GB2108355A - Electro-acoustic transducer - Google Patents

Abstract

An electro-acoustic transducer is constructed by assembling together a drive unit 1 comprising a permanent magnet 2, a chassis part 4 and suspensions 14, 16 suspending a coil former 6 with the coil 10 in operative relationship with the magnet, and a diaphragm unit 20, in which a diaphragm is received within a second chassis part 21. The diaphragm may carry a second coil 32 and magnet 36, on the other side thereof from the former 6, for damping the diaphragm or for applying d.c. bias. The diaphragm may be partially filled with foamed plastics material (fig 3 - not shown). <IMAGE>

Description

SPECIFICATION Electro-acoustic transducer The invention relates to an electro-acoustic transducer.

A conventional moving coil electro-acoustic transducer comprises a diaphragm movably suspended by a first suspension in a chassis, a former carrying voice coil secured to the diaphragm and suspended in the chassis by a second suspension for reception in an annular recess in a magnet fixed to the chassis.

For use of the transducer as a loudspeaker, an electric signal is supplied to the voice coil to effect movement of the diaphragm.

The clearance between the moving voice coil and the magnet is desirable as small as possible to make effective use of the available magnetic field and there is consequently a danger of contact between the coil or its former and the magnet, due for example to unbalancing ofthe diaphragm by dust accumulated thereon, which would result in serious distortion of the sound output.

The invention therefore provides that the voice coil former is suspended at two positions spaced axially therealong, so as to be at all times accurately centred within the magnet. The transducer thus comprises three suspensions instead of the usual two, the additional suspension being associated with the voice coil former.

Loudspeaker diaphragms of different diameters can be driven by drive units comprising the magnet and voice coil of identical construction, and to facilitate loudspeaker manufacture, the invention also provides a drive unit comprising a magnet and suspension means mounting a voice coil former in a recess therein, the drive unit being connectable in a predetermined relationship with a diaphragm unit comprising a chassis and a diaphragm suspended therein, to thereby form a complete loudspeaker.

The loudspeaker so formed may be conventional except for the modifications necessitated by and resulting from the connection between the diaphragm and the voice coil former and between the chassis and the magnet, but preferably, in accordance with the invention, the voice coil former is suspended in the drive unit with the coil in working relationship with the magnet so that the unit can be tested as a whole before assembly with the diaphragm unit. The suspension means can then advantageously comprise for example two axially spaced suspensions as described above, the suspen sions acting between the voice coil former and the magnet or a chassis part secured thereto.

A selection of connectable diaphragm and drive units of different sizes and/or characteristics can then be manufactured and stored, preferably after testing when the facility for driving this is provided, and loudspeakers can then be readily made up by combining various units as described.

It is desirable for various reasons to provide for modification of the diaphragm movement in an electro-acoustic transducer, and this can conveniently be done by provision of an auxiliary coil on the diaphragm. According to the present invention therefore, a moving coil electro-acoustic transducer is provided with an auxiliary coil and a co-operating magnet on the opposite side of the diaphragm from the voice coil.

The diaphragm is conventionally frusto-conical, with the voice coil former extending from the smaller diameter end; the auxiliary coil can be provided on an extension of the voice coil former into the interior of the diaphragm, and the auxiliary magnet can be suspended on supports extending from the rim of the chassis at the larger diameter end of the diaphragm. Alternatively, the diaphragm can be rendered rigid by foamed plastics material therein and the auxiliary coil former can be partially embedded in the foamed material. A conventional transducer can then be readily modified to incorporate the auxiliary coil feature of the invention, as can the transducers of the invention previously described.

The invention is further described below, by way of example, with reference to the accompanying drawing, Figs. 1,2,3 and 4 of which are schematic sectional side views respectively of a drive unit, a first diaphragm unit, a second diaphragm unit, and a third diaphragm unit, for use in an electro-acoustic transducer in accordance with the invention. In Figs.

2,3 and 4, like parts, or parts with a like function are indicated by the same reference numerals.

The drive unit 1 illustrated in Fig. 1 is for forming part of an electro-acoustic transducer intended for use as a loudspeaker. The unit 1 comprises a permanent magnet 2 secured to a chassis part in the form of a sleeve 4 fitting over one end of the magnet 2 and extending from the magnet to an outwardly extending flange 5 at its free end. A hollow cylindrical former 6 with a domed closed end 8 carries a voice coil 10 received in an annular recess 12 in the magnet. The former 6 is guided for accurately axial movement by a pair of axially spaced like annular suspensions 14, 16 extending between the former and the sleeve 4. The former 6 is thus held in a working relationship with the magnet 2, in that it is capable of axial movement relative to the magnet in response to a current in the coil 10.The two spaced suspensions 14, 16 ensure that the movement is reliably axial, whether the unit is being tested on its own or in use in combination with a diaphragm such as is shown in Fig. 2, Fig. 3 or Fig. 4.

The diaphragm unit 20 of Fig. 2 comprises a chassis part 21 having a plurality of spaced ribs 22 extending between an inner ring 24, the free face of which generally corresponds in its inner and outer diameters to the free face of the flange 5, and an outer ring 25. A diaphragm received within the unit 20 has a frusto-conical portion 26 and a domed portion 28 closing its smaller diameter end. The domed portion 28 corresponds in shape to the domed closed end 8 of the former 6. The diaphragm portion 26 is suspended in the chassis part 21 by an The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy. annular suspension 30 between the larger diameter end of the portion 26 and the ring 25.

