructed as a magnetised system, the magnetic fluid being positioned in a magnetic field in an air gap, and the shaft being supported by the magnetic fluid in such a way that mechanical contact between the two parts of the bearing is avoided. A pure fluid friction between the bearing parts is thereby obtained.

From the articles "Effects of Magnetic Fluids on Loudspeakers" (Bottenberg, Melillo and Raj, AES Journal January/February 1980, pp. 17-25) and "Special Focus - Ferro-fluids" (Mike Klasco, Voice Coil March 1992) , among others, it is known that the presence of magnetic fluid in the air gap formed as a circular cylindrical shell, in an electrodynamic loudspeaker has a radially centering effect on the voice coil and a preserving action on the circular cylindrical form of the voice coil.

For experimental use, loudspeakers are known where the voice coil "floats" in magnetic fluid filled into the air gap formed as a circular cylindrical shell of the loudspeaker. These loudspeakers are produced from known, series production high frequency loudspeakers with the air gap filled with magnetic fluid, and where the movement of the moving system constituted by the voice coil may in a generally known way be controlled by means of a mechanical edge suspension. The production of the experimental loud-speakers is made by removing the edge suspension, and there will then only be the magnetic fluid left for control of the movement of the moving system. Such loudspeakers are described among others in the Danish magazine "High Fidelity" no. 4, 1989 no. 10, 1989 p. 21; and nos. 7-8, 1990 p. 23.

However, these loudspeakers are unsuitable for practical use as the axial retaining of the moving system is effectuated merely by the surface tension of the magnetic fluid together with the effect of the attractive force of the magnetic system on the magnetic fluid which by its adhesion adheres to the parts of the moving system which are moved outside the air gap. This poor retaining entails that such a loudspeaker is extremely fragile because the moving system may fall out of its bearing by minor impacts. Such impacts can eg. be too powerful electrical signals or signals with direct-current elements applied on the voice coil, physical contact with the diaphragm or transport of the loudspeaker in an unfavourable position. Furthermore, these loudspeakers have the dis¬ advantage that the magnetic fluid is not retained in a sufficiently effective manner in the air gap, either because, as mentioned, it adheres to the parts of the moving system which are moved outside the air gap, the fluid migrating away from the air gap along the surfaces of the pole shoes or of the voice coil form by a kind of capillary effect, or because drops of the fluid will simply break away by heavy electrical signals. The said capillary effect may eg. appear on unevenly machined surfaces with tool marks. It is further assumed that an inappropriate graduation of the strength of the magnetic field in the peripheral areas may cause a too small withdrawal force on the magnetic fluid which has left the air gap by migration or for another reason.

The moving system must be controlled to make a purely linear movement along the axis of motion and must be influenced by a resilient action defining a position of rest in axial direction for the system. The system must thus be secured against capsizing, by which is meant angular deflection between the symmetry axis of the voice coil and the symmetry axis of the pole shoes. The system must further be secured against radial displacement, by which is meant displacement of the axis of the voice coil away from convergence with the symmetry axis of the pole shoes. Furthermore, the system must be secured against rotation arc r.d the axis of motion in order that the flexible lead-;:, wires trans- mitting current from termination fa li ies cr. the body of the loudspeaker to the voice coil are net tightened and thus influence the movement of the moving system.

In this connection, it is a drawback of the above mentioned loudspeakers for experimental use that the axial position of the voice coil is indeterminate because there is no longer the mechanical, axially acting return spring constituted by the edge suspension. Furthermore, the voice coil is by these experimental loudspeakers not secured sufficiently against capsizing (also known as "rocking"). It is known that undesired capsizing may occur during use of the loudspeaker if the moving system is not suspended in a sufficiently stable manner. Such capsizing may result in harmonic distortion and intermodulation distortion.

By the invention there is procured a loudspeaker of the initially stated type where the above described drawbacks are eliminated, the moving system being suspended without the use of mechanical suspensions, but with a more precise guidance than it is known from the said loudspeakers for experimental use. The moving system is thus free from frictional hysteresis and at the same time, the desired resilience and the desired stability against capsizing are obtained.

