GB1572023A

GB1572023A - Moving coil loudspeakers

Info

Publication number: GB1572023A
Application number: GB1913177A
Authority: GB
Original assignee: Tannoy Products Ltd
Current assignee: Tannoy Products Ltd
Priority date: 1977-05-06
Filing date: 1977-05-06
Publication date: 1980-07-23
Also published as: DK197578A; JPS53138724A; DE2819548A1

Description

(54) IMPROVEMENTS RELATING TO MOVING COIL LOUDSPEAKERS (71) We. TANNOY PRODUCTS Ll- MITED, a British Company, of St. John's Road. Tylers Green, High Wycombe, Buckinghamshire, HP 10 8HR do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relate to moving coil loudspeakers of the type including a pair of co-axial speech coils. one of which drives a cone for reproduction of the lower frequencies, while the other drives a high-frequency diaphragm at the rear of the loudspeaker.

The output of the diaphragm passes along a horn extending through the centre of the magnetic assembly and emerges through the centre of the cone. An example of such a construction is described and illustrated in British patent specification no: 893.838.

In order to allow for the passage of the horn through the centre of the magnetic assembly, it is necessary to use a ring magnet and, as described in the patent specification just mentioned, the air gaps for the two speech coils are arranged in series in a single magnetic circuit. so that onlv a single magnet is required to provide the magnetic flux for both air gaps. Despite the fact that such a ring magnet is not very efficient in that a relatively large proportion of the total magnetic flux bypasses the air gaps and is thus wasted, i.e. it constitutes stray flux, such a form of construction has hccome largely standardised over the years.

According to the present invention, separate ring magnets are provided to supply the magnetic flux to each respective air gap, the two magnets being arranged so that faces of similar polarity are adjacent one another. It is found that the mutual reaction of the two magnetic fields on one another modifies the configurations of the lines of force in such a way that a larger proportion of the magnetic flux passes around the magnetic circuit atld a correspondingly smaller proportion constitutes stray flux representing wastage of the power of the magnet. Expressed in terms of classical magnetic theory, the lines of force in the two fields may be regarded as repelling one another in an axial direction so that lines which would have emerged from the respective magnetic circuit and thus having constituted stray flux, are prevented from doing so and are instead constrained to pass around the ulagnetic circuit and thus contribute to the working flux ill the respec tive air gap.

Accordingly. although two separate ring magnets are required in place of one, the increase of efficiency more than compensates for the additional components required. In previous constructions using a single ring magnet, an annular end plate of high permeability material is required at each end of the magnet. The central ing in each plate may then conveniently define the outer edge of the respective 'iir gap. In a construction in accordance with the present invention. each ring magnet preferably forms part a separate magnetic assembly comprising a pair of annular plates of high permeability material with the magnet sandwiched between them. The opening in the outer plate of each assembly, i.e. the plates which are the equivalents of the end plates ill previous constructions. Oily then define the edge of the respective air gap in the same way as previously. By providing two separate magnetic assemblies in this way, these can be magnetised quite independently of one another prior to assembly and may be fixed in position separately from one another. Once assembled, the inner plates of the two assemblies lie immediately next to one another and although, in theory, a single central plate could perfoml the same function, the division of the construction into two separate magnetic assemblies is more convenient for the reasons alreadv described.

A construction of loudspeaker in accordance with the invention is illustrated in the accompanving drawing which is a longitudinal sectional view.

The basic structure of the loudspeaker is quite similar to that illustrated in British patent specification no: X93.3X referred to above and will thus be described onlv relativelv briefly. Thus a cone I is supported bv a surround 2 which is attached at its outer edge to a frame member 3 and the cone is driven bv a speech coil 4 working in an air gap between an annular plate 5 and a central cvlindrical member 6. Similarly. at the rear of the loudspeaker. a diaphragm 10 enclosed bv a cover 11 is driven by a speech coil 12 working in an air gap between an annular plate 13 and a cyclindrical central member 14. Sound generated by a diaphragm 10 passes along passages 15 in the member 16 and thence along a horn member 17 extending through the centre of the magnetic assembly. As so far described. the construction and its mode of operation is similar to the earlier construction referred to above.

In contrast with previous constructions. however. the air gaps for the speech coils 4 and 17 are supplied with magnetic flux by separate respective ring magnets 20 and 21 which have faces of similar polarity adjacent one another. In this particular example. the south polar faces are adjacent one another as indicated by S while the north polar faces N are directed outwardly. Each of the magnets 20 and 21 forms part of a separate magnetic assembly comprising respectively the annular plate 5. the magnet '() and an inner annular plate 23 and. in respect of the magnet 21 the magnet itself between the annular plate 13 and an inner annular plate 24. the plates 23 and 24 bering in close proximity to one another.

The assembly including the magnet 2(1 is held in position by clamps. one of which is seen at 25. while the assembly including the magnet 21 is held in position by longer bolts. one of which is seen at 29. Prior to assem bly. the two magnetic assemblies may be glued together and then can be magnetised quite independently of one another. After magnetisation the', are fitted in position. in succession. bv means of the clamps 25 and bolts 29.

The magnetic flux from the ring magnet 20 passes from the magnet into the plate 5, across the air gap for the speech coil 4 and thence into the member 6. It then passes in an axial direction along the member 6, into the plate 23 and thence back to the magnet 2(). Similarly. the tlux from the magnet 21 passes into the plate 13. across the air gap for the speech coil 12. through the member 14. through the plate 24 and thence back to the magnet. There are thus two independent magnetic circuits which react on one another so as to tend to compress the lines of magnetic force in an axial direction. This effect is most marked at the edges where strav flux occurs and in this region the effect of the compression of the lines of force is to constrain them to remain within each respective magnetic structure. In other words by reducing the proportion of stray flux. a larger proportion of the magnetic flux from each magnet is caused to pass around the magnetic circuit and thus to contribute to the working flux in the respective air gap. thereby increasing the overall efficiency.

This effect arises from the proximity of the similar polar surfaces of the two magnets as described previously. The improvements obtained is independent of the nature of the magnetic material used for the magnet. but particularly good results are obtained with anisotropic barium ferrite.

Claims

WHAT WE CLAIM IS:

1. A moving coil loudspeaker of the type including a pair of co-axial speech coils. one of which drives a cone for reproduction of the lower frequencies. while the other drives a high-frequency diaphragm at the rear of the loudspeaker and in which separate ring magnets are provided to supply the magnetic flux to each respective air gap. the two magnets being arranged so that faces of similar polaritv are adjacent one another.

2. A moving coil loudspeaker according to claim 1 in which each magnet forms part of a separate magnetic assembly comprising a pair of annular plates of high permeability material with the magnet sandwiched between them. the opening in the outer plate of each assembly defining the edge of the respective air gap.

3. A moving coil loudspeaker according to claim 2, in which each magnetic assembly is fixed in position separately from the other