GB2123651A

GB2123651A - Transducers

Info

Publication number: GB2123651A
Application number: GB08317151A
Authority: GB
Inventors: Stanley Kelly
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-06-29
Filing date: 1983-06-24
Publication date: 1984-02-01
Also published as: GB2123651B; GB8317151D0

Abstract

A moving conductor transducer, for example a loudspeaker, with a magnetic assembly 10-16 presenting at least two elongate approximately parallel air gaps and having a thin diaphragm 21 with a generally planar portion supporting conductors 34, 35 thereon is described. At least one linear conductor 34, 35 is disposed to coact with the magnetic field in each air gap, an actuating current being passed through the conductors 34, 35 in the different gaps in series such that the conductors, and with them the diaphragm, all move in the same direction. The conductors 34, 35 are preferably in the form of flat spirals. Supplementary magnets may be provided to reduce flux leakage and hence increase the flux density in the air gaps. <IMAGE>

Description

SPECIFICATION Transducers This invention relates to loudspeakers and similar transducers of the moving conductor type.

A well-known type of loudspeaker comprises an annular coil disposed in a transverse magnetic field, produced by a suitable magnetic assembly, and connected to a diaphragm, usually conical in shape. Energising current is fed to the coil, and the resultant force is transmitted to the diaphragm at the line of contact between the coil former and the diaphragm. Because there is a finite velocity of propagation of the sound waves through the diaphragm, there will be a progressive time, and hence phase, delay between the motion at the driving point and the other parts of the diaphragm, and the phase lag will increase with increase of the distance from the driving point and with the frequency of the energising current.At frequencies for which the wavelength of the transverse vibration in the diaphragm is large compared with the diaphragm dimension the diaphragm can be considered to vibrate as a rigid surface, but as the frequent increases this is no longer the case, and the diaphragm vibrates in a series of different modes, the nature of which depends upon the mechanical characteristics of the diaphragm material and its dimensions. Such modes of vibration impair the fidelity of reproduction from the loudspeaker.

Various attempts have been made to produce loudspeakers, especially those for use at higher frequencies, in which the diaphragm moves as though it were a rigid member, independent of frequency. One design which has been used with success is the so-called ribbon loudspeaker, or microphone, which consists of a single conductor in the form of a thin strip supported at its ends and positioned in a transverse magnetic field developed between the pole pieces of a magnetic assembly, normally a permanent magnetic assembly, so that when current is passed through the conductor it moves in a direction and with a magnitude which is determined by the direction of current flow and the direction of the magnetic field, the intensities of the current and the magnetic field, and the mechanical restraints upon the conductor.Because the force is imposed uniformly on the conductor over its area there is no phase shift over the area and the frequency response is theoretically independent of frequency; subject to the limitations due to the physical size of the conductor used as a diaphragm such a system offers a transducer of wide frequency response, good transient response, and average sensitivity compared with the equivalent moving coil loudspeaker.

A disadvantage of this type of transducer is the low impedance of the conductor used as the diaphragm. In a typical case, the conductor might be 60 mm. long, 8 mm. wide and 3 ,t4m. thick, and using aluminium based material for the conductor the resistance will be about 7 xl 0-2 ohms. To drive such a transducer from a conventional power amplifier requiring a load impedance in the range of 4 to 1 6 ohms a matching transformer must be used. The resistive and core losses in a suitable transformer and the contact resistance between the conductor and its terminal connections introduce a total system loss of about 6 to 8 dB.

The present invention has for its object to provide a construction of transducer of this type in which such losses are substantially reduced and, depending upon the service for which the transducer is intended, may permit a matching transformer to be dispensed with.

In accordance with the invention there is provided a transducer of the type comprising a generally linear conductor disposed for movement in a magnetic field in a gap between magnetic poles, wherein said transducer has two such gaps side by side with a moving conductor in each gap, and wherein current is adapted to flow through a conductor in the first gap and then serially through a conductor in the second gap, so that the conductors move in the same direction.

Advantageously, the current is then caused to flow through a further conductor in the first gap, and then serially through a further conductor in the second gap; further conductors can be arranged in the respective gaps, with the current flowing serially through all the conductors for the same movement of the conductors.