According to the invention, this is obtained by a loudspeaker which is characterised by the features stated in the characterising part of claim 1. By the features stated in claims 2 and 3, a simple and low-cost design of the loudspeaker according to the invention is obtained, magnetic fluid, which is relati¬ vely expensive, being positioned in one air gap only. By the features stated in claims 4 and 5, it is obtained that the magnetic fluid serving to obtain an axial resilience is not exposed to hea ir.g from the voice coil whereby the viscosity of the fluid would be affected which would entail a modified spring charac¬ teristic. Furthermore, a good stability arair.st capsi- zing of the moving system is obtained as it is supported by the magnetic fluid in two separated a r gaps.

By the features stated in claim 6, there are obtained partly the possibility of distributing the means to displace magnetic fluid over two secondary air gaps which provides a larger freedom to their design and partly the possibility of a stable suspension of loudspeakers with large diaphragms such as low frequency loudspeakers.

By the features stated in claim 7, it is obtained that the diaphragm in a low frequency loudspeaker may be suspended stably and that at the same time, magnetic fluid is only to be positioned in two air gaps. By the features stated in claim 8, a desired well- defined position of rest for the moving system is obtained.

By the features stated in claims 9 and 10, a desired, well-defined position of rest in the rotating direction for the moving system is obtained.

By the features stated in claim 11, it is obtained that the desired, well-defined positions of rest in both axial direction and the rotating direction for the moving system may be obtained by use of one secondary air gap.

By the features stated in claims 12 and 13, a simple and appropriate fastening of the means to displace the magnetic fluid is obtained. By the features stated in claims 14 and 15, certain, desired spring characteristics are obtained in a simple way.

By the features stated in claim 16, a more even graduation of the force of the magnetic field in the air gaps is obtained so that large field strength gradients are avoided. Such gradients may entail that the magnetic fluid is poorly retained in the air gaps.

By the features stated in claims 17 and 18, a smaller adhesion of the magnetic fluid to the pole shoes and the adjacent faces is obtained, thus further securing that the fluid is retained in the air gap.

By the features stated in claim 19, both a desired degree of damping and centering of the moving system are obtained. The invention will in the following be explained in more detail by means of embodiment examples under reference to the drawing, in which

Fig. 1 shows a longitudinal section in a high frequency loudspeaker according to the invention with the moving system in position,

Fig. 2 is a section of Fig. 1 showing the means of the loudspeaker for displacing magnetic fluid, on a larger scale, Fig. 3 corresponds to Fig. 2 and shows how the means displace magnetic fluid when the moving system is displaced backwards (towards the back of the loud¬ speaker) , Fig. 4 corresponds to Fig. 2 and 3, and shows how the means displace magnetic fluid when the moving system is displaced forwards (towards the front of the loud¬ speaker) ,

Fig. 5 shows means mounted on the voice coil form for displacing magnetic fluid when the moving system is displaced either forwards or backwards or is rotated in one of the rotating directions, seen from the exterior of the voice coil,

Fig. 6 shows the means in Fig. 5 seen in the direction of the tangent line of the voice coil form, Fig. 7 shows a longitudinal section in a preferred embodiment of a high frequency loudspeaker according to the invention with the moving system removed and without magnetic fluid, Fig. 8 shows a longitudinal section of a preferred embodiment of a medium frequency loudspeaker or a low frequency loudspeaker according to the invention with the moving system removed and without magnetic fluid, and Fig. 9 shows a longitudinal section of the secon¬ dary pole shoes and the voice coil form in a preferred embodiment of a loudspeaker according to the invention, on a larger scale.

