WO1994027413A1

WO1994027413A1 - Loudspeaker with short circuit rings at the voice coil

Info

Publication number: WO1994027413A1
Application number: PCT/DK1994/000186
Authority: WO
Inventors: Lars Goller
Original assignee: Scan-Speak A/S
Priority date: 1993-05-10
Filing date: 1994-05-10
Publication date: 1994-11-24
Also published as: GB9522905D0; GB2292651A; GB2292651B; DK54093D0; DK124495A; DK176300B1; DE4493008T1; DE4493008B4; AU6720694A

Abstract

In loudspeakers (figure 1) with so-called 'symmetric drive' (SD) there is mounted about the central magnet pole piece (6) a copper cylinder (22), and a corresponding cylinder (24) is mounted in front of this pole piece, projecting outwardly as far as corresponding to maximum displacement of the voice coil (16). According to the invention (figure 2) the same or even a better compensation result is achievable with the use of a considerably reduced amount of copper, viz. by means of a short copper ring (22B, 24B) at each side of the air gap (14) and a somewhat longer, yet still relatively short copper ring (22A) about the root portion of the pole piece (6).

Description

LOUDSPEAKER WITH SHORT CIRCUIT RINGS AT THE VOICE COIL.

The present invention relates to a loudspeaker of the conventional type, where the voice coil of the loud¬ speaker diaphragm has an axial length larger than that of the annular air gap, in which the coil operates, and where there is mounted, at both sides of the air gap, a cylinder of electrically conducting material, also call¬ ed a copper cylinder. These cylinders are used in order to create such surroundings for the coil which are, electrically, practically the same wherever the coil is located inside its range of oscillation; it is conventi¬ onal, therefore, that the copper cylinders extend beyond the opposed ends of the coil, inwardly around the cen¬ tral magnet core and outwardly in the air outside the central pole piece, respectively.

It is a main effect of this arrangement that the magnetic field will be stabilized against changes caused by the current in the voice coil, inasfar as the copper cylinder will act as a short circuit winding, which, by the inductive coupling with the voice coil, will gene¬ rate an inversely directed magnetic flux that will re¬ lieve the modulating effect of the voice coil on the flux in the permanent magnetic field.

Another main effect is that the current or current changes in the voice coil will meet an impedance which is largely independent of the position of the coil, i.e. whether it is located way in over the central iron core, where the impedance would otherwise be high, or way out from this core, where the impedance would be low. The large movements of the coil are caused by the low fre¬ quencies in the reproduced signal, and the simultaneous reproduction of the high frequencies would otherwise be highly influenced by the coil being located in the iron or air filled area, respectively. When the said 'short circuit winding' is present all over the operation length of the coil, the coil will present the same impe- dance in all of its positions, because the influence from the surroundings will be widely neutralized by the presence of the short circuit winding.

It has been found that the copper cylinders should have a considerable wall thickness in order to be really operative. Previously, it was found advantageous to make the copper cylinders extend continually through the air gap, with a wall thickness so small that the air gap should be made only slightly broader, but the fact is that the electrical resistance in the copper cylinder will hereby be high enough to substantially weaken the desired compensation effect.

When the copper cylinders should thus be both long and thick-walled, the use of them will involve a notice¬ able price and weight increase, and besides the problem occurs that the voice coil has to be placed on a tube core, which projects rearwardly rather far from the diaphragm, because the latter cannot be displaced in¬ wardly any further than to the outer end of the copper cylinder, this being permanently projecting as far as corresponding to the outer maximum amplitude of the voice coil. The associated considerable length of the tube core is a problem seen from a stability point of view.