It is especially advantageous if the total area of the conductors in each gap approximately equals the effective area of the gap, so that the conductors operate as a current sheet, or substantially so. This effect can be procured by providing a diaphragm assembly which comprises a thin sheet of insulating material which carries strip-like conductors on both of its surfaces, the conductors being arranged in two groups, the conductors of each group cooperating with the magnetic flux in one of the gaps; the conductors of each group, on both sides of the thin sheet, are arranged almost to overlap, or substantially so, which is possible because it is not necessary to leave an insulating gap between adjacent edges of the conductors, as would be necessary if all the conductors were on the same side of the sheet.

The invention also includes an improved magnetic assembly, for increasing the magnetic flux in the air gaps, and improving sensitivity and performance.

Features and advantages of the invention will appear from the following description of an embodiment thereof, given by way of example, and the accompanying drawings, in which: Figure 1 is a diagrammatic cross sectional view of a transducer, showing the major parts of the transducer; Figure 2 is a diagrammatic plan view showing one conductor of the transducer of Figure 1; Figure 3 is a diagrammatic plan view similar to Figure 2 but showing a second conductor of the transducer; Figure 4 is a diagram showing in plan view the disposition of the conductors of a transducer, such as those of Figures 2 and 3, in relation to the magnetic air gaps of the transducer;; Figure 5 is a diagram showing in cross section the disposition of the conductors of a transducer in relation to each other, to the diaphragm member and to the pole pieces of the magnetic air gaps, and Figure 6 is a graph showing the field strengths that can be obtained at different positions with respect to a medial position, using different magnet arrangements.

Figure 1 is a diagrammatic cross sectional view of a loudspeaker according to the invention, showing the essential parts of the magnet system and the other principal parts of the loudspeaker.

The main magnet system includes two permanent magnets 10 and 11, which are magnetised in the horizontal direction as seen in Figure 1, with similar poles, south poles as indicated, inward. A centre pole piece 12 of soft magnetic material is in contact with the inner adjacent faces of the two magnets, with its upper edge 1 4 projecting above the top surfaces of the magnets 10 and 11, and two outer pole pieces 1 5 and 1 6 of inverted Lshape contact the outer surfaces of the magnets; the upper surfaces of these pole pieces lie in the same horizontal plane.The pole pieces 12, 1 5 and 1 6 thus define two parallel air gaps 1 7 and 1 8 which are elongated and linear, with magnetic flux in the gaps mainly horizontal and in opposite directions; as shown, the centre pole 12 will be a south pole and the two outer poles 1 5 and 1 6 north poles, but the actual direction is not important except for the phasing of the loudspeaker.

Lying on top of the pole pieces is a diaphragm member 20, which consists of a thin sheet 21 of plastic insulating material, such as a polyimide, which is sandwiched between two thin sheets 22 and 23 of insulating material, such as impregnated fibrous material; sheets 22 and 23 are formed with registering central openings at 24 so as to leave exposed a central portion of sheet 21.Above the pole pieces 12, 1 5 and 1 6 is a further composite magnetic member 25 which in effect is in the form of a flat plate with parallel gaps 26 and 27 which register with and help to define the air gaps 17 and 18, and a portion 28 between the gaps 26 and 27 in register with the top surface of pole piece 1 2. By this construction the diaphragm sheet 21 is accurately positioned in the two air gaps 1 7 and 1 8. For a purpose further described hereinafter the magnetic assembly may include two further permanent magnets 29 and 30. The parts are secured together, for example by a rear housing 31 and a horn member 32, of rectangular cross section and of which plate 25 may form part, and bolts 33 which clamp the parts together.

That part of the diaphragm sheet 21 which is positioned in the air gap carries the conductors which are adapted to cooperate with the magnetic field, and an illustrative configuration of the conductors is shown in Figures 2 and 3, but for a simplified explanation of the operation it is convenient first to refer to Figures 4 and 5. Figure 4 is a diagram showing in plan view a configuration of conductors, and Figure 5 is a diagram showing in cross section the disposition of pole pieces and conductors. Figures 4 and 5 are not to scale, and are intended to be iliustrative only. In Figure 4 the pole pieces are indicated at 12, 1 5 and 16, and in Figure 5 the magnets are indicated at 10 and 11.The diaphragm member 21 is not shown in Figure 4, but is assumed to carry two conductors 34 and 35, of which the former is shown by the full line and the latter by the broken line. Conductor 34 is shown as having the form of a rectangular spiral of three turns, beginning at an outer terminal 36 and finishing at an inner terminal 37. Of this spiral three of straight sides lie in the air gap 1 7 between the pole pieces 1 5 and 12, and the other three sides lie in the air gap 1 8 between pole pieces 12 and 16.If a current is fed to terminals 36 and 37 the current will flow in one direction, considered in relation to the magnet assembly, in the first three sides and in the reverse direction in the other three sides, but because the magnetic field is in opposite directions in the two air gaps, as described above, the conductors, and with them the diaphragm sheet 21, will move in the same direction in response to the current.