In Fig. 1 is shown a high frequency loudspeaker according to the invention. The main elements of the loudspeaker are constituted by a magnetic system and a moving system. The loudspeaker is in the embodiment shown in Fig. 1 rotationally symmetrical about the axis of motion 15 of the moving system. In the embodiment shown in Fig. 1, the magnetic system consists of an annular permanent magnet 3 which is rotationally symmetrical around the symmetrical axis 15 of the loudspeaker and magnetized in the direction of this axis. One pole of the magnet is connected to a magnetic yoke 5 on which an inner pole shoe 4, 8 is positioned. The second pole of the magnet is connected to an annular, outer primary pole shoe 1 and an annular, outer secondary pole shoe 12. There will thus be two air gaps formed as circular cylindrical shells, ie. a primary air gap 9 and a secondary air gap 16. The magnetic system is joined by glueing.

Magnetic fluid 11, 13 is positioned in each air gap 9, 16. The magnetic fluid 13 in the primary gap 9 is divided into a quantity placed between the exterior face of the voice coil 2 and the outer primary pole shoe 1, and a quantity placed between the interior face of the voice coil form 6 and the inner primary pole shoe 4. The magnetic fluid 11 in the secondary air gap 16 is likewise divided into a quantity placed between the outer secondary pole shoe 12 and the exterior face of the voice coil form 6 with the means 10, and a quantity placed between the interior face of the voice coil form 6 and the inner secondary pole shoe 8.

By this division of the magnetic fluid 11, 13 in each air gap 9, 16 in two quantities, an effecient self-centering of the moving system is obtained. This centering effect is known and discussed in the technical literature as already mentioned. The effect is based on the phenomenon that due to its ferromagnetic properties and due to its mobility arising from its fluidity, the magnetic fluid moves towards the most powerful area of the magnetic field.

Supposing that in Fig. 1, the diaphragm 7 is affected by a downward force displacing the moving system such that it capsizes, the voice coil 2 is displaced downwards and the magnetic fluid 13 in the primary air gap 9 is displaced. As the fluid is thus displaced from the areas where the magnetic field is most powerful, it tends to move back again and thus tries to bring the voice coil 2 back. By a longer impact the magnetic fluid is displaced mainly in direction of the periphery whereby each of the two quantities of fluid positioned on the outer and inner side, respectively, of the voice coil form will have different layer thickness measured in radial direction. Where the layer thickness is least, the average power of the magnetic field is higher as the average distance to the pole shoe is smaller and therefore the fluid moves in this direction, thus leading to a uniform layer thickness of the fluid in the entire periphery and on both sides of the voice coil form and thus a centering of this.

The used magnetic fluid is preferably a magnetic oil such as Ferrofluid type APG 513 from Ferrofluidics Corporation, Nashua, New Hampshire, USA. This oil is developed for use in a known way in the primary air gap in electrodynamic loudspeakers and it is a very good approximation to a Newtonian liquid which is important in order to avoid acoustic distortion. In the embodiment shown in Fig. 1, the moving system is constituted by a voice coil form 6 shaped as a tube section on which a voice coil 2 is coiled opposite the primary air gap 9 and a diaphragm 7 is mounted at the front end. Current is led to the voice coil via not shown lead-in wires from not shown termina¬ tion facilities positioned on the body of the loud¬ speaker. The moving system may move in the direction of the axis of motion 15.

Furthermore, opposite the secondary air gap 16 on the outer side of the voice coil form, means 10 are fastened to the voice coil form 6 which means displace magnetic oil 11 in the secondary air gap 16 when the moving system is displaced in the longitudinal direc¬ tion. On the the means 10 there may be designed a stop 14 which prevents the moving system 2, 6, 7 from moving unintentionally outside its working area. If, eg. a rather heavy current should be applied to the voice coil or the moving system 2, 6, 7 is in another way exposed to very large stress, the means 10 may be pressed all through the magnetic fluid whereby the moving system comes outside its working area and the loudspeaker ceases to function.

The said displacement of magnetic fluid in the secondary air gap is shown in Figs. 2-4 which are sections on a larger scale of Fig. 1.

In Fig. 2 the moving system and thus the voice coil form 6 and the means 10 are in the position of rest. The magnetic fluid 11 retained by the magnetic field is also in a position of rest.