With the present invention it has been found that it is not necessary for the copper cylinder to extend all over the working length of the voice coil in order to effectively fulfill its purpose. It is sufficient that a short cylinder piece is mounted about the root of the central part of the magnetic system, whereby the associated inductive coupling with the voice coil will ensure that the coil will represent the same impedance whether, at a given moment, it is moving towards or away from the iron filled area at the central core portion of the magnet system. However, it will still be required to stabilize the induction of the voice coil, such that the induction will be substantially the sane, no matter where the voice coil is located relative to the air gap, and to this end it has been found that it is fully suf¬ ficient to mount a compact copper ring immediately at either side of the air gap, i.e. copper rings which, axially, are substantially shorter than the axial dimen¬ sion of the voice coil, while being suitably thick in order not to substantially limit the current induced in these rings. Hereby it becomes sufficient that the rings occur as relatively short units, and the totality of three ring parts will thus be able to show a combined axial length considerably shorter than the operation length of the voice coil. This will condition a both lighter and cheaper design of the loudspeakers.

In the prior art there are proposals nominally approaching the invention from different sides. Thus, according to DE-C-30,46,5S1, it is known to use a root ring of copper about the inner end of the pole piece, but without the ring being supplemented by copper rings at the effective magnet gap, and besides without it being further specified how the ring is located relative to the maximum amplitude of the voice coil. Correspond¬ ingly, from DK-C-148,050, it is known to use relatively short copper rings or cylinders at the respective outer and inner sides of the foremost, central pole piece, but without this being related to an additional ring of copper at the root of the central magnet core, and besi¬ des with a somewhat deviating purpose.

In the following the invention is described in more detail with reference to the drawing, in which:-

Fig. 1 is a sectional view of the coil and magnet system of a known loudspeaker type, while

Fig. 2 is a corresponding view of a loudspeaker according to the invention, and

Fig. 3 is a similar and authentic view thereof.

The known loudspeaker of Fig. 1 has a magnet system comprising a ring magnet 2 with a rear/lower pole plate 4, from the center of which a pole piece 6 projects forwardly/upwardly, and with a foremost/upper pole plate 8 made with a central hole 10, inside which, and flush with the pole plate 8, there is mounted an inner pole plate 12 on the outer end of the central pole piece 6. In the annular gap 14 between the pole plates 8 and 12 is located a voice coil 16, mounted on a cylindrical carrier core 18 connected with an outer loudspeaker diaphragm 20, which is held by a carrier chassis (not shown) in such a manner that the coil tube 18,16 is guided to carry out purely axial oscillations in the air gap 14.

• In its position of rest the voice coil 16 is loca¬ ted so as to project equally to both sides of the plate portions 8 and 12. The pole piece 6 is surrounded by a cylinder 22 of copper, and a similar cylinder 24 is mounted so as to project forwardly/upwardly from the central pole piece 12. Therefrom, both of the copper cylinders 22 and 24 project beyond the ends of the voice coil 16, such that during operation they will steadily be located inside the oscillation area of the voice coil.

In connection with the invention, confer Fig. 2, it has been recognized that the total length of the copper cylinders 22 and 24 can be reduced essentially, because it is sufficient to make use of a shortened root cylin¬ der 22A extending from the pole plate 4 forwardly/up¬ wardly approximately to the inner/lower end of the voice coil 16, and two axially short copper rings 22B and 24B mounted at the respective opposite sides of the central pole plate 12. When comparing Figs. 1 and 2 it will be seen that it is hereby possible to use a noticeably reduced amount of copper, whereby the loudspeaker can be lighter and less expensive.

Moreover, it will be possible to shorten the core tube 18 of the voice coil, since the diaphragm 20 can now be moved closer towards the magnet system, and such a shortening involves great advantages in several res¬ pects.

It has been found that with the use of a system according to Fig. 2 the advantages obtained are quite as obtained with the system of Fig. 4, when only the copper rings are made thick enough, i.e. with a thickness of several millimetres, to carry the actual counter elec- tromotoric forces without offering any noticeable resis¬ tance. The root cylinder 22A should not extend into the voice coil in the rest position thereof, as the only task of this cylinder is to compensate for the varying iron filling of the coil. The optimal length of the cylinder can be experimentally determined, and it has been found that the most correct compensation is obtain¬ ed when the cylinder is located outside the voice coil, even in the maximum inwardly displaced position thereof. Then a variation of the degree of coupling between the coil and the cylinder will occur during the entire cour¬ se of oscillation, as a pure function of the variation of the iron filling of the coil, and just hereby a very efficient compensation is achievable.

