GB2489535A

GB2489535A - Electromagnetic drive arrangement for a loudspeaker with a planar, flexible diaphragm

Info

Publication number: GB2489535A
Application number: GB1109335.8A
Authority: GB
Inventors: Michael Peter Cooke
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-03-26
Filing date: 2011-06-03
Publication date: 2012-10-03
Anticipated expiration: 2031-06-03
Also published as: GB2489535B; GB201105139D0; GB201109335D0

Abstract

The drive means comprises a linear conductor 1a borne on the diaphragm 2a and a magnet arrangement 3 mounted in proximity to the linear conductor and arranged to provide a force that is substantially perpendicular to the surface of the diaphragm, substantially proportional to a current within the linear conductor and arranged to provide a substantially constant force over the full range of the diaphragm movement. The magnet assembly is arranged such that north and south poles define a magnetic axis that lies in a plane parallel to the plane of the diaphragm. The linear conductor is arranged such that it extends beyond the end faces of the magnetic poles (fig. 9 for example).

Description

Loudspeaker

Technical Field of the Invention

The present invention relates to a loudspeaker. In particular, the present invention relates to loudspeakers that utilise a planar, flexible diaphragm.

Background

Ideally a High Fidelity loudspeaker should cover the full audio range from 20Hz (or lower) to 20KHz (or higher) without audible resonances or distortion. For good stereo imaging the point the sound comes from should not appear to move with frequency. A fundamental problem with achieving this is that to give an even distribution of sound into the listening environment, the physical width of the radiating surface must be small at high frequencies, but in order to move enough air to reproduce bass frequencies at reasonable volumes it must be much larger.

There have been many approaches to this problem, but they mainly fall into two groups.

One approach is to have two or more drive units each sized for a smaller range of frequencies and an electrical cross-over to switch between them at some point in the frequency range. A problem with this approach is that for a significant proportion of the audio range, you have more than one driver producing sound and that a driver working at the top of its range has a different sound dispersion pattern from one at the bottom of its range. High-order crossovers and multiple drive units are often used to ameliorate this problem, but this results in an expensive solution, phase anomalies and a (difficult to drive) reactive load for the amplifier. Unless the effective acoustic centres of the drivers are coincident in all three axes, there will also be blurring of the stereo image. Another approach is to have a driver with a flexible radiating surface and to drive it over a small area. At low frequencies the whole surface will move with tmpistonic motion". At high frequencies only the area near to the driving point will move significantly. The problem with this is that the switch between these modes happens in the audible range and that the driver will inevitably have a number of audible resonances. Various techniques have been applied to even out these resonances such as those used in NXT type speakers (e.g. EP1055351, EP1068770), but a smooth response and even sound dispersion over the full audio frequency range is still not achievable.

In order to avoid the problems of either needing multiple drive units or having to handle the transition out of pistonic motion, inventors such as Lane (US1773910) and Burton (U84924504) have proposed designs where a line region on a flexible diaphragm is driven so that the diaphragm never moves as a piston. A difficulty to overcome with such designs is that in order to have good high frequency response the driven area must have low mass relative to its radiating area, but in order to generate sufficient air movement at low frequencies It must have a large excursion. To avoid generating harmonic distortion, all the forces on the diaphragm must be substantially constant or linear over the full range of this excursion. A constant electromagnetic force for a given input current is easily achieved with conductors moving through a narrow magnetic gap (e.g. US4924504 figure 11), but the conductors and magnetic gap must overlap throughout the full excursion for the force to be constant. The mass of conductors and support needed to achieve large enough low frequency excursions is higher than desirable for good high frequency response. An additional problem is that for a line source, the relatively stiff structure will have structural resonances along its length within the audio range. Also it can't drive the full height of the diaphragm as its stiffness would restrict movement if it came too near to the support frame.

To avoid excessive mass, the conductors may be applied directly to the diaphragm as shown in US4924504 figure 10. In this design however, the force in the conductors increases substantially as the conductors get closer to the magnets. Whilst in U84924504 it is stated that this is an advantage as it scombats the Mylar diaphragm's dynamic compliance", in practice for good low frequency response the tension in the diaphragm must be too low for the increase in tension force to match that of the magnets. A further problem is that the magnets are substantially in the way of sound radiated directly from the conductors, so frequency response and sound dispersion anomalies will be caused at the highest frequencies.

The object of the present invention is to provide a full range line source loudspeaker with improvements implemented to overcome or substantially mitigate the frequency response and distortion problems described above.