In Fig. 3 the moving system 2, 6, 7 is displaced towards the backside of the loudspeaker (to the left in Figs. 1-4) which corresponds to the voice coil 2 being subject to a current in one direction. The front means 10 (the one on the right in Figs. 1-4) is thus dis¬ placed into the quantity of magnetic fluid 11 posi¬ tioned between the outer secondary pole shoe 12 and the exterior face of the voice coil form 6 with the means 10. A corresponding part of this quantity of magnetic fluid is displaced away from its position of rest against the magnetic attraction which results in a reaction force affecting the front means 10 and trying to press this - and thus the moving system 2, 6, 7 - back to the position of rest. This effect is quite analogous to the buoyancy influencing a body when immersed in a fluid which is affected by gravitation, however, the magnetic attraction affecting the magnetic fluid 11 is not directed in one exact direction, but follows the lines of magnetic force in the secondary air gap 16.

In Fig. 4 the moving system 2, 6, 7 is displaced in the opposite direction against the front side of the loudspeaker (to the right in Figs. 1-4) corresponding to the voice coil 2 being subject to a current in the other direction. The rear means 10 (the one to the left in Figs. 1-4) is displaced into the magnetic fluid 11 which results in a reaction power influencing this rear means 10 and trying to press the moving system 2, 6, 7 back to the position of rest. In this way, a system with the same effect as a bidirectional spring is established.

In figs. 5 and 6 a preferred embodiment of the means 10 is shown. These are here designated 20 and are in Fig. 5 seen from the exterior side of the voice coil form and in Fig. 6 seen in the direction of the tangent of the voice coil form. The voice coil form is typically provided with several such means, eg. three. The means 20 is - besides being designed to displace magnetic fluid when the moving system is displaced either backwards or forwards - also designed to displace magnetic fluid when the moving system is turned in one of the rotational directions.

The means 20 is designed as a thickening of the wall of the voice coil form 6 and runs along a closed curve. The means 20 is defined by an internal limita- tion curve 17 and an external limitation curve 18. The means cooperates with a secondary outer pole shoe 19 corresponding to the pole shoe 12. The active face of the pole shoe 19 facing the voice coil form 6 is concave and formed with a circular cylinder surface corresponding to the run of the voice coil form 6 at a level perpendicular to the axis of motion 15 of the moving system 2, 6, 7. The pole shoe 19 has, seen in the viewing direction of Fig. 5, an extent corresponding to the internal limitation curve 17 of the means 20. In Fig. 6 the pole shoe 19 and the magnetic fluid 11 are shown in longitudinal section.

In Fig. 7, a preferred embodiment of a high frequency loudspeaker according to the invention is shown with the moving system 2, 6, 7 out of place. The means 10 are here not provided with stops 14 so the moving system may as a matter of course be guided in position when assembling the loudspeaker after the assembling and the magnetization of the magnetic system. The voice coil form 6 is here manufactured from aluminium as it is generally known in the production of loudspeakers and it is therefore designed with a slot 25 in order not to function as a short-circuit turn. When assembling the loudspeaker, the magnetic fluid 11, 13 is injected preferably after magnetization of the magnet and before insertion of the moving system as it would otherwise be difficult to place the fluid in between the internal face of the voice coil form 6 and the inner primary pole shoe 4.

However, it is also possible to inject the magnetic fluid after insertion of the moving system if the voice coil form 6 is designed with holes such that there is interconnection between the two fluid quantities 11 (Fig. 2) and/or between the two fluid quantities 13 (Fig. 1) on the interior and exterior side, respec¬ tively, of the voice coil form 6. Hereby is obtained that it is only necessary to insert the syringe needle or alike by means of which the fluid is injected, to the interspaces between the exterior face of the voice coil 2 and the outer primary pole shoe 1, and between the outer secondary pole shoe 12 and the exterior face of the voice coil form 6, respectively, as by the estab¬ lished connections the fluid finds its way by itself into the interspaces between the interior face of the voice coil form 6 and the inner primary pole shoe 4, and between the interior face of the voice coil form 6 and the inner secondary pole shoe 8, respectively. Tests have shown that the above described self- centering effect is influenced very little by the creation of these connections between the interior and exterior sides of the voice coil form. The connections may be constituted by holes in the voice coil form 6, preferably designed in between the quantities of magnetic fluid 11 as shown in Fig. 2. Alternatively, the connection may be constituted by the slot 25 (Fig. 7) . The inner pole shoes 4, 8 are in Fig. 7 designed integrally with the yoke 5 as they have the form of a circumferential projection on a central elongation 21 of the yoke 5. Perforations 22-24 together with the slot 25 serve to prevent airflows from being trapped behind the barriers constituted by magnetic fluid. Such trapped quantities of air may partly, by their heat expansion, displace the magnetic fluid from its position of rest and partly act as an uncontrolled spring for the moving system. Tests have shown that such a trapped resilient quantity of air may have unfortunate acoustic properties. The perforation 22 counterbalan¬ ces the alternating pressure occuring behind the diaphragm 7 by the intentional movements hereof, and is designed with wide dimensions in order that the resulting alternating airflow does not give rise to noises.

