CA2109711A1

CA2109711A1 - Loudspeaker

Info

Publication number: CA2109711A1
Application number: CA 2109711
Authority: CA
Inventors: Lawrence R. Fincham
Original assignee: Individual
Current assignee: KEF Audio UK Ltd
Priority date: 1991-05-31
Filing date: 1992-05-29
Publication date: 1992-12-10
Also published as: EP0516471A1; NO934298L; WO1992022175A1; GB2256560B; GB9111737D0; NO934298D0; NZ242962A; GB2256560A

Abstract

A loudspeaker construction is disclosed in which a conical diaphragm (15) is caused to generate and radiate sound by means of interaction between a drive coil on a coil former (24) secured to the neck of the diaphragm and a magnet assembly (14) secured to a chassis (10) of the loudspeaker. Electrical energisation of the drive coil produces a drive force acting on the drive coil to move the diaphragm. In order to reduce or prevent radiation of sound from a front ring (11) of the chassis and from a baffle board on which the chassis is mounted arising from reaction to the drive force acting on the magnet assembly, the magnet assembly is mechanically isolated or de-coupled from the chassis. This is accomplished by securing the magnet assembly to the chassis by means of resilient mountings (41) to reduce or prevent transmission of reaction force from the magnet assembly to the chassis.

Description

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Magnetk drlve system for loudspeaker This invention relates to loud~peaker drive units for sound reproduction and in particular to a construction of magnetic drive s~stem for such loudspeaker driYe units.
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Loudspeaker dxive units include a sound radiating di~phragm, commonly of conical ~orm, which is caused to vibrate by a drive ~ystem to generate the radiated sound waves. The mouth of the diaphragm i9 mounted-by means of 10 a compliant suxround to a chassi-~ of the loud~pea~er drive unit to permit a~ial movement of the ~one relative to the chassis. Usually the drive system comprises a magnet a3~embly rigidl~ mounted on the chas~is of the l oudspeaker drive unit . The magnet assembly includes an annular air gap in a magnetic path such that there is magnetic flux across the air gap. The diaphragm carries a cylindrical coil which is located concentrically with the annular air gap and at l~ast a part of the length o~ the coil is within the annular air ~ap and i5 eloctrom~gnetically coupled with the magnetic flu~ in the annular air gap.
~leatric aurrent is passed through the coil and hence a force is generated which cau~e~ the diaphragm to move away from its rest position. Accordingly if the current in the :~coil is a waveform: representing .qou~d:~ signals, ~the di~phragm ~will be driven to move in a m~nner such as~ to ;reproduce: those :~sou~d~si~nals.~ It will abe~- appreciated :- that :-when:.the diaphragm is-driven in thi manner the movement of th- d~iaphr-gm cauaes pre~ure variations in ~: the: air adjace~t thereto and this req~ires a: substantial :~ 30 magnitude of-. power- to be input to the coil of the loudspea~er drive unit in order to drive the~diaphragm to produce the required magnitude of sound level. Also, the diaphragm has a finite mass which, although it is low, requires significant magnitude of force to ov~rcome the inertia of the ma~s of the diaphragm. As a result, when an electric current is passed through the coil to produce a force to drive the diaphragm the magnet assembly is W092/22175 PCT/GB92/~70 2109711 2 ~ j su~jected to an equal and oppo~ite re-action force.

In common constructions of loudspeaker drive unit the chas~is of the speaker is in the form of a generally ~onical bas~et comprising a front annular flange and a number of 1 egs, extending rearwardly and inwardly from the flange, to which the magnet a~sembly i5 riyidly -qecured.
The mouth of the conical diaphragm is mounted by a compliant surround to the front annular flange of the cha~sis and this front fl ange f onm5 a mounting means for the loudspeaker drive unit. In order to provide the reyuirecl acoustic characteri~tic of a loudspeaker, the loudspealker drive unit is moun~ed on a ~affle board forming a wall of a ~ox or cabinet. A circular aperture is pro~ided in the baffle board and the fro~t ~hassis f 1 ange is secured to the board surrounding the aperture with the remainder of the chassis and the magnet assembly of the loudspeaker drive unit extendinq into *he interior of the box. When the coil of the loud~peaker drive unit '20 i8 energi~ed to vibrate the diaphra~m to generate sound radiation the egual and opposite re-action force-~ to whiih the magnet structure is ~ubje~ted are tran~mitted through . the chassis to the baffle board and ~s a consequence there . is a tendency for the baffle board to vibrate a~d radiate 25 :sound.~ As a result:the overall sound produced by the : . .i loudspeaker drive unit~when mounted on the baffle board of the box-compri~es the ~om~ination o~ the sound radiated by :the ~diaphra~m togethér with any sound radiated by the ^ chassis flange and the baffle board due to vibration thereof resulting~rom the re-action forces a~ting on the magnet a3~embly.

