Magnetic circuit for planar diaphragm type loudspeaker

A magnetic circuit for a planar diaphragm type loudspeaker having a square planar diaphragm and square voice coil in which four magnetic circuits are arranged in a square corresponding to the edges of the planar diaphragm. Pairs of plates are disposed opposite to one another upon which are attached a number of magnets with the opposed poles opposite one another along the inner walls of the oppositely disposed plates. Each of the magnets is rectangular in shape. A yoke connects the pairs of plates. Holes for permitting air flow are formed either in the yoke or in the pairs of oppositely disposed plates in patterns such as to not disturb the magnetic field. The magnetic circuit units made up by the magnets, pairs of plates, and yokes are joined at their ends by magnetic couplings. An electrical conductive member may be disposed over portions of the magnets to prevent the occurrence of third harmonic distortion due to current distribution.

BACKGROUND OF THE INVENTION 
The present invention relates to magnetic circuits and more particularly to 
a magnetic circuit suitable as the driver unit of a planar diaphragm type 
loudspeaker. 
It is known in the art that a planar diaphragm loudspeaker has a relatively 
flat acoustic pressure characteristic throughout its frequency range and 
accordingly can reproduce sounds with stable acoustic images. A variety of 
loudspeakers employing such planar diaphragms have been proposed in the 
art. Since planar diaphragm type loudspeaker systems of relatively large 
diameter can be readily manufactured, planar diaphragms are suitable for 
manufacturing high power loudspeaker systems. 
However, employment of a large diameter diaphragm involves problems such as 
the provision, at a low manufacturing cost, of a driver unit which can 
sufficiently operate a large diaphragm. More specifically, since a large 
diameter bass-range diaphragm needs a large driver unit, it has proven 
rather difficult to assemble such a driver unit. For instance, large 
magnetic circuits require large plates and magnets. For a square planar 
diaphragm, a large, frame-shaped magnetic circuit is required. Such a 
large magnetic circuit is intricate in construction. Accordingly, it is 
difficult to assemble and process such a large magnetic circuit. 
In a conventional linear type magnetic circuit, the magnetic flux from the 
magnet is introduced through the plate to the field system. That is, the 
conventional linear type magnetic circuit employs a circular external 
magnet type magnetic circuit. Accordingly, its magnetic flux leakage 
coefficient and electromotive force loss are undesirably high and its 
efficiency is low. 
SUMMARY OF THE INVENTION 
Accordingly, in order to overcome these difficulties, an object of the 
invention is to provide a linear type magnetic circuit having a high 
efficiency and which is arranged along a square voice coil of a square 
planar diaphragm type loudspeaker. 
The foregoing object and other objects of the invention have been achieved 
by the provision of a linear type magnetic circuit in which plural 
magnetic circuit units are arranged in the form of a polygon, such as for 
instance a square, and are mechanically and magnetically connected to one 
another with gaps at the corners of the polygon. Each of the magnetic 
circuit units has a number of small magnet units instead of a single 
large, linear type magnet. The small magnetic units are arranged linearly 
so as to have the same effect as that of the large linear type magnet. 
The nature, principle and utility of the invention will become more 
apparent from the following detailed description when read in conjunction 
with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred examples of a magnetic circuit for planar diaphragm type 
loudspeakers according to the invention will be described with reference 
to the accompanying drawings. 
FIG. 1 is a rear view of a loudspeaker unit employing a square planar 
diaphragm and constructed with a first preferred embodiment of a magnetic 
circuit of the invention and FIG. 2 is a perspective view of a portion II 
thereof. Referring first to FIG. 2, the loudspeaker unit has a voice coil 
bobbin 31 whose opposite end portions are bent at 90 degree angles with 
respect to the body of the voice coil bobbin 31 forming flanges 31a and 
31b which are parallel to the sides of the square planar diaphragm 3. The 
flange 31a is secured to the diaphragm 3. A square voice coil 32 is 
coupled to the rear side of the flange 31b and is positioned in the air 
gap of the magnetic circuit 2. In this connection, it should be noted that 
the magnetic circuit 2 is made up of a plurality of magnetic circuit 
units, which are also designated by reference numeral 2, as will become 
more apparent from the discussion which follows. 