In an alternative embodiment of the loudspeaker in Fig. 7, the primary air gap is so narrow that the means 10 cannot pass this, and thus the moving system 2, 6,

7 cannot be guided into the magnetic system 1, 5, 12 after the assembling hereof. Therefore, the voice coil form 6 with the voice coil 2 and the means 10 are brought in position before the outer annular primary pole shoe 1 is mounted on the rest of the magnetic system. After the mounting of the pole shoe 1 and the magnetization of the magnet 3, magnetic oil is filled into the air gaps, the secondary air gap 16 being accessible through the perforation 23, and the primary air gap 9 being accessible from the front side of the loudspeaker (from the right in Fig. 7) . Subsequently, the diaphragm is mounted on the voice coil form 6.

In Fig. 8, a preferred embodiment of a low frequen¬ cy loudspeaker according to the invention is shown. The non-numbered parts of the figure correspond exactly to the parts shown in Fig. 7.

As a low frequency loudspeaker must necessarily be provided with a very large diaphragm surface compared to a high frequency loudspeaker, it is not appropriate to design the diaphragm in the same way as in the high frequency loudspeakers shown in Figs. 1 and 7. Instead the diaphragm 26 has the form of a rigid conical surface which is firmly connected with the voice coil form 6 and suspended in a yielding manner at its front edge 27.

The front suspension may be designed as in generally known low frequency loudspeakers, but is in the shown preferred embodiment designed with magnetic fluid in magnetic fields in secondary air gaps. A front magnetic system 28, 29 is positioned along the front edge 27 of the diaphragm 26 and cooperates with a diaphragm bearing 30 in the form of a short, circular cylindrical tube section which is rigidly connected with the front edge of the diaphragm.

In Fig. 8, there are shown two alternative designs of the magnetic system 28, 29 for the secondary air gap at the front suspension of the diaphragm. The number 28 refers to an embodiment corresponding to the design of the secondary air gap 16 in the high frequency loudspeakers in Figs. 1 and 7. One or more magnets 31 distributed along the frcr.t edge of a body 32 corresponding to the front edge cf the diaphragm 26 are provided with a circum erential, secondary, inner pole shoe 33 and a circumferential, secondary, outer pole shoe 34 in order to form a secondary air gap 35. In this air gap the diaphragm bearing 30 is positioned, a not shown quantity of magnetic fluid being positioned between the bearing 30 and each pole shoe

33, 34. The bearing 30 is centred by itself by the already stated effect of the magnetic fluid. As it is shown in a section of Fig. 8, there may on the bearing be positioned means 36 which as to design and effect correspond to the means 10 in Figs. 1 and 7, however, the bearing may also be designed without such means, in that case the necessary restoring force is performed by the means 10. The number 29 refers to an alternative embodiment of the magnetic system for the front suspension of the diaphragm. This alternative embodiment may also be used for the secondary air gap 16 in the high frequency loudspeakers shown in Figs. 1 and 7.