High quality reproduction of sound requires that the relationship between the magnitude of the electrical input drive signal and the magnitude of the level of sound output is as linear as possible and is repre~ented by a smooth curve as a function of frequency. However the `"~:

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effect of the radiation of sound due to the reaction force~ is to distort this smooth curve and to result in distortion of the reproduced sound. To overcome this distortio~ in reproduction it has been proposed to mount the front flange of the loudspeaker drive unit chassis to the baffle board by means of re~ilient or compliant mountings. However in order to obtain the required coupling of the diaphragm with the surrounding air it is re~uired that the mouth of the diaphragm is sealed to the baffle board in a manner which prevents leakage of air between the mouth of the diaphragm and the ~affle board.
Accordinçlly if the loudcpeaker drive unit is mounted by means of resilient or compliant mountings it is necessary to provide a compliant sealing ring to seal between the lS front of the cha~ and the baffle board. While this method may enable the cha sis of the loudspeaker drive ~nit to b,e mechanically isolated from the baffle board so that any sound radiation from the baffle board due to reaction forces acting on the magnet assembly is elimi~ated or reduced the front flange of the chassis itself presents a ~igni~icant sound radiating surface which can radiate sound which will interfere with that radiated by the diaphragm and result in di~tortio~ of the reproduced ~ound.
According to the inv~ntion a loudspeaker drive unit oomprising a ~ cbassis; a~maqnet as~embly includinq an annular air gap, mag~etic flux being generated in the air gap by a magn~t; a diaphra~m having a periphery thereof secured by compliant means to the chaYsi~;- and a drive coil ~ecured to the diaphragm and located in the magnetic flux ~of the air gap so that pa~sage of electric current through the drive coil generates a drive force acting between the magnet assembly and the diaphragm to cause the diaphragm to vibrate i5 characterised in that the magnet aqsembly is mechanically isolated or de-coupled from the chassi~ to prevent or reduce tran~mission to the chassis r . ~ r ~ .

P~T/~ ~ / on9 25 MAY l99 of reaction force arising from said drive force.

An embodiment of the inYention will now be described by way of example with reference to the drawing which shows a central cros~ section through a loudspeaker drive unit.

Referring to the drawing, a chassis 10 of a loudspeaker drive unit comprise3 a front annular flange 11 from which legs 12 extend rearwardly and inwardly. The rear ends of the legs 12 are joined by a rear member in the form of an annular plate 13. The chassis may be of cast construction but is conveniently presqed from sheet metal. Preferably three legs 12 equi-angularly spaced on the flange 11 are provid0d! but if desired a larger ~umber of le~s may be lS provided. A magnet a~embly 14 is secured to the annular~
plate 13. The magnet assembly 14 comprises an annular magnet 17, a fir t pole element 18 having a aircular flange 19 magnetically coupled to one pole face o~ the magnet 17 and a cylindrical centre pole 20 extending from the flange 19 through the magnet 17 and a second pole el~ment comprising a f ront plate 21 magnetically coupled to the other pole fac~ of the magnet 17. The front plate 21 is of annular form with a central aperture therein of greater diameter than the diameter of the centre pole 20.
The centre ~ole 20 extend~ through the aperture in the fro~t plate 21 to define,~betwoe~ them, a~ annular air gap concentric wi~h a centre line 22. The larger diameter front edge of the conica~l diaphragm 15 is secured to Itbe~
front flange 11 ~of the chassis 10 by means of a compliant ~urround 23. A cylindricai coil former 24 is seoured to the rear smaller diameter of the conical diaphragm 15.
Tho~ coil former 24 carrie~ a drive coil for the coniaal diaphragm. The inner diameter of the coil former 24 is slightly larger than the diameter of the centre pole 2Q
and the outer diameter of the drive coil on the coil former is slightly smaller than the diameter of the aperture in the frsnt plate 21. The coil former extends UI~ UTE $HI~

