Annular gap magnet system, particularly for low frequency loudspeakers

An annular gap magnet system, in particular for a low frequency loudspeaker (Woofer), in which a coil is movable with a large stroke in the working air gap. A braking air gap provided in the region of the inner or lower pole plate produces a magnetic resistance in the pole plate so that a part of the magnetic flux flows over the braking air gap. This magnetic flux is opposed to the stray magnetic flux below the working air gap and excites a counter magnetic field which opposes further inward movement of the moving coil. In this way an impact of the moving coil against the inner or lower pole plate is prevented. In a low frequency loudspeaker the membrane carrying the moving coil may, therefore, be suspended extremely softly.

BACKGROUND OF THE INVENTION 
The invention concerns an annular gap magnet system, particularly for low 
frequency loudspeakers (Woofers) in which a moving coil moves with a large 
stroke in the working air gap, with a cylindrical pole core of soft iron 
and an annular permanent magnet arranged at a distance from the pole core 
between an outer (herein called the upper) pole plate limiting the working 
air gap and an inner (herein called the lower) pole plate. The invention 
also concerns low frequency loudspeakers and electromagnetic drives having 
an annular gap magnet system of this type. 
DESCRIPTION OF THE PRIOR ART 
In low frequency loudspeakers it is desirable to produce an extremely soft 
suspension of the membrane carrying the moving coil. This has the result 
that the moving coil after having left the main magnetic field in the 
working air gap of the magnet system is drawn further inward by the 
internal stray field below the working air gap and, in particular, if 
overloaded it strikes against the lower pole plate. 
In order to prevent such an impact of the moving coil against the lower 
pole plate it has been necessary hitherto to make a compromise in that the 
membrane and the moving coil are suspended stiffer than is desirable from 
the point of view of the acoustic quality of the loudspeaker. 
The basic object of the invention is to design an annular gap magnet system 
or a loudspeaker of the type described in the introduction in such a way 
that even with an extremely soft suspension of the membrane and the moving 
coil, impact of the moving coil against the lower pole plate is prevented 
with certainty even when the loudspeaker is overloaded. 
SUMMARY OF THE INVENTION 
This object is solved according to the invention in that in or on the 
inner, that is the lower, pole plate, at a distance from the outer, that 
is the upper, pole plate which is at least equal to the thickness of the 
upper pole plate, there is provided a braking air gap surrounding the pole 
core in its lower region as an axial extension of the working air gap, and 
that, in the region of the lower end of the braking air gap in the lower 
pole plate there is provided a magnetic resistance of a magnitude such 
that the magnetic flux through the braking air gap and the stray flux 
above the braking air gap, both of which are directed oppositely to the 
magnetic flux in the working air gap and also oppositely to the stray flux 
below the working air gap, are at least equal in sum to the oppositely 
directed stray flux below the working air gap. 
By means of said magnetic flux through the braking air gap and the stray 
flux above the braking air gap, both of which are directed oppositely to 
the magnetic flux in the working air gap and to the internal stray field 
in the region surrounding the working air gap, there is produced a 
magnetic counter force, of well defined magnitude, which prevents impact 
of the moving coil against the lower pole plate even when the loudspeaker 
is overloaded and, in particular, independent of the softness of the 
membrane suspension. 
The magnetic resistance can be generated by a reduction in the 
cross-section of the lower pole plate. It is, however, also possible to 
provide a connecting element without or with low magnetic conductivity 
between the lower pole plate and the pole core. 
For the purpose of easy tuning of the loud speaker to any given desired 
acoustic quality and/or for reasons of economy, there may be provided 
adjacent the lower pole plate a soft iron ring which limits the braking 
air gap at least over a part of its axial length and which is in 
magnetically conducting connection with the internal circumference of the 
permanent magnet ring. The depth of the braking air gap and its distance 
from the upper pole plate may be varied by the insertion of soft iron 
rings of different heights. 
It is, however, also possible for the dimensions of the braking air gap to 
be limited over part of its axial length by the annular permanent magnet, 
where the annular permanent magnet is conveniently constructed from two 
permanent magnets in series, of which that magnet which is situated facing 
away from the working air gap forms by means of its external circumference 
the external limit of the dimensions of the braking air gap at least over 
part of its axial length.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 there is illustrated schematically a low frequency loudspeaker 
having an annular gap magnet system 1, a conical membrane 2 and a membrane 
cage 2a. A cylindrical body 4b on which a moving coil 4a is mounted is 
attached rigidly to the membrane. A centering membrane 2b is arranged 
between the inner (i.e. the lower, as shown,) end of the membrane 2 and 
the membrane cage 2a. The loudspeaker cage 2a is rigidly attached to the 
magnet system in the usual way. The annular gap magnet system 1 has a 
cylindrical pole core 5 of soft iron and an annular permanent magnet 6 
with a thickness D which is fixed concentric with the pole core 5 between 
an annular upper pole plate 7 of thickness d.sub.2 and a lower pole plate 
8. A working air gap 3, into which the moving coil 4a dips, is formed 
between the pole core 5 and the internal circumference of the upper pole 
plate 7, situated concentric with the pole core. 