One or more magnets 37 distributed along the front edge of a body 32 corresponding to the front edge of the diaphragm 26 are provided with a circum¬ ferential, secondary, rear pole shoe 38 and a circum- ferential, secondary, front pole shoe 39 in order to form a secondary air gap 40. The diaphragm bearing 30 is positioned up against this air gap, a not shown quantity of magnetic fluid being positioned between the bearing 30 and the pole shoes 38, 39. The magnetic fluid is thus placed between three circumferential elements, ie. the pole shoes 38, 39 and the bearing 30, and there is only fluid on the exterior of the bearing. The bearing 30 is also here centred by itself by the earlier stated effect of the magnetic fluid. There may as shown on the bearing 30 be positioned a means 41 whose effect corresponds to that of the means 10 in Figs. 1-4 and 7, but is positioned between the pole shoes 38, 39 such that the thickness of the means increases in the direction away from each pole shoe towards the other pole shoe to a maximum thickness in between the pole shoes. When cooperating with the two pole shoes 38, 39, such a means 41 designed on the exterior of the bearing 30 has the same effect as the means 10 in Figs. 1-4 and 7 when cooperating with the outer, secondary pole shoe 12. The bearing 30 may also be designed without such an means 41, as mentioned above.

Just as the low frequency loudspeaker shown in Fig. 8 has special magnetic systems 28, 29 each with their magnet or set of magnets 31, 37 for secondary air gaps 35, 40, the low frequency loudspeakers (Figs. 1, 7) according to the invention may be designed each with their magnet or set of magnets for the primary air gap 9 and the secondary air gap 16, respectively. Corre¬ spondingly, this also applies for the part of a lo frequency loudspeaker according to the invention which corresponds to the low frequency loudspeakers in Figs. 1 and 7 and which is seen farthest to the left in Fig. 8.

In the present invention where the magnetic fluid performs the very suspension of the moving system, it is of utmost importance that no magnetic fluid is lost from the air gaps in the course of time. However, tests have shown that even with good magnetic fluids adapted to the use in air gaps of loudspeakers, difficulties may arise as regards retaining the magnetic fluid in the air gaps. These difficulties seem to be due to the fact that the magnetic attraction cannot fully surmount the adhesive power of the magnetic fluid to the surfaces on and close to the pole shoes. The magnetic fluids available on the market are often oil-based, and like other oils, such magnetic oils may leave an oil film on faces which have been wetted with the oil. Just as an ordinary oil may stick to a vertical face in spite of the impact of the gravity, a magnetic oil has proven to be able to stick to a face in spite of the magnetic attraction. Furthermore, as initially stated, the oil can migrate away from the air gap by surface or capil¬ lary effect.

In the embodiment shown in Fig. 9 of a set of secondary pole shoes 8, 12 in a loudspeaker according to the invention, two arrangements are used to ensure that the oil is retained in the air gap 16. The pole shoes 8, 12 are rotationally symmetrical around the axis of motion 15 of the moving system.

Firstly, in the areas at their edges, the pole shoes 8, 12 are designed with roundings 42, 43 to obtain an essentially even run of the working surfaces of the pole shoes such that local magnetic field concentrations are avoided, which concentrations may occur at pole shoes with more or less sharp edges or corners, as it is known. The object of the embodiment shown in Fig. 9 is a magnetic field which in the direction of the axis of motion 15 decreases evenly in strength in a direction away form the centre of the air gap. The run of the roundings 42, 43 of the pole shoes 8, 12 are preferably designed to obtain a specific, desired graduation of the magnetic field force. A further advantage of this embodiment of the pole shoes 8, 12 is that the flow velocity of the oil in the oscillating movement is reduced substantially when the oil reaches the "trumpet-shaped" mouth constituted by the roundings 42, 43. It is assumed that an uneven graduation of the magnetic field force, which may eg. occur by pole shoes with sharp edges as shown in Figs. 1-4 and 7-8, may partly entail dynamic instability in the fluid when the loudspeaker is influenced by an electrical alternating signal such that the fluid "plashes" or that drops may actually break away, and may partly imply that the field force at a small distance from the air gap is simply too small such that a sufficient, magnetic attraction of the fluid towards the air gap is not maintained. Secondly, the area of the pole shoes 8, 12 in and close to the air gap 16 is designed with a surface 44 which repels the used magnetic fluid. This fluid repellent surface is preferably procured by means of a coating 44 with a plastic material containing fluo- rine. A suitable coating is eg. the one made by the firm Accoat A/S, Kvistgaard (Denmark) under the designation Accofal 2G. It is a smooth, modified fluorocarbon coating with low friction and good non-stick properties. The film thickness is preferably chosen to be 50 μm. Even though Fig. 9 only shows coating 44 on the part of the pole shoes 8, 12 which is nearest to the air gap, it will usually be most suitable that the entire part in question is treated, eg. the yoke 5 with the central elongation 21 and the inner pole shoes 4, 8.