Statements of Invention

According to a first aspect of the present invention there is provided a loudspeaker comprising: at least one substantially planar, flexible diaphragm; drive means for driving the diaphragm, the drive means comprising conductors operably connected to the diaphragm and at least one magnet arrangement mounted in proximity to the linear conductors, with the North and South magnetic poles of the magnet arrangement substantially parallel to the diaphragm. The magnet arrangement and conductors are arranged to provide a net force that is substantially perpendicular to the surface of the diaphragm, substantially proportional to a current within the linear conductors and substantially constant over the full range of movement of the diaphragm.

Further preferred features of the invention relate to variants of the magnet arrangement, conductors and diaphragm(s).

Description ofthe invention.

Figure 1 shows a loudspeaker according to the invention in its most basic form.

Figure 2 shows a horizontal sectional view cut through the loudspeaker of figure 1.

Figures 3 and 4 show sectional views of alternative embodiments to the basic loudspeaker.

Figures 5 and 6 show arrangements of conductors and magnets used in the prior art.

Figure 7 shows schematically the flux lines for the magnet arrangement used in the loudspeaker of figures 2 and 4.

Figure 8 shows schematically the flux lines for the magnet arrangement used in the loudspeaker of figure 3.

Figures 9 to 15 show alternative arrarig'ements of magnets and conductors and a method for calculating the optimum positionin of magnet and conductors for each arrangement.

Figures 16 and 17 show further options of magnet arrangement in a plane perpendicular to that of figures 2 to 15.

Referring to figures 1 and 2. the audio signal generates a current in ribbon-like conductors (Ia) fixed to a diaphragm. A bar of magnets (3) is mounted adjacent to the conductor (Ia) to generate a magnetic field that intersects the conductors so as to provide a force substantially perpendicular to the diaphragm surface, substantially proportional to the current in the conductors (1 a). The diaphragm (2a) is tensioned such that a sound wave originating from the conductor (Ia) travels through the diaphragm at a speed much lower than that of sound in air (typically less than l00° of it). What this means is that a wave travelling through the diaphragm is largely inaudible at any significant distance from the diaphragm as the positive and negative parts of the signal are physically so close together on the diaphragm that when radiated into air they substantially cancel out. Only the original audio signal generated at the conductor is therefore heard. If we imagine either the positive or negative portion of an audio signal travelling through the diaphragm however, the distance it gets before being cancelled out by the opposite polarity increases as the frequency decreases, so a larger area of the diaphragm becomes active, the lower the frequency. If the width "\I'J" of the diaphragm is selected such that for the speed of sound through the diaphragm, half a wave of a (low) frequency to be produced will fit within it, then the full diaphragm area becomes active at this frequency. Typically a polymer diaphragm of the order of 1 3microns thick can easily be tensioned to become fully active at about 20Hz within a width of the order of 0.5-1 metres.

In order to prevent waves reflecting off the frame (4) and causing resonances, a damping arrangement such as an air cavity (5) at the edge of the diaphragm lined with an open cell acoustic foam (6) may be used. In the basic design of figure 2, the flux density decreases substantially as the diaphragm moves away from the magnets (3). This makes it difficult to achieve a constant force in the conductor without sacrificing efficiency. As shown in figure 3, this effect can be reduced by adding another bar of magnets (10) on the opposite side of the diaphragm (2a).

To improve the force and linearity of large diaphragm excursions without requiring a second set of magnets, figure 4 shows that a second diaphragm (2b) and conductors (Ib) may be added. As one set of conductors (Ia) gets closer to the magnet, the other set (Ib) gets further away. One drawback of this approach is that the diaphragms must be physically separated by a distance of the order of 20-30mm. What this means is that at around 5-8KHz a wave through the air from one diaphragm would at least partially cancel out that from the other, In order to avoid this problem, acoustic foam (11) or any other suitable sound absorbing material may be added adjacent to the magnets (3) to damp out or block the highest frequencies travelling between the two diaphragms (2a12b).