In FIG. 1, as in FIGS. 2-6, the pole core 5 and the lower pole plate 8 are 
illustrated as if designed to be of unitary construction. Normally, the 
pole core 5 and the pole plate 8 are two separate bodies which are, for 
example, connected rigidly together by screws or rivets. This is indicated 
in FIG. 1 by the dashed line 5a. 
The moving coil 4a is designed in such a way that is moves in the working 
air gap 3 of the magnet system with a suitable stroke, for example the 
typical large stroke of a low frequency loudspeaker (Woofer). 
In or on the lower pole plate 8 there is provided a cylindrical annular 
braking air gap 9 with depth d.sub.3 which surrounds the pole core 5 as an 
axial extension of the working air gap and into which the moving coil can 
dip at its lower end. 
The open end 9a of the braking air gap 9 is situated in all embodiments 
shown by way of example at a distance d.sub.1 from the lower side of the 
upper pole plate 7, said distance being at least equal to the thickness 
d.sub.2 of the upper pole plate 7, but preferably larger. 
The braking air gap 9, which extends within the lower pole plate 8, 
produces a decrease in the area of the cross-section of the lower pole 
plate at its lower end in a manner which results in an increase in the 
magnetic resistance 10e. 
FIG. 1 shows in dashed lines the magnetic flux which is produced by the 
braking air gap 9 and the magnetic resistance 10e. Because of the magnetic 
resistance 10e, the magnetic flux from the lower pole plate 8 to the pole 
core 5 passes to a large extent 10a through the braking air gap 9 and the 
stray magnetic flux 10d above the open end 9a of the braking air gap 9 
flows substantially from the inner rim and the adjacent upper side of the 
pole plate 8 to the pole core 5. The magnetic flux 10a and the stray flux 
10d forming the braking flux are, in sum, at least equal to the internal 
stray flux 10c between the pole core 5 and inner rim and the adjacent 
lower side of the upper pole plate 7 beyond the working air gap 3 and are 
preferably larger. If the moving coil slips into the magnet systems, it 
leaves the magnetic field in the working air gap. It is then driven 
further inwardly by the stray flux 10c. Counteracting the drive caused by 
the stray flux 10c is the sum of the braking fluxes 10a and 10d. In this 
way the moving coil 4a is actively braked and is thus prevented from 
striking against the lower pole plate. 
In other embodiments only the annular gap magnet system is illustrated in 
each case. Similar parts or parts with similar function are, in each case, 
given the same reference numbers as in FIG. 1. Therefore, in each case, 
only those characteristics by which the magnet systems differ from the 
embodiment shown in FIG. 1 are described in the following. 
In FIG. 2 the lower pole plate 8 has on the left-hand side of the centre 
line a recess 8a in its lower side which extends radially outward from the 
braking air gap 9 for economy of material or reduction in weight. To the 
right of the centre line a modification is illustrated in which the lower 
pole plate is formed by a ring 8b and a plate 8c. The pole core 5 is fixed 
centrally on the plate 8c the thickness of which determines the magnetic 
resistance. 
While, in the embodiments shown in FIGS. 1 and 2, the whole depth d.sub.3 
of the braking air gap 9 lies within the lower pole plate 8, in the 
embodiment shown in FIG. 3a only a part d.sub.3 of the depth of the 
braking air gap is formed within the lower pole plate, in particular its 
lower end. A soft iron ring 12 is arranged on the upper side of the lower 
pole plate 8 and its external surface is applied with magnetic 
conductivity against the inner surface 6a of the permanent magnet ring 6, 
while with its internal surface it limits the dimensions of the braking 
air gap over part of its depth. 
In a similar manner in the embodiment shown in FIG. 3b, a soft iron ring 
12a is provided, which in this case has a height such that by means of its 
internal surface it limits the dimensions of the braking air gap 
externally over its whole depth d.sub.3. The soft iron ring 12a is here 
set into a suitable recess 13 in the lower pole plate 8. 
In contrast to the embodiments of FIGS. 1 to 3a and 3b, in the embodiments 
according to FIGS. 4 and 5 no soft iron ring is provided between the 
annular lower pole plate 8 and the pole core 5. In the embodiment of FIG. 
4 there is provided, between the internal surface of the lower pole plate 
8 and the outer surface of the pole core 5, a ring 14 of limited height by 
means of which the two bodies are connected to one another. The ring 14 
consists of a non-magnetic material such as, for example, brass, 
aluminium, synthetic material or the like. In the embodiment shown in FIG. 