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2 1 ~ 9 7 1 1 rearwardly from the diaphragm 15 through the air gap between the centre pole 20 and the front ~late 21 such that the drive coil is located in the region o~ the air gap and is subjected to magnetic f1ux generated by the S magnet 17 in the air gap. Centering means 2$ is secured to the coil former 24 to maintain the coil former, and the drive coil carried there~y, concentric relative to the-centre line 22 of the annular air gap. The centering means may comprise, as shown in the drawing, - an annular sheet formed with concentric corrugations therein whereby the co~nical diaphragm lS is free to move in an a~ial direction on the compliant mounting ring 23 relative to the cha~sis 10 while being maintained concentric relative to the axis 22. An outer edge of the centering means 2S
15 is secured to a ~urface of an annular member 27 which is attached to the magnet assembly 14.

When the coil of the drive unit i9 energised with a waveform representing sounds to be reproducod a force is generated by the inter-action of el~ctrical current in the drive coil and the magnetic flu~ in the air gap which drives the conical diaphragm to move such as to radiate sound. The force acting on the diaphragm produces an equal and opposite re-action force acting on the magnet : ~ 25~ assembly~ 14 whiah tends to be transmitted through the ha~sis to the front flange of:the chassis and to any baffle board on which the drive unit is ~ounted. This transmitted force tends to cause the front flange of the drive unit chassis and the baffle board to ~ibrate and to : 30 radiate sound which interferes wi~th ~ound ~diated by the ~: conical diaphragm and as:a result cau~es distortion of the reproduced sound. In order to prevent or at least reduce this distortion of the reproduced sound ~ue to the re~
action force actin~ on the magnet ascembly, the magnet assembly 14 is mounted on the chassis 10 in such a manner that the magnet assembly is mechanically isolated or de-coupled from the chassis 10. The mechanical isolation or ~'~

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de-coupling of the magnet ass~mbly 14 from the cha~sis 10 is ac~ieved by means of resilient mountings 41, only one of which is illustrated in the drawing, for the magnet assembly. Each mounting 41 comprices a resilient bush S member located in an aperture in the rear plate 13. The resilient bush member comprises an inner tubular eleme~t 35 and an outer annular element 36 interaonne~ted by a web 37. The outer annular element 36 ha~ an annular groove in the periphery thereof to receive the edge of the.plate 13.
A rigid tubular member 38 extends through the inner tubular element 35. As shown in the drawing, ~he tubular member 38 is carried by the member 27. The front plate 21 of the malgnet assembly extends out to the locations of the mounting: 41 and is s~cured to the tubular m~mbers 38 by screws 35~ and nuts 40. Thus the outer annular element of the resilient bush members is held by the plate 13 and the inner tubular element 35 is secured to the front plate 21 by means of the tubular spigot 38 engagi~g in the tubular element 35 and by the tubular ~pigot 38 bein~ s~cured to the ~ront plate 21 by the screws 39 and nuts 40. For ease of a3sembly the nut~ 40 are held captive in recesses in the member 27. Accordingly it will be appreciated that the magnet assembly 14 is resiliently connected to the chassis by means of the webs 37 of the resilient bush members. The webs permit movement of the magnet as~embly r~lative~to the chaqsis 10 in the direction of the axis 22 and it is in this direction that vibration of the magnet aQsembly due to the ~re-action forces' ~ will occur.
Therefore the re~ilient bush members mechanically isolate or ~de-~ouple the magnet assem~ly from.the chassis 10 in respect of vibration of the magnet assembly resulti~g from re-action forces when the diaphragm 15 i5 driven. However the webs of the bush members resist movement of the magnet assembly in directions transverse to the axis 22 and hence 3S the axis 22 of the magnet assembly and the air gap therein is maintained substantially concentric with the axis of the chassis 10 regardless of the orientation of the V '~ a~

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It is preferred to secure the magnetic assembly to the ;~
chassis by means of three resilient bush members 41 and to -locate the resilient bush members in the rear member 13 adjacent to the legs 12. ;