Claims

C L A I M S

1. An electrodynamic loudspeaker with at least one magnet (3) , a primary inner pole shoe (4) and a primary outer pole shoe (1) , where between the primary pole shoes is formed a primary air gap (9) , preferably formed as a circular cylindrical shell, with a primary magnetic field, in which parts of a moving system consisting of a voice coil form (6) , a voice coil (2) and a diaphragm

(7, 26) are suspended, displaceable along a axis of motion, preferably being rotationally symmetrical around the axis of motion and resiliently suspended in the direction of the axis of motion, and where in the interspaces between the voice coil (2) and each of the primary pole shoes (1,4) is placed a quantity (13) of a magnetic fluid which is retained in these interspaces by means of the primary magnetic field, characterized in that the loudspeaker comprises: a number of secondary pole shoes (8, 12, 33- 34, 38-39), which pole shoes are distributed in the direction of the axis of motion and/or distributed in the circumferential direction, and where between the secondary pole shoes is formed a number of secondary air gaps with secondary magnetic fields, in which there is placed magnetic fluid (11) , retained in these air gaps by means of the secondary magnetic fields, and means (10, 20) which are connected with the moving system (2, 6, 7, 26) in a manner to transmit forces in the direction of the axis of motion, and which, in order to obtain resilience, displace magnetic fluid (11) in the secondary air gap or air gaps between the secondary pole shoes (8, 12, 33-34, 38-39) when the moving system is displaced in the direction of the axis of motion.

2. A loudspeaker according to claim 1, charac¬ terized in that the secondary pole shoes (8, 12) are generally positioned in the same plane perpendicular to the axis of motion in order to obtain one set of secondary pole shoes.

3. A loudspeaker according to claim 2, charac- terized in that the set of secondary pole shoes is identical and coincident with the primary pole shoes.

4. A loudspeaker according to claim 2, charac¬ terized in that the plane in which the secondary pole shoes (8, 12) are positioned is separated in the direction of the axis of motion from the primary pole shoes (1, 4) .

5. A loudspeaker according to claim 4, charac¬ terized in that the number of secondary air gaps is one and that the secondary air gap is formed between an inner secondary pole shoe (8) , which is formed integral¬ ly with the inner primary pole shoe (4) , and an outer secondary annular pole shoe (12) magnetically connected with the same magnetic pole or magnetic poles as the outer primary annular pole shoe (1) , the secondary air gap thereby forming a magnetic shunt for the primary air gap (9) .

6. A loudspeaker according to claim 1, charac¬ terized in that the secondary pole shoes (8, 12, 33-34, 38-39) are generally distributed in two planes perpendi- cular to the axis of motion in order to form two sets of secondary pole shoes.

7. A loudspeaker according to claim 6, charac¬ terized in that one of the sets of secondary pole shoes is identical and coincident with the primary pole shoes.

8. A loudspeaker according to any of the preceding claims, characterized in that the means (10, 20) displace magnetic fluid (11) in the secondary air gap or air gaps between the secondary pole shoes (8, 12, 33- 34, 38-39) when the moving system is displaced in any of the two directions of the axis of motion in order to obtain resilience in both directions of the axis of motion, such that this resilience defines a specific position of rest in the direction of the axis of motion for the moving system (2, 6, 7, 26) .