5 a similar effect is produced due to the fact that the annular lower pole 
plate 8 and the pole core 5 are fixed on a plate 15 of non-magnetic 
material. Since there is no longer any bridge of soft iron present, the 
whole magnetic flux passes through the braking air gap in the embodiment 
shown in FIGS. 4 and 5. 
In the embodiment shown in FIG. 6, the annular permanent magnet 6 is made 
up of two partial magnet rings 6b and 6c, each with thickness D.sub.1 or 
D.sub.2, which in sum corresponds to a thickness D of the permanent magnet 
6 of FIG. 1. The upper partial magnet 6b has an internal diameter which is 
equal to the internal diameter of the magnet 6 according to FIG. 1. The 
lower partial magnet 6c has an internal diameter which is equal to the 
external diameter of the braking air gap 9. Thus it forms with its 
internal surface 6d, the external surface of the braking air gap 9 which 
extends as an annular cavity 9b into the lower pole plate 8 so as to 
determine the reduction in cross-section which determines the magnetic 
resistance. The thickness d.sub.1 of the magnet 6b is chosen in such a way 
that the condition that the distance between the open end 9a of the 
braking air gap 9 and the lower side of the upper pole plate 7 is at least 
equal to the height of the working air gap and thus to the thickness 
d.sub.2 of the upper pole plate is again satisfied. 
The advantage of this embodiment, as in the embodiments of FIGS. 3a and 3b, 
resides in the fact that for a predetermined height of the braking air gap 
the thickness of the lower pole plate can be made less than in the 
embodiments of FIGS. 1 and 2. The weight of the magnet system is thereby 
decreased. 
In FIG. 6, to the left of the pole core, there is illustrated the stray 
magnetic flux which, in this embodiment with its lower region 10d directed 
towards the pole core, flows substantially radially through the braking 
air gap 9. 
In a ring magnet system according to the embodiment of FIG. 3b having 
dimensions as follows: 
d.sub.2 =8 mm 
d.sub.1 =14 mm 
d.sub.3 =10 mm 
the magnetic flux density B was measured over the total height d.sub.1 
+d.sub.2 +d.sub.3 by means of a Hall probe, where the measurements were 
limited to a total depth of 30 mm since useful results of measurement 
could no longer be obtained in the neighbourhood of the base of the 
braking air gap. The results of measurement are shown diagrammatically in 
FIG. 7. 
Above the abscissa, the magnetic flux is directed away from the pole core 
and below the abscissa it is directed towards the pole core. As can be 
seen in the diagram, the magnetic flux density is substantially constant 
over the thickness d.sub.2 of the upper pole plate 7, that is over the 
height of the working air gap 3. Over the height d.sub.1, that is between 
the lower side of the upper pole plate 7 and the open end 9a of the 
braking air gap 9, the density of magnetic flux resulting from the stray 
field 10c falls fairly steeply. Thus, the magnetic flux density becomes 0 
at the point Y, that is at a distance of 9 mm from the lower edge of the 
upper pole plate 7. From the point Y onwards the stray field 10d is 
effective. Here the flux density rises again with oppositely directed 
magnetic flux and, at about the region of the open end 9a of the braking 
air gap, reaches its maximum, the magnitude of which depends on the 
magnitude of the magnetic resistance in the lower pole plate. The flux 
density then remains substantially constant over the depth of the braking 
air gap in the region measured. In the diagram the flux density is shown 
in Tesla (T). 
As a comparison measurement, measurements were made of the magnetic flux 
density in an annular gap magnet system of conventional type, that is 
without the braking air gap. 
The flux density in the working air gap is the same as in the magnet system 
with braking air gap. Below the working air gap a flux density was 
measured which corresponds to the dashed curve shown in the diagram. This 
curve falls less steeply and remains above the abscissa in the whole 
region. Immediately on the upper side of the lower pole plate, that is at 
the point X, the flux density is still about 0.3 T. Thus, in a normal 
magnet system no magnetic counterfield is built up which limits the inward 
movement of the moving coil. In fact, up to the upper side of the lower 
pole plate 8, there exists a magnetic field which promotes the inward 
movement of the moving coil and which is the cause of the impact of the 
moving coil against the lower pole plate when the loudspeaker is 
overloaded. 
In contrast, in the annular gap magnet system with the braking air gap as 
described above, impact of the moving coil does not occur even at maximum 
overload of the loudspeaker. The inward movement of the moving coil is, in 
fact, braked by the counter magnetic field generated above the braking air 
gap and is thus limited. 
Annular gap magnetic systems according to the invention are not only useful 
with loudspeakers, but can be used with their full advantage also with 
electromagnetic drives demanding a relatively large undamped stroke. For 
instance said moving coil can be constructed as a driving element for a 
writing element.