While in general the low fre~uency diaphragm is of conical form, the diaphragm may be of other forms -as may be ~;~
desired. For example, the diaphraqm may be planar with the periphery thereof mounted by compliant means to a chas-~is of the drive unit and the drive coil secured to a central region of the diaphragm. - ~

15 Hereinbefore, the chassis of the loudspeaker drive unit ~ ~;
has been described as a unitary structure with the magnet a~embly mounted thereon by mean~ of resilient mountings `
secured; to the magnet a sembly and to the plate 13.
However if desired the chassis may comprise two part~
which are connected together but me~hanically isolated or decoupled from one another by resilient mounting means.
One part of the chassis include~ the front flange and the other part of the chassis comprises a sub-chassis which includes the rear member. The front flange may have leqs 25~ ~e~tending rearwardly and the rear me~ber may have le~s e~t-nding; forwardly~and connection between the two parts of the chassis may be effected by resilient mounting means secured to correspondinq pairs of legs of the cha~sis and sub-chassis. With a two part chassis in which resilient connections-are provided between front and rear parts, the ~- magnet assembly may be rlgidly secured to the rear sub-chassis. ~ -. :... .
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Claims

1. A loudspeaker drive unit comprising a chassis (10); a magnet assembly (14) including an annular air gap, magnetic flux being generated in the air gap by a magnet;
a diaphragm (15) having a periphery thereof secured by compliant means (23) to the chassis; and a drive coil (24) secured to the diaphragm and located in the magnetic flux of the air gap so that passage of electric current through the drive coil generates a drive force acting between the magnet assembly and the diaphragm to cause the diaphragm to vibrate; characterised in that the magnet assembly (14) is mechanically isolated or de-coupled from the chassis (10) to prevent or reduce transmission to the chassis of reaction force arising from said drive force.

2. A loudspeaker drive unit as claimed in claim 1 further characterised in that the mechanical isolation or de-coupling of the magnet assembly (14) from the chassis (10) is effected by resilient mountings (41) securing the magnet assembly to the chassis.

3. A loudspeaker drive unit as claimed in claim 2 further characterised in that the resilient mountings (41) are effective to permit relative movement between the magnet (14) assembly and the chassis (10) in a direction aligned with the direction in which the reaction force acts on the magnet assembly and are effective to restrain relative displacement of the magnet assembly (14) and the chassis (10) in directions transverse to the direction of action of the reaction force.

4. A loudspeaker drive unit as claimed in claim 2 or 3 further characterised in that the chassis (10) comprises a front annular member (11) to which the diaphragm (15) is secured by the compliant means (23); a rear member (13); a plurality of legs (12) connecting the rear member to the front member; a plurality of resilient mountings (41) secured to the rear member; and the magnet assembly includes a front plate (21) surrounding the air gap and defining an outer boundary of the annular air gap; and including means (40) securing the front plate to said resilient mountings.

5. A loudspeaker drive unit as claimed in claim 4 further characterised in that the resilient mountings (41) are located in the rear member (13) respectively one adjacent each leg (12) of the chassis (10).

6. A loudspeaker drive unit as claimed in claim 2 or 3 further characterised in that the chassis (10) comprises a front annular member (11) to which the diaphragm (15) is secured by the compliant means (23); in which the magnet assembly (14) is secured to a rear sub-chassis member (27) and in which the rear sub-chassis member (27) is connected to the chassis (10) by resilient means (41) effective to mechanically isolate or de-couple the magnet assembly (14) and rear sub-chassis member (27) from the chassis (10).

7. A loudspeaker drive unit as claimed in claim 6 further characterised in that the chassis (10) includes first legs (12) extending rearwardly; the rear sub-chassis member (27) includes second legs corresponding to said first legs and extending forwardly; and in which the resilient means (41) is secured to corresponding first and second legs.

8. A loudspeaker drive unit as claimed in any one of claims 2 to 7 further characterised in that each resilient mounting (41) comprises an inner element (35); an outer annular element (36) extending around the inner element and a resilient web (37) interconnecting the inner and outer elements.

9. A loudspeaker including a box and characterised by a loudspeaker drive unit as claimed in any preceding claim;

said loudspeaker drive unit being mounted on a wall of the box by a rigid connection between the chassis of the drive unit and the wall of the box.