WO1995024815A1

WO1995024815A1 - Electromagnetic loudspeaker

Info

Publication number: WO1995024815A1
Application number: PCT/GB1995/000424
Authority: WO
Inventors: Jonathan Neil Smith
Original assignee: Jonathan Neil Smith
Priority date: 1994-03-08
Filing date: 1995-02-28
Publication date: 1995-09-14
Also published as: GB9404585D0; AU1817695A

Abstract

With particular reference to Figure 6, an electromagnetic loudspeaker has an element (14, Figure 4) movable for sound generation, comprising an electromagnetically driven sheet of untensioned material. The movable element (14) is flat and is mounted by its edges in the lower portion of the frame (2) by a 'roll surround' (13, Figure 5). The movable element comprises a sheet (17) of COBEX polymer having thickness 0.5 mm. A conducting pattern (18) (Figure 4) and magnets (20, Figure 6) co-operate to provide electromagnetic means for driving the sheet (17). Spaces (12a and 12b) provide room for two rows of an array comprising four rigid rod members (21), said rod members (21) being spaced inwardly of the sheet (17) of its edges. Each rod member (21) is connected by one end to a mounting point on the sheet (17) (not shown), and by the other end, via an elastomeric washer (22, Figure 7) of neoprene located at the rear of the array of magnets (20), to a metal mounting ring (23) attached to the rear of the array of magnets (20). Twelve more rod members (21) shown are also spaced inwardly of the sheet (17) from its edges, and outwardly of the first-mentioned rod members (21) in spaces (12a and 12b). The sixteen rod members (21) of this example, in combination with the 'roll surround' (13), approximates to an all-over mounting for the sheet (17). The neoprene washers (22) serve as spring means and provide damping, and the 'roll surround' (13) serves as an additional spring means and provides further system damping.

Description

ELECTROMAGNETIC LOUDSPEAKER

This invention relates to an electromagnetic loudspeaker and particularly, but not exclusively, to a flat loudspeaker driven electromagnet!cally.

Various flat loudspeakers have been proposed in the past in which a movable element is provided comprising a sheet of flexible material mounted by its edges so as to be substantially flat.

The sheet usually bears a conducting pattern which, when provided with a current from the system amplifier, drives the sheet to generate sound by interaction with the magnetic field of an array of magnets located behind the sheet. Each magnet is typically of opposite polarity (N or S) to the magnets on either side adjacent thereto, and the magnets are spaced at various intervals in the prior art designs, for example in the order of half of the wavelength in air of sound to be produced or less in some designs, and in others much wider.

In these prior art devices the sheet is typically polymer film between 1 and 25 microns thick.

Any slackness in such a.film will give rise to serious sub-harmonic, non-linear motions at low frequencies. This is due to the varying spacing from the magnets resulting from the slackness and due to the effects of gravity or other factors. As the film is tightened, however, it starts to behave like the surface of a drum, with the edge constraints approaching the 'fixed' condition, such that acoustic modes across the surface dominate behaviour and the frequency response becomes very resonant.

Insufficient damping is inherent in the prior art designs, so that resonances in the film are not well controlled. Any motion ⁽flexural wave etc) which does start in the material 'rings on' for a while, giving a very poor transient response for the loudspeaker.

Standard theory also shows how the frequency at which bass roll-off occurs increases in such thin sheet devices as the mass of the driven element approaches zero and air loading effects become more important. A typical frequency at which bass roll-off becomes pronounced is 800 Hz.

Instead of mounting the film by its edges, mounting on a layer of foam interposed between the film and the magnets has also been tried. However, all the foams tried proved to be far too rigid, except for the highest frequencies. Foam is considered to be too rigid even for the much thicker sheet used as the driven element in the present invention, which has relatively high mass.

A very rigid element, in the form of a plate bearing the conducting pattern, is somewhat better than a thin film. However this still has drawbacks. One drawback is that the effectively 'free' edges implicit in a rigid element make the plate resonant. The high stiffness factor then makes it nearly impossible to get the damping up to an adequate level, so that any lateral vibration induced in the plate 'rings'.