The magnetic circuit unit 2 may be provided with a single magnet assembly 
constituted by large magnets of different polarity arranged linearly 
confronting one another. However, in the case of a large bass-range driver 
unit, the magnet assembly is necessarily bulky. Accordingly, its 
manufacturing cost is high since it is necessary to construct it with a 
high precision, and in addition it is rather difficult to magnetize such a 
large magnet assembly. Moreover, if a large magnet assembly were used, it 
is subject to breakage during storage because of the weight and magnetic 
force of the magnet. 
In order to overcome these difficulties according to the invention, as 
shown in FIG. 2, a pair of elongated plates 21 are disposed opposite each 
other and a number of small magnet units 22 are arranged along the edge 
portions of the plates 21 with different polarity units disposed opposite 
one another. The opposite edge portions of the plates 21 are coupled 
through a yoke 23 to form a rigid unit. However, the unit constituted by 
the plates 21 and the yoke 23 may be replaced by a single member having a 
U-shape in section. 
Since the saturated magnetic flux density of the magnet assembly depends 
upon the material of the plates 21, the sectional area of the plates 21 
should be determined in accordance with the material of the plates 21. 
With the magnetic circuit formed as described above, unless the gap in the 
field system is overly increased, the theoretical magnetic flux leakage 
coefficient is unity and hence the magnetic flux density at the magnet 
operating point is equal to the magnetic flux density of the field system 
whereby a magnetic circuit having a very high efficiency is provided. 
In the above-described embodiment, the magnetic circuit units can be 
readily assembled together with high accuracy as they are constructed as 
unified components. Because they are of simple configuration, the plates 
can be formed by stamping at low manufacturing cost. 
Second and third examples of a magnetic circuit unit 2 of the invention are 
shown in FIGS. 3 through 6. As is clear from FIGS. 3 through 6, the 
magnetic circuits 2 are of the internal magnet type. In FIGS. 3 through 6, 
those components which have been previously described with reference to 
FIGS. 1 and 2 are similarly numbered. 
The second and third examples are intended to eliminate a difficulty in 
that the flow of air produced by the piston motion of the diaphragm 3 and 
the motion of the voice coil 32 adversely affects the field system. The 
air in the space behind the diaphragm 3 and the voice coil 32 is 
compressed and expanded by the movement of the diaphragm 3 and the voice 
coil. These movements are disturbed by the viscous resistance of the air 
as a result of which characteristic distortion is sometimes caused. In 
order to overcome this difficulty, the second and third embodiments have 
been developed. 
A plurality of air holes 24 of suitable size and configuration are formed 
in the plates 21 or the yoke 23. The dimensions and positions of the holes 
are chosen such that their presence does not make the magnetic flux 
patterns irregular. The air holes 24 can be formed in either or both of 
the plates 21 and the yoke 23. 
As a result of the provision of the air holes 24, the flow of air produced 
by the piston motion of the diaphragm 3 is relieved from the narrow space 
in the field system as shown in FIG. 5. Moreover, a cooling effect is 
produced by the flow of air. If it is desired to increase the cooling 
effect, then the air holes 24 should be formed confronting each other. If 
no air holes were provided, as the voice coil moved, air would be sucked 
into and discharged from the region of the magnetic circuit thus 
generating frictional noises. However, the generation of such noises is 
eliminated because the interior of the magnetic circuit is effectively 
opened to the atmosphere through the air holes. 
FIGS. 7 and 8 show a fourth embodiment of the magnetic circuit unit 2 which 
is an external magnet type of a magnetic circuit unit. In this case, one 
surface of each magnet unit 22 is exposed and therefore it is desirable 
that a protective film 27 be bonded to the outer surface of the magnetic 
circuit. 