When the coil supercedes its operative maximum displacement, the phenomenon 'clipping' occurs, re¬ ferring to the top of the signal, being compressed, whereby the associated curve of the signal approaches a rectangular shape. As well known, this implies an in¬ creased content of overtones relative to the base tone, meaning distortion.

The invention provides for an improvement even of this phenomenon, in that the voice coil, when oscillat¬ ing excessively, will surround but a small amount of copper in the system 22b, 24b, whereby the effect of the copper with respect to a minimising of the induction of the voice coil will be reduced. Thereby the impedance of the coil will increase at high frequencies, i.e. the formation of the distorting overtones will be counterac¬ ted such that the associated distortion will be reduced considerably.

Thus, it will be appreciated that the "shortage of copper" according to the invention relative to the prior art does not imply only a saving, but even a qualitative improvement in the sound reproduction of the loudspeak¬ er.

Fig. 3 is a sectional view of one of the loudspeak¬ ers having been used for measurements in connection with the invention, i.e. this figure is authentic as far as the dimension relations are concerned. It will be noted that there is a substantial axial distance between the root ring 22A and the inner end of the voice coil and that the ring 22B is slightly thinner than the ring 24B; this reduced thickness is owing to the fact that the root ring 22A to some degree contributes to the function of the ring 22B, which contribution is reduced when the coil moves outwardly, such that the outer ring 24B ide¬ ally should be somewhat thicker than the ring 22B. In this example the thickness of the ring 24B is only about 1/6 of the length of the voice coil. None of the rings 22B and 24B should extend beyond the voice coil in the neutral position thereof, as this would compromise the reduction of the clipping distortion mentioned above. The length of the voice coil of this loudspeaker is 18 mm. It has been found sufficient for the root ring 22A to have a length of 8 mm, this being less than the half of the distance between the inner pole ring 22B and the outer/lower end of the root ring. The total length of the copper rings is about 13 mm, while in a correspond¬ ing system according to Fig. 1, it would be some 40 mm.

For the invention it is not decisive whether the copper rings are placed inside or around the voice coil, although the efficiency is higher when they are placed inside the coil. Many parameters will influence the exact dimensioning of the parts for obtaining the desir¬ ed ideal result, and for every new loudspeaker design it will normally be required to carry out a series of test with parts of different dimensions until both the gene¬ ral impedance stabilisation and the minimized clipping distortion is established. Thereafter, the solution as found may be used as a standard for all loudspeakers of that particular design.

Claims

C L A I M S :

1. A loudspeaker of the type in which the voice coil of the driver diaphragm is longer than the axial extension of the annular air gap of the associated mag¬ net system, and in which there is mounted, about the central pole piece of the magnet system, a cylinder of copper or a similar material, while such a cylinder member is also mounted in front of the pole piece, sub¬ stantially concentrically with the voice coil, which cylindrical parts have a wall thickness sufficient for an effective conduction of compensating induction cur¬ rents, characterized in that the copper cylinder about the central pole piece is divided into two mutually separated parts, viz. a root part (22A) at the root area of the pole piece and an inner pole ring part (22B) at the inner side of the annular gap, respectively, said root part (22A) being located outside the inner end of the voice coil in the position of the rest thereof, preferably even in the operatively maximum inwardly displaced position thereof, while the pole ring part (22B) is noticeably shorter than half the length of the voice coil, whereas the corresponding outer pole ring part (24B) , located at the outside of the annular gap, likewise is noticeably shorter that half the length of the voice coil.

2. A loudspeaker according to claim 1, in which the axial dimension of the inner pole ring part (22B) is somewhat smaller than that of the outer pole ring part (24B) .

3. A loudspeaker according to claim 1, in which the axial dimension of the root part (22A) is smaller than the distance between this part and the inner pole ring part (22B) .