These problems are further compounded by the fact that the array of magnets inevitably produces an alternating field which is not perfectly regular, and because the spacing between magnets and the plate will vary depending upon the orientation of the speaker. These factors cause the surface to be driven with locally varying forces as a current is supplied, ie shear forces exist. These then drive the flexural resonances to which the plate is susceptible.

According to one aspect of the present invention, an electromagnetic loudspeaker has an element movable for sound generation, the element comprising an electromagnetically driven sheet of untensioned material mounted by an array of mounting points attached to spring means, the mounting points being spaced inwardly of the sheet from its edges.

The mass of the driven sheet is preferably between 0.5 kg/m² and 2.0 kg/m².

Preferably the sheet is also mounted by its edges, and where the speaker is to be enclosed to operate as a monopole radiator, edge mounting is a requirement.

Where edge mounting is used, this preferably comprises a 'roll surround' similar to that used in conventional cone speakers to permit 'semi-free' motion and to increase 'system' damping so as to help to control surface resonance in the movable element. Such a roll surround is typically omega shaped in cross section, and is formed from a rubberised cloth which gives high damping.

The sheet is conveniently mounted so as to be substantially flat. However, for some applications it may be preferable to have an arrangement in which the sheet is curved in one or more dimensions.

The sheet is preferably electromagnetically driven by means comprising a conducting pattern provided on the sheet and co-operating with an array of magnets of corresponding pattern fixed behind the sheet adjacent magnets presenting opposite polarity (N or S), such that the sheet is driven by applying a suitable current to the conducting pattern.

The inventive mounting of the sheet by the array of mounting points, or by the array in combination with edge mounting of the sheet, can approximate to an ideal case of 'all over' mounting which the prior art has been unable to achieve. The more mounting points provided for a given sheet rigidity, the closer the approximation. The inventive mounting also allows the movable element to comply to a degree with local unevenness of drive force, without causing high amplitude resonances of the structure.

It would be possible for the inventive speaker to be constructed with a movable element comprising a strip of sheet material sufficiently narrow so that having the mounting points spaced from the edges would not be necessary for approximating to 'all over' mounting. Thus according to a second aspect of the invention an electromagnetic loudspeaker has an element of untensioned sheet material of strip form which is movable for sound generation, an arrangement of discrete mounting points being provided along the lengthwise edges of the element for mounting the element, and the mounting points being attached to spring means.

Preferably the magnets are spaced at less than , and most preferably less than 1/10 of the wavelength in air of the sound to be produced.

I have realised that a relatively thick, high mass polymer sheet is desirable for the movable element of the invention.

Although a sacrifice in terms of efficiency has to made for such a sheet, in comparison with the thin films of the prior art, which are tending towards zero mass, the bass roll-off is lowered towards a reasonable frequency.

A preferred movable element for the invention thus comprises polymer sheet material of thickness substantially in the range 0.2 to 2 mm.

However, it will be appreciated that the mass per unit area of the sheet and the spring stiffness and damping in the direction of movement are co-variant. They should preferably be scaled to achieve a 'Q' for the system substantially in the range 0.6 to 1.7, and most preferably substantially in the range 0.8 to 1.0. The overall spring strength for the spring means is prefe ly in the range 33000 to 130000 S^~2 for each kilogram of the movable element.

Desirably the spring means incorporates damping, and this also reduces transmission of sound back to the fixed structure of the speaker.

Preferably the spring means comprises elastomeric material havi'i high inherent da- ng. Desirably each of the mounting points 3 attached to the ing means by a rod member. The rod members are preferably intec ly formed with the movable sheet as a unitary moulding, and tlu ^ conveniently be hollow.