The conductor 35 is similarly in the form of a three-turn spiral, but starting at terminal 37 and finishing at terminal 38, with three sides of the spiral lying in the gap between pole pieces 1 5 and 12 and the others in the gap between pole pieces 12 and 16. In like manner, a current passing from terminal 37 to terminal 38, or from terminal 36 to 38, will cause the diaphragm sheet 21 to move in the same direction as before.

In Figure 4 the conductors are shown as lines, for the purpose of illustration, but Figure 5 shows them more as they are in practice, consisting of flat, ribbon-like conductors. It will be seen from Figure 5 that, firstly, conductor 34 is disposed on the upper side of sheet 21 and conductor 35 on the lower side, terminal 37 making connection between the adjacent ends of the conductors, and, secondly, that in each gap the conductors on the upper and lower sides of member 21 are of a width such that their adjacent edges are close to each other, though separated by the thickness of member 21. As a result, the conductors in the two gaps approximate a current sheet, imparting a uniform driving force to the member over the area of the conductors and minimising undesirable modes of vibration of the member.

The number of turns in each spiral conductor, the width and thickness of the conductors and the lengths of the sides of the spirals can be chosen in accordance with the desired physical characteristics of the loudspeaker and its electrical impedance. In a practical case, a loudspeaker having an impedance of eight ohms was provided by a diaphragm member 21 which was polyimide sheet 10 um. thick; the two conductors, each of seven turns, had an effective length, that is, the length of each side immersed in the magnetic field, of 50 mm., a width of 0.270 mm.

and a thickness of 25 cm. The total length of conductor between terminals 36 and 38 was 1.4 metres. The impedance of the loudspeaker was substantially resistive up to frequencies in excess of 60 KHz. It will be understood that these figures are given by way of example only, and that other conductor lengths, thicknesses and widths, and numbers of turns can be adopted in accordance with the desired properties of resistance, power handling and frequency response.It is, however, desirable that in selecting the widths of the conductors and the numbers of turns in the spirals the conductors on the upper and lower surfaces of the diaphragm member 21 should together fully occupy the available width of the member in the air gaps, or substantially so, in order to achieve uniformity of drive to the diaphragm and the most efficient dissipation of heat, but lesser figures such as 90, 80 or even 70% can be tolerated in some cases. Overlap is permissible, but undesirable.

Some explanation is required of the function of the magnets 29 and 30, the use of which is optional, but which improve the performance of the loudspeaker.

Considering the one magnetic circuit including magnet 10, pole pieces 12 and 1 5 and air gap 17, the working magnetic flux in the air gap will be given by the relationship: L. H .T.W.

F G where: L is the magnetic path length H is the magnet magnetomotive force under working conditions T is the pole thickness W is the pole width G is the gap width.

The following constants have been determined experimentally: For a strontium ferrite magnet, the magnet can be approximated over the normal working range by a constant M.M.F. generator of 3880 Oersteds in series with an internal reluctance of 0.97/A, where A is the magnet area.

Thus, the flux F is determined not only by the mechanical dimension of the assembly but also by the working M.M.F. of the magnet, and this is a function of the total circuit reluctance, which in turn can be resolved in three component factors: (a) the reluctance of the gap (the working flux) (b) the reluctance of the external leakage due to the surface area of the magnets (c) the reluctance of the leakage flux from the poles adjacent the gap.

Of these factors, (a) is under the control of the designer, and is equal to G/WT, as defined above.

Factor (b) is a function of magnet area and is equal to L/(2 1 44/A+293.A2), and factor (c) is equal to 1/3.5W; it is independent of gap width and thickness.