Extending from the diaphragm portion 26, at its join with the portion 28, and in the direction towards the ring 25, there is provided a tubular former 29 which is shown as, but need not be, of the same diameter as the former 6 of Fig. 1. The former 29 extending into the frusto-conical portion 26 carries an auxiliary coil 32 received within an annular recess 34 in a permanent magnet 36 suspended within the portion 26 by a ring 38 connected by two or more spaced ribs 35 extending between the ring 38 and an auxiliary ring 39 secured to the outer face of the ring 25.

The magnet 36 although shown as smaller than the magnet 2 could be of equal size if preferred, and could be held in place by ribs of any convenient shape and number. The structure shown can be applied to the modification of a conventional transducer which could otherwise have the former 29 integral with a conventional position former.

The units 1 and 20 can be separately manufactured with different sizes and/or characteristics, only the diameters of the rings 24 and the flanges 5 being kept the same. A loudspeaker can then be obtained by assembling the units together as by fasteners extending through aligned holes in the ring 24 and the flange 5 are indicated at 40.

In use, the voice coil 10 will be connected to the output of an amplifier carrying a main signal to be converted to sound. The coil 32 can also be con nected in this way but can instead be fed with an a.c.

signal, which may or may not be devised from the main signal, and which may be in or out of phase with the main signal, to control the frequency response of the transducer. Instead, a d.c. signal can be fed to the coil 32 to apply a steady bias to the diaphragm assembly.

The coil 32 is however preferably used as a magnetic brake to control the damping of the diaphragm assembly. The damping can be selectively adjustable by way of a circuit component remote from but electrically connected to the coil 32, or the coil 32 can be connected to one or more circuit components carried by the magnet 36, which may be presettable to a desired value.

Although reference has been made to coils 10,32 as single coils, it will be evident that either may comprise two or more separate coils sach to be connected in one of the ways described above.

The modified diaphragm unit 50 of Fig. 3 has the diaphragm portions 26 and 28 partially filled with foamed plastics material 51 to provide a rigid diaphragm with a sound radiating surface 52 which may be concave as shown or flat. The former 29 is secured in this unit by being partially embedded in the material 51.

The simplest form of diaphragm unit is shown in Fig. 4. The unit 60 has no secondary coil 32 or magnet 36 but simply provides a diaphragm with a frusto-conical portion 26 and a domed portion 28 for connection to the closed end 8 of the drive unit 1 when the ring 24 is secured to the flange 5.

The embodiments of the invention described can be modified in various ways within the scope of the invention. The invention does not exclude the use of diaphragm assemblies of shapes otherthan as shown, nor the use of different diaphragm suspension arrangements, for example, electro-magnetic suspension means.

Claims (16)

1. A drive unit for an electro-acoustic transducer, the drive unit comprising a magnet, a coil former carrying a coil, and suspension means suspending the coil former with the coil in operative relation with the magnet.

2. A drive unit as claimed in claim 1 wherein the suspension means comprises two suspension elements spaced apart axially of the coil former.

3. A drive unit as claimed in claim 2 wherein the suspension elements extend between the coil former and a chassis part having the magnet secured thereto.

4. A drive unit as claimed in claim 3 wherein the chassis part comprises a sleeve having the magnet received in one end thereof.

5. A drive unit as claimed in any preceding claim wherein the former has the end thereof protruding from the magnet closed by an outwardly domed end portion.

6. An electro-acoustic transducer comprising a drive unit as claimed in any preceding claim assembled with a diaphragm unit comprising a diaphragm unit suspended in a chassis, the diaphragm being connected to the coil former and the chassis being connected to the magnet or chassis part.

7. An electro-acoustic transducer comprising a chassis, a magnet mounted on the chassis, a diaphragm assembly having a diaphragm with an outer periphery and a coil former secured to the diaphragm within the outer periphery, a coil on the coil former, and suspension means suspending the diaphragm assembly in the chassis with the coil in operative relation with the magnet, the suspension means comprising a first suspension element at the diaphragm outer periphery and second and third suspension elements at or in the vicinity of the coil former.

8. An electro-acoustic transducer as claimed in claim 6 or 7 wherein the diaphragm carries a second coil on the opposite side thereof from the first-mentioned coil, the second coil being in operative relation with a second magnet carried by the chassis.

9. An electro-acoustic transducer having a diaphragm suspended in a chassis, first and second coils carried by the diaphragm on opposite sides thereof, and first and second magnets carried by the chassis in operative relation with the first and second coils respectively.

10. An electro-acoustic transducer as claimed in claim 8 or9 wherein the diaphragm is of hollow shape and partially filled with foamed plastic material, the second coil being carried on a coil former partially embedded therein.

11. A method of assembling an electro-acoustic transducer having the steps of providing a first sub-assembly comprising a first chassis part with a magnet and a coil former suspended so as to have a coil carried thereby in operative relationship with the magnet, providing a second sub-assembly compris ing a second chassis part and a diaphragm suspended therein, and assembling together the first and second sub-assemblies with the diaphragm connected to the coil former and the first chassis part connected to the second chassis part, to constitute the transducer chassis.

12. A method as claimed in claim 11 having the step of testing the first sub-assembly prior to the assembly step.

13. A drive unit for an electro-acoustic transducer, the drive unit being substantially as herein described with reference to Figure 1 of the accompanying drawings.

14. An electro-acoustic transducer substantially as herein described with reference to Figure 1 in association with Figure 2, Figure 3 or Figure 4 of the accompanying drawings.

15. A method of assembling an electro-acoustic transducer substantially as herein described with reference to Figure 1 in association with Figure 2, Figure 3 or Figure 4 of the accompanying drawings.

16. An electro-acoustictransducer assembled by the method of claim 11,12or15.