Preferably the spring means comprises a respective separate element of the elastomeric material for each rod member, which is conveniently in the form of a washer. Where the spec -er is to be mounted substantially vertically, each of the separate elements of elastomeric material is most preferably a washer of elliptical or similar shape, with the minor axis of the shape substantially vertical.

Instead of using the combination of rod members with elastomeric washers which act as the spring means, a spring means in the form of filaments or strips made into zig-zags, 'W', helix, coil, or other suitable shape could be used, and could be attached directly to the movable sheet. However, where the filament or strip is not made of a material which has suitable inherent damping, an additional damping means associated with said mounting points would be desirable.

The conducting pattern may comprise a single conducting wire or a number of single wires disposed in spaced-apart relationship, but in order to achieve a more desirable level of impedance for the speaker, a group or groups of two or more wires may be laid in place of a single wire arrangement, the wires w*" n any group being connected in series and laid next to each other, but with insulation between them. The wire or wires of the conducting pattern are preferably attached to the sheet by a resilient adhesive having damping properties to improve the damping further. In an alternative embodiment the conducting pattern comprises a 'printed circuit', and damping means broadly equivalent to the resilient adhesive in damping effect is preferably provided with this.

In a preferred embodiment the speaker is mounted in an enclosure. However, the speaker may desirably be arranged as a dipole radiator. Preferably a separate tweeter is provided and, if desired, a separate sub-woofer could be provided.

By way of example only a specific embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a perspective view of the loudspeaker of the embodiment;

Figure 2 is a front view showing the frame of the loudspeaker separately;

Figure 3 is a front view showing the mounting of the flat loudspeaker together with a tweeter;

Figure 4 is a rear view of the movable element of the flat loudspeaker;

Figure 5 is a section view of the movable element taken on line 5-5 of Figure 4, but omitting the conducting pattern;

Figure 6 s a view of the array of magnets of the embodiment from the front, that is to say, from the side facing the movable element; Figure 7 is a side view of an assembly comprising a mounting rod, neoprene washer and mounting ring; and

Figure 8 is a front view of the assembly of Figure 7

With reference to Figure 1, an electromagnetic loudspeaker has a timber plinth 1 with bevelled edges, and a frame 2, also of timber. A handle 3 is provided on the frame and may be used for carrying the loudspeaker.

In the embodiment the loudspeaker is a dipole radiator, having sound coming from both front surface 15 and rear surface 16. These surfaces are therefore provided with a covering of cloth 7 which is substantially permeable to sound. However, for use as a monopole radiator, the rear of the loudspeaker would typically be enclosed by a rigid panel .

With reference to Figure 2, the space within the frame 2 is partitioned into a upper and lower portion by a cross-bar 8.

Beneath the cloth 7 at the front 15, the loudspeaker has an element 14 movable for sound generation, comprising an electromagnetically driven sheet of untensioned material. The movable element 14 is flat and is mounted by its edges in the lower portion o" the frame 2 by a 'roll surround' 13, as shown in Figures 4 and 5, whilst a separate tweeter is mounted in the upper portion. The movable element 14 has a mass of 62 grams (0.675 Kg/m²). It comprises a sheet 17 of COBEX polymer having thickness 0.5 mm. Upon the rear surface of sheet 17 a conducting pattern 18 is adhered ιy a vinyl rubber adhesive proc, ced by 3M, whic^~ has suitable dampin properties. The conducting pattern is fosmed from a single copper wire 11 which is laid in a convoluted arrangement of parallel lengths which are directed alternately upwards and downwards on the rear surface of the sheet. The lengths are equi-spaced except for two larger spaces ^signated 12a. Leads 19 connect the ends of the wire to the system amplifier (not shown).

Behind the movable element 14 an array of permanent bar magnets 20 is mounted, as shown in Figure 6. Each magnet 20 comprises a ferrite strip (of about 3.0mm thickness) having opposite magnetic polarity to its nearest neighbour or neighbours. The opposite polarity is indicated in Figure 6 by the alternation of hatching angle between the adjacent magnets. The magnets are spaced so that each is disposed beneath a space between adjacent lengths of the wire 11, with the two wider spaces 12b between magnets being disposed beneath the wider spaces 12a of the conducting pattern 18.