This reluctance can also be expressed in terms of gap efficiency E, expressed as the ratio of the total flux carried by the centre pole piece to the gap working flux: E=T/(T+3.5G) This factor is particularly significant when the gap width is greater than the pole thickness; for example, if G is equal to 5 mm. and T is 2 mm., E is 2/(2+3.5x5)=0.1026, so that about 90% of the flux available is wasted.

The supplementary magnets 29 and 30 are used to reduce the leakage flux from the gap and improve efficiency and performance, and their function is primarily to produce a magnetic flux which is parallel to and in the same direction as the leakage flux, thereby to modify the field pattern to effect the stray flux is reduced and the working flux in the air gap is increased. With this object the two supplementary magnets are arranged respectively adjacent the pole pieces 25 and 26, with the direction of magnetisation generally vertical, as seen in Figure 1, and with poles opposing the poles of the main magnet, that is to say, with the pole piece 1 5 (or 16) having the same polarity as the adjacent face of magnet 29 (or 30).The magnets 29 and 30 can be of square or rectangular cross section, but it has been found advantageous to shape the magnets with the upper inner corners cut away, as shown in Figure 1, so that the lower pole faces are of larger area than the upper ones. The magnets are placed in close contact with the surfaces of the respective pole pieces 1 5 or 16 provided by plate 25.

The magnitude of improvement that can be obtained with the use of the supplementary magnets in a practical case is indicated in Figure 6, which is a graph showing flux density at positions above and below the central piane of the pole pieces 1 5 and 16, substantially where the diaphragm and conductors are located. In the loudspeaker to which Figure 6 pertains, the excursions of the diaphragm were only a very small fraction of a millimetre, so that the relevant part of the graph is that close to the 0 mm.

ordinate. In Figure 6, curve A relates to the loudspeaker without the supplementary magnets, curve B to the same loudspeaker with square section magnets and curve C to the loudspeaker with the shaped magnets. It will be seen that the use of the supplementary magnets results in an increase of flux density from about 3.7 Tesla to about 4.8 Tesla, an improvement of approximately 30%.

Claims

1. A transducer of the type comprising a generally linear conductor disposed for movement in a magnetic field in a gap between magnetic poles, wherein said transducer has two such gaps side by side with a moving conductor in each gap, and wherein current is adapted to flow through a conductor in the first gap and then serially through a conductor in the second gap, so that the conductors move in the same direction for a current flowing serially through them.

2. A transducer comprising a magnetc assembly arranged to produce magnetic flux in two approximately linear air gaps, side by side, and a diaphragm including a plurality of conductors disposed in each of said air gaps, said conductors being connected so that the diaphragm moves in the same direction for a current passing serially through the conductors.

3. A transducer according to either of the preceding claims, wherein said air gaps are arranged closely side by side and said conductors are provided respectively by the sides of a straight-sided spiral conductor.

4. A transducer according to claim 3, wherein said spiral conductor is supported on one surface of a diaphragm member, and comprising a further straight-sided spiral conductor on the other surface of said diaphragm member, the sides of the further spiral being also disposed respectively in said air gaps.

5. A transducer according to claim 4, wherein said spiral conductors are serially connected.

6. A transducer according to any of the preceding claims, wherein the conductors disposed in each gap have a total area approximately equal to the effective area of said air gap.

7. A transducer according to claim 6, wherein the conductors are thin strip-like conductors on opposite sides of a thin sheet of insulating material, with adjacent edges of the conductors on opposite sides of the sheet substantially meeting.

8. A transducer according to any of the preceding claims and including a magnetic assembly which presents on a surface thereof a centre pole and two outer poles defining two linear air gaps between them, and a diaphragm member extending across said surface and supporting said conductors in said air gaps.

9. A transducer according to claim 8, wherein the flux in said gaps is in opposite directions.

10. A transducer according to claim 2 and any of claims 3 to 9 as appendant thereto, and comprising further permanent magnetic means arranged to produce magnetic flux parallel to and in the same direction as the flux in said gaps, and in the vicinity of said gaps, whereby to increase the flux in said gaps.

11. A transducer according to claim 10, and comprising two permanent magnet means adjacent and parallel to said gaps.

12. A transducer according to claim 11, wherein said permanent magnetic means are permanent magnets each shaped to have a pole face of greater area in a region nearer the respective gaps and a pole face of lesser area in a region more distance from said gaps.

1 3. A transducer substantially as described with reference to the accompanying drawings.