9. A loudspeaker according to any of the preceding claims, characterized in that the loudspeaker comprises means (20) which are connected with the moving system (2, 6, 7, 26) in a manner to transmit forces in the rotation direction about the axis of motion and which displace magnetic fluid (11) placed in the secondary air gap or air gaps between the secondary pole shoes (8, 12, 33-34, 38-39) when the moving system is rotated around the axis of motion.

10. A loudspeaker according to claim 9, charac¬ terized in that the means (20) displace magnetic fluid (11) in the secondary air gap or air gaps between the secondary pole shoes (8, 12, 33-34, 38-39) when the moving system is rotated around the axis of motion in any of the two rotational directions in order to obtain resilience in both rotational directions such that this resilience define a position of rest in the rotational direction for the moving system (2, 6, 7, 26) .

11. A loudspeaker according to claims 8 and 10, characterized in that the number of secondary outer pole shoes (12) is more than one and that the secondary air gaps are formed between an inner secondary pole shoe (8) formed integrally with the inner primary pole shoe (4) and a number of concave secondary outer pole shoes (12) formed with circular cylinder surfaces, and that corresponding to at least one of these secondary outer pole shoes (12) , there are means (20) which displace magnetic fluid (11) in the secondary air gap or air gaps between the secondary pole shoes (8, 12, 33-34, 38-39), when the moving system is rotated around the axis of motion in any of the two rotational directions in order to obtain resilience in both rotational directions such that this resilience define a position of rest in the rotational direction for the moving system (2, 6, 7, 26) .

12. A loudspeaker according to any of the claims 1-8 in which the voice coil form (6) has the form of a tube section (6) with preferably circular cross section, characterized in that the means (10) displacing the magnetic fluid are connected to the moving system (2, 6, 7) in a manner to transmit forces in the direction of the axis of motion by being fastened to the tube section (6) , and that the means (10) have the form of thickenings of the tube wall with a material thickness which is increasing in the direction of the axis of motion away form the area or areas of the tube (6) which in the position of rest is situated opposite the secondary outer pole shoe or pole shoes (12) .

13. A loudspeaker according to any of the claims 9-12 in which the voice coil form (6) has the form of a tube section (6) with preferably circular cross section, characterized in that the means (10, 20) displacing magnetic fluid are connected with the moving system (2, 6, 7) in a manner to transmit forces both in the direction of the axis of motion and in the rotatio¬ nal direction around the axis of motion by being fastened to the tube section (6) , and that the means (10, 20) have the form of thickenings of the tube walls with a material thickness which is increasing in the direction away from the area or areas of the tube (6) which in the position of rest is situated opposite the secondary outer pole shoe or pole shoes (12) .

14. A loudspeaker according to claim 12 or 13, characterized in that the runs of the thickenings in the direction away from the areas corresponding to the position of rest are designed to displace magnetic fluid in a manner to obtain resilience with specific, desired spring characteristics.

15. A loudspeaker according to claim 14, charac- terized in that the spring charateristics are linear.

16. A loudspeaker according to any of the preceding claims, characterized in that at least one of the pole shoes (1, 4, 8, 12, 33-34, 38-39) on the face facing the moving system (2, 6, 7, 26) is designed with roundings (42, 43) in order to obtain reduced magnet field gradients in the direction of the axis of motion.

17. A loudspeaker according to any of the preceding claims, characterized in that at least one pole shoe (1,

4, 8, 12, 33-34, 38-39) is coated in order to obtain a liquid repellent surface.

18. A loudspeaker according to claim 17, charac¬ terized in that the coating comprises an application of a plastic material (44) containing fluorine, such as a material contaning polytetrafluoroethylene (PTFE) .

19. A loudspeaker according to any of the preceding claims, characterized in the magnetic fluid in the secondary air gap or air gaps is an oil containing suspended, submicroscopical or colloidal, magnetic particles, with a viscosity at 27°C of 50-700 CP, preferably about 100-200 CP, a saturation magnetization of 200-700 gauss, preferably about 200-400 gauss, and an evaporation coefficient at 175°C of 1-5 x 10 g/cm²s.