Alternatively or additionally, a capacitor connected in parallel with and/or an inductor connected in series with the conductors (1 b) facing away from the listener may be used to attenuate the high frequency electrical signal to it. As the signal to the conductors (Ia) facing the listener is always present, the problems normally inherent in a cross-over are much reduced. A further technique which may be used instead of, or in addition to an electrical attenuation circuit is to increase the mass and/or damping of (or associated with) the rear conductors (1 b). This may be done for example by increasing the thickness of the conductor and/or by adding a layer of non-conductive material. It may seem that removing the sound from one of the diaphragms will result in a reduced sound output at high frequencies, but in practice it has been found that there naturally tends to be a rise in output of each diaphragm at these sorts of frequencies, so the resulting frequency response is reasonably flat. If an electrical attenuator with a capacitor in parallel to the rear conductors (1 b) is used, it is also possible to boost the current to the front conductors (Ia) to increase high frequency output at the expense of lower electrical impedance at high frequencies.

The direction of magnetic force in a conductor is in a direction perpendicular to both the current and the magnetic flux. This means that to move a diaphragm back and forth with a current in a vertical conductor, the magnetic flux must be travelling largely horizontally in the plane of the diaphragm at least where the conductor is, In a ribbon loudspeaker, a horseshoe shaped magnetic path (12) as shown in figure 5 is used to provide magnetic flux lines (ISa) in the correct direction, but this is only practical for a relatively narrow gap or the magnets become massive, so it is not practical to fit a wide diaphragm between the poles of such a magnet. In uquasi..ribbonn or uorthodynamicll drivers, the magnets are placed as shown in figure 6. The magnet poles face the diaphragm and are either side of the conductor, giving flux lines (13b) which are curved, but approximately in the plane of the diaphragm (2a) near the conductor. The magnets alternate with those on one side of a conductor (Ia) having North poles facing the diaphragm and the ones on the other side having South poles facing the diaphragm. A perforated steel magnetic return path (14) is provided to complete the magnetic circuit. This arrangement is suitable for loudspeakers where most of the diaphragm area is directly driven, but when it is desired to drive a narrow region of the diaphragm, the problem is that the width of the magnet arrangement is relatively large and will block and/or refract high frequencies.

With the advent of high flux rare earth magnet materials such as Neodymium Iron Boron, it is possible to mount a relatively narrow magnet in front of or behind the diaphragm as shown in figure 7, to give flux lines (1 3c) through the conductor without the need for a separate magnetic return path. Sound is able to travel around such a narrow structure with relatively little diffraction. A problem with such an arrangement however is that the flux lines (13c) concentrate as the diaphragm gets nearer to the magnet which would increase the force on a conductor in this region for a given current. In order to maximise the amount of flux interacting with the conductors when they are distant from the magnets, it is advantageous for the conductors to go beyond the pole faces of the magnets. In order to counteract the increase in force when the conductors approach the magnets, the conductor can have a gap (15) adjacent to the highest flux region which is generally located midway between the pole faces of the magnet. This means that as the gap (15) approaches the magnet, the flux within this gap does not generate force, with this ulost force increasing as the gap (15) gets closer to the magnet. In order to improve the net force and linearity, a second diaphragm 2b and associated conductors may be disposed on the opposite side of the magnet. Alternatively, magnets can be disposed on both sides of the diaphragm (2a) as shown in figure 8. A particular advantage of this arrangement being that the flux lines (13d) are relatively strght between the magnets, so bending forces on the conductors are minimised.

Figure 9 shows a method to calculate the optimum positioning of the conductors for an arrangement similar to figure 7. The left hand graph shows the integral of flux density in the direction of the plane of the diaphragm with distance from the centre line of the magnet (i.e. the slope of the line is proportional to flux density in the direction of the plane of the diaphragm). As the force on a conductor is proportional to flux density multiplied by current, the slope of the graph also represents the local umagnetic pressur& perpendicular to the diaphragm on a conductor carrying a given current per unit width. The (thick) top line (1 7) is for when the diaphragm is closest to the magnet and the (thin) bottom line (18) is for when the diaphragm is furthest from the magnet. The dotted lines in between these two lines represent intermediate positions of the diaphragm. If a conductor extends from an inner boundary (19) to an outer boundary (20), the net force on the conductor perpendicular to the diaphragm is proportional to the difference in the height of this line between the two boundaries. By optimal positioning of the outer boundary (20) of the conductor outside of the magnetic pole face (21) and the inner boundary (19) of the conductor away from the centre line (16) of the magnet, it can be seen that Fl when the diaphragm is nearest the magnet and F2 when the diaphragm is furthest from the magnet can be made substantially the same and thus harmonic distortion of the sound produced will be minimised. In order to achieve such linearity with a single magnet and diaphragm however it can be seen that the outer boundary (20) of the conductor must be relatively wide and the force (El, F2) is much less than the highest possible force (F3) that could be achieved. The graph on the right shows the effect of adding a second diaphragm and conductors on the opposite side of the magnet. The thick line (22) is the sum of the top and bottom lines (17, 18) on the left hand graph (i.e. when one diaphragm is closest to the magnets and one is furthest away) and the dotted lines represent intermediate positions.