The conducting pattern 18 and magnets 20 co-operate to provide electromagnetic means for driving the sheet 17.

The wider spaces 12a and 12b are to provide room for the middle two rows of an array comprising four rigid rod members 21, said rod members 21 being spaced inwardly of the sheet 17 of its edges. Each rod member 21 is connected by one end to a mounting point on the sheet 17 (not shown), and by the other end, via an elastomeric washer 22 of neoprene located at the rear of the array of magnets 20, to a metal mounting ring 23 attached to the rear of the array of magnets 20. Twelve more rod members 21 shown are also spaced inwardly of the sheet 17 from its edges, and outwardly of the first-mentioned rod members 21 in spaces 12a and 12b.

The sixteen rod members 21 of this example, in combination with the 'roll surround' 13, approximates to an all-over mounting for the sheet 17. The neoprene washers 22 serve as spring means and provide damping, and the 'roll surround' 13 serves as an additional spring means and provides further system damping.

The overall spring strength is approximately 67000 S^-2 for each kilogram of the movable element 14, and the spring/damping arrangement achieves exceptionally low distortion for the loudspeaker, which is generally below 0.1% (allowing for material vagaries) and sometimes below 0.01%.

Other properties relating to the movable element 14 of the embodiment are as follows:

stiffness of suspension in displacement direction: 180,000 N/metre to 720,000 N/metre per Kilogram driven sheet. This represents the sum of the stiffnesses for the individual mounts. For a 70g sheet the stiffness is substantially 40,000 N/metre.

damping in displacement direction: 35 kg/s

target Q: 0.85

Flexural properties of the movable element 14 of the embodiment perpendicular to displacement are as follows:-

elastic modulus (flexural): 2500 GPa

Fundamental free plate resonance 4.3 Hz

Q for flexural waves: 4 (target •= 1)

Where convenient, the flat loudspeaker is desirably considerably more elongated than in the embodiment described, in order to achieve improvements such as a better horizontal radiation pattern of sound. Thus, for example, a flat loudspeaker in accordance wif- the invention might be made about 1 metre high, 2 centimetres thi.._f, and 8 centimetres wide.

Instead of using ferrite bar magnets 20, an array of thin (say 1.0mm to 3.00mm) Neodymium Iron Boron magnets may be employed. Magnetic arrays may be placed on both sides of the sheet 17, or on only one side thereof.

The conducting pattern 18 may be replaced by wiring in parallel. Conducting patterns may be stacked in layers. Copper wiring may be replaced by wiring of aluminium, or another good conductor.

Claims

1. An electromagnetic loudspeaker having an element movable for sound generation, the element comprising an electromagnetically driven sheet of untensioned material mounted by an array of mounting points attached to spring means, the mounting points being spaced inwardly of the sheet from its edges.

2. A loudspeaker as claimed in claim 1, wherein the mass of the driven sheet is between 0.5 Kg/m² and 2.0 Kg/m².

3. A loudspeaker as claimed in claim 2, wherein the mass of the driven sheet is about 1.0 Kg/m².

4. A loudspeaker as claimed in claim 2, wherein the mass of the driven sheet is about 0.675 Kg/m².

5. A loudspeaker as claimed in any one of claims 1 to 4 wherein the sheet is also mounted by its edges.

6. A loudspeaker as claimed in claim 5 wherein the edge mounting comprises a roll surround.

7. A loudspeaker as claimed in claim 6 wherein the roll surround is omega-shaped in cross-section.

8. A loudspeaker as claimed in any one of claims 5 to 7 wherein the edge mounting is of damping material.

9. A loudspeaker as claimed in claim 8 wherein the edge mounting is formed from a rubberised cloth.

10. A loudspeaker as claimed in any one of claims 1 to 9 wherein the sheet is substantially flat.

11. A loudspeaker as claimed in any one of claims 1 to 10 wherein the sheet is curved in one or more dimensions.

12. A loudspeaker as claimed in any one of claims 1 to 11, wherein the sheet is electromagnetically driven by means comprising a conducting pattern provided on the sheet and a co-operating array of magnets of corresponding pattern fixed behind the sheet, adjacent magnets presenting opposite polarity such that the sheet is driven by applying electrical current to the conducting pattern.