A similar, but slightly different graph would be achieved for the arrangement of figure 8 where an additional magnet is added on the opposite side of the diaphragm. We can now see that the lines are much closer together and therefore the outer boundary (23) of the conductor so that F4 is equal to F5 can be much closer to the axis. This is advantageous in that the closer this outer boundary is to the axis, the better the dispersion of the highest frequency sounds will be. Also as F4 and F5 are much larger than Fl and F2, the loudspeaker will generate a louder sound for a given current input and therefore be more efficient. If it is desired for the conductor to cross the centre line (16), it can be seen that within a similar width as the left hand graph, it is possible to achieve a close to constant force (F6).

If a very narrow conductor is required in order to disperse the very highest frequencies adequately, a low or zero flux density zone (24) can be created on the centre line near the magnets, by providing two magnets with their poles facing in the same direction, but with a gap (24) between them as shown in figure 10. The gap (24) causes the flux lines (13e) near the magnets to follow a short path from one side of each magnet to the other whilst the flux lines (131) further from the magnet follow a path through both magnets. The effect of this is that the boundary (25) of the conductor for which a substantially constant force (Fl) is developed can be much closer to the centre line. Also, without using a second diaphragm and conductors an approximately constant force (F8) can be generated over a limited range of movement.

A disadvantage with the arrangement shown in figure 10 is that the force achieved for a given current per unit width in the conductor is reduced compared to the arrangement of figure 9. As shown in figure ii, some of this disadvantage can be reduced by providing a bridge (26) between the magnets from a ferromagnetic material (e.g. mild steel). The forces (F9 and FiG) achieved are improved relative to those (F7 & F8) on figure 10. The ferromagnetic bridge also has a practical advantage in that the magnets naturally want to stick to it, so it is a good way to support the magnets.

Figure 12 shows that instead of a ferromagnetic bridge, a step can be formed in the magnets to create a gap (27) only near the conductors. The force (Fl 1) achievable is increased relative to the arrangements of figures 10 and 11. An additional advantage is that the force is relatively linear all the way from the conductor boundary (28) to the centre line (16), which means that the conductor may be even narrower if desired.

In the magnet arrangements shown in figures 7 to 12, the current in all conductors must be in the same direction (i.e. either all in to the page or all out of it in the orientation shown). Usually the input connectors to the loudspeaker will be at the bottom of the loudspeaker, so one or more return current paths are needed, which may conveniently be through one or both of the magnet bars, or a conductor mounted to them.

In order for a loudspeaker to have an electrical impedance in the 4-8 ohm range (which most amplifiers are nominally designed for) whilst having thick enough conductors to be robust, it is advantageous to have several conductors in series. To avoid needing to make multiple return connections, it is convenient to have the return conductors on the diaphragm and preferably generating additional force on it. As shown in figure 13, to achieve this, additional magnets in a mirrored configuration about the centre line (16) may be added such than one set of conductors (Ic, id) can have current in the opposite direction to the other set (le, if). These can be put onto the diaphragm in either the form of a rectangular spiral (29) or two U shapes (30, 31). It can be seen that both the inner conductors (Ic, le) and outer ones (id, If) can be positioned such the total force (F13+F14) generated is constant over the full range of diaphragm movement. It can also be seen that forces (F13, F14) in the individual conductors can also be substantially constant throughout the diaphragm movement. This allows for the possibility to only drive the inner conductors at the highest frequencies to achieve improved dispersion of high frequency sounds. Also illustrated in this figure is that ferromagnetic bridge pieces (32, 33) can also be stepped back from the diaphragm (i.e. a combination of the solutions of figures 11 and 12).

Figure 14 illustrates that if the outer magnets (34, 35) are made thicker than the inner ones, the flux density at a distance from the magnets can be increased without much effect on that close to the magnets, reducing the gap that is necessary between conductors to get a constant force over the full diaphragm movement. The forces (F15, FIB) on the conductors are also increased compared to those (F13, F14) in figure 13.

In figure 15, it can be seen that if the outer magnets (36, 37) are thick enough it is possible to achieve a substantially constant force (F17) on the conductor without the need of a gap in the conductor adjacent to the inner magnets.

Whilst figures 9 to 15 show magnets arrangements with constant cross-section, low flux regions close to the magnets may also be created by spacing of the magnets along the lengths of the conductors. For example figure 16 illustrates an arrangement of magnets (38, 39) which will achieve a similar effect to those in figure 12. Figure 17 shows an arrangement of magnets (40, 41) and ferromagnetic bridge pieces (42) which will achieve a similar overall effect to those of figure 14. When mirrored magnet arrangements are used, the magnets either side of the centre line tend to repel each other, so tension elements (43) such as rods, or screws may be placed at intervals along the ferromagnetic bridge piece to minimise bending of the magnets and ferromagnetic bridge pieces.

Claims

SCLAiMS 1. A loudspeaker comprising: a substantially planar, flexible diaphragm drive means for driving the diaphragm, the drive means comprising a linear conductor operably connected to the diaphragm and a magnet arrangement mounted in proximity to the linear conductor and arranged to provide a force that is substantially perpendicular to the surface of the diaphragm and substantially proportional to a current within the linear conductor, the magnet arrangement comprising a north pole having an end face and a south pole having an end face, the north and south poles defining a magnetic axis wherein the magnetic axis is substantially parallel to the plane of the diaphragm, and the linear conductor intersects at least one of: a plane that passes through the end face of the north pole, and; a plane that passes through the end face of the south pole, the planes being substantially perpendicular to the magnetic axis.
2. A loudspeaker as claimed in Claim 1, wherein the north pole end face and south pole end face are substantially perpendicular to the diaphragm.
3. A loudspeaker as claimed in Claim I or 2, wherein the loudspeaker comprises a plurality of linear conductors and at least one conductor of the plurality of conductors intersects at least one of a plane that passes through the end face of the south pole and a plane that passes through the end face of the north pole, the planes being substantially perpendicular to the magnetic axis.
4. A loudspeaker as claimed in any preceding claim, wherein the or the plurality of conductors are parallel to the diaphragm and are generafly perpendicular to the magnetic axis.
5. A loudspeaker as claimed in any preceding claim, wherein two or more conductors are located between the planes that pass through the end face of the magnet arrangement.
6. A loudspeaker as claimed in any preceding claim, wherein the loudspeaker comprises a plurality of magnetic arrangements.
7. A loudspeaker as claimed in Claim 6, wherein two or more conductors are located between the planes that pass through the end face of at least one magnet arrangement
8. A loudspeaker as claimed in Claim 6 or 7, wherein magnetic arrangements are spaced apart by a ferromagnetic bridge.
9. A loudspeaker as claimed in Claim 8, wherein the ferromagnetic bridge is formed from mild steel.
10. A loudspeaker as claimed in any preceding claim, wherein the diaphragm defines a centre line that runs within the plane of the diaphragm and perpendicular to the magnetic axis
11. A loudspeaker as claimed in Claim 10, wherein the loudspeaker comprises at least two pairs of magnetic arrangements, each pair of magnetic arrangements being spaced apart by a ferromagnetic bridge and wherein, for each pair of magnetic arrangements, the magnetic arrangement that is further from the centre line is larger than the magnetic arrangement that is closer to the centre line.
12. A loudspeaker as claimed in any preceding claim, wherein the loudspeaker comprises a plurality of magnetic arrangements arranged in a mirrored configuration about the centre line.
13. A loudspeaker as claimed in Claim 6, wherein magnetic arrangements are disposed either side of the diaphragm.
14. A loudspeaker as claimed in any preceding claim, further comprising more than one diaphragm, the diaphragms being disposed either side of the magnetic arrangement.
15. A loudspeaker as claimed in any preceding claim, wherein the magnetic arrangement comprises a north pole region comprising the north pole and a south pole region comprising the south pole and a waist region between the north and south pole regions, the waist region having a smaller cross sectional area than the cross sectional areas of the north and south pole regions.
16. A loudspeaker as claimed in any preceding claim, wherein the or the plurality of conductors are parallel to the diaphragm and are generally perpendicular to the magnetic axis and wherein the loudspeaker comprises a plurality of magnet arrangements, the magnet arrangements being spaced along the length of each conductor such that neighbouring magnet arrangements are separated by a small gap.
17. A loudspeaker as claimed in any preceding claim, further comprising damping means arranged around the periphery of the diaphragm to damp waves arriving at the periphery from the drive means wherein the diaphragm is tensioned such that the speed of sound within the diaphragm is lower than the speed of sound in air.