13. A loudspeaker as claimed in claim 12 wherein the magnets are spaced at less than half of the wavelength in air of sound to be produced.

14. A loudspeaker as claimed in claim 13 wherein the magnets are spaced at less than 1/10 of said wavelength.

15. A loudspeaker as claimed in any one of claims 1 to 14, wherein the magnets comprise Neodym um Iron Boron magnets.

16. A loudspeaker as claimed in claim 15, wherein the Neodymium Iron Boron magnets are about 1.0mm to 3.0mm in thickness.

17. A loudspeaker as claimed in any one of claims 1 to 16 wherein the movable element comprises polymer sheet material of thickness substantially in the range of 0.2 to 2.0 mm.

18. A loudspeaker as claimed in any one of claims 1 to 17 wherein the mass per unit area of the sheet of material and the spring stiffness and damping in the direction of movement achieve a Q-value substantially in the range 0.6 to 1.7.

19. A loudspeaker as claimed in claim 18 wherein the Q-value is substantially in the range 0.8 to 1.0.

20. A loudspeaker as claimed in any one of claims 1 to 19 wherein the overall stiffness of the mounting points and spring means is in the range 180,000 Newtons/metre to 720,000 Newtons/metre for each kilogram of the movable element.

21. A loudspeaker as claimed in any one of claims 1 to 20 wherein the spring means incorporate damping.

22. A loudspeaker as claimed in any one of claims 1 to 21 wherein the spring means comprise elastomeric material.

23. A loudspeaker as claimed in claim 22 wherein the elastomeric material comprises Neoprene.

24. A loudspeaker as claimed in claim 22 or 23 wherein the spring means comprise a plurality of annular members.

25. A loudspeaker as claimed in claim 24 wherein the annular members are of elliptical shape.

26. A loudspeaker as claimed in any one of claims 1 to 24 wherein each of the mounting points is attached to the spring means by means of a rod member.

27. A loudspeaker as claimed in claim 26 wherein the rod members are integrally formed with the movable sheet as a unitary moulding.

28. A loudspeaker as claimed in claim 26 or 27 wherein the rod members are hollow.

29. A loudspeaker as claimed in any one of claims 1 to 28 wherein the spring means are of filamentary or strip form.

30. A loudspeaker as claimed in claim 29 wherein the spring means are made of material having inherent damping.

31. A loudspeaker as claimed in any one of claims 12 to 30 wherein the conducting pattern comprises a single conducting wire.

32. A loudspeaker as claimed in any one of claims 12 to 30 wherein the conducting pattern comprises a number of single conducting wires disposed in spaced-apart relationship.

33. A loudspeaker as claimed in any one of claims 12 to 30 wherein the conducting pattern comprises a group or groups of two or more conducting wires, the wires within any group being connected in series, and laid next to each other, with insulation between them.

34. A loudspeaker as claimed in any one of claims 32 to 33 wherein the wire or wires of the conducting pattern are attached to the sheet by a resilient adhesive having damping properties.

35. A loudspeaker as claimed in any one of claims 12 to 30 wherein the conducting pattern comprises a printed circuit.

36. A loudspeaker as claimed in any one of claims 1 to 35 mounted in an enclosure.

37. A loudspeaker as claimed in any one of claims 1 to 35 arranged as a dipole radiator.

38. An electromagnetic loudspeaker, substantially as hereinbefore described, with reference to the accompanying drawings.