Fifth and sixth embodiments of an internal magnet type magnetic circuit 
unit 2 of the invention are shown in FIGS. 9 and 10, respectively. In 
these two embodiments, provision is made for reducing third harmonic 
distortion due to current distortion. More specifically, in the magnetic 
circuit in FIG. 9, electrically conductive sheets 25 made of copper or 
aluminum are bonded to the opposed surfaces of the magnet units 22. The 
electrically conductive sheets 25 may be applied to one or both of the 
opposed surfaces. In the magnetic circuit in FIG. 10, caps 26 of 
electrically conductive material such as copper or aluminum are disposed 
over the end portions of the plates 21 and the magnet units 22. 
The above-described magnetic circuit units 2 are arranged in such a manner 
that adjacent units 2 are at 90.degree. angles as shown in FIG. 11 and are 
mechanically and magnetically connected with connectors 4 as shown in 
FIGS. 12(a), 12(b), 13, 14(a), 14(b), 15(a) and 15(b). 
The connector 4 shown in FIGS. 12(a) and 12(b) is made of magnetic metal 
and has insertion recess portions 41 at both ends and a coupling portion 
42 through which the insertion recess portions are coupled to each other. 
One end portion of the magnetic circuit unit 2 is inserted into the 
insertion recess portion 41. The coupling portion 42, which is bent in 
conformance with the angle formed by adjacent magnetic circuit units 2, 
has a hollow part 42a communicating with the air gaps in adjacent magnetic 
circuit units 2 connected thereto. A corner portion of the voice coil 32 
is accommodated in the hollow part 42a of the coupling portion 42 so that 
the leakage magnetic flux of the magnets 22 is introduced into the hollow 
part 42a as a result of which the effective length of the voice coil is 
increased. 
The connector 4 shown in FIG. 13 is made up of a mechanical connecting 
member and a magnetic connecting member. More specifically, the connector 
4 includes a mechanical connecting member 43 which is positioned obliquely 
with respect to the end portions of adjacent magnetic circuit units 2 
which are arranged at right angles, and also includes an elbow type 
connecting member the ends of which are in close contact with the end 
faces of the adjacent magnetic circuit units 2, thus magnetically 
connecting the magnetic circuit units 2. It should be noted that the 
connecting member additionally performs a heat radiating function cooling 
the corresponding corner portions of the voice coil 32 which does not 
extend into the air gap between the opposed magnets 22. The mechanical 
connecting member 43 may be made of magnetic material. However, it should 
be connected so that it does not short any of the magnets 22. 
The connector 4 shown in FIGS. 14(a) and 14(b) is an L-shaped metal member 
45 which is secured to the inner walls of adjacent magnetic circuit units 
2 with screws. The connector has a flange 45a which is placed in contact 
with the upper edges of the magnetic circuit units 2 which are of the 
internal magnet type as shown in FIGS. 7 and 8. 
The connector 4 shown in FIGS. 15(a) and 15(b) has two grooves 46a which 
are perpendicular to each other and which are adapted to receive the end 
portions of adjacent magnetic circuit units 2 and a guide 46b used for 
positioning the inner walls of adjacent magnetic circuit units 22 at the 
respective corners. Thus, using four such connectors, four magnetic 
circuit units 2 arranged in a square can be accurately positioned relative 
to one another and rigidly connected to one another. The connector 4 is 
preferably made of magnetic material. 
An example of a 4-way planar diaphragm type loudspeaker employing a 
magnetic circuit of the invention is shown in FIG. 16. A plan view of this 
loudspeaker from the rear side is shown in FIG. 17. The loudspeaker 
includes a bass-range diaphragm 3A, a mid-range diaphragm 3B, a 
treble-range diaphragm 3C, and a super-treble-range diaphragm 3D. In FIG. 
16, reference numeral 14 designates a groove formed in a connection arm 13 
used to position the above-described magnetic circuit unit 2, 17 a groove 
which is formed in a coupling arm 16 and which is used to position a 
mid-range magnetic circuit unit 2'. The coupling arm 16 provided between 
an inner frame 12 and a central frame 15 supports the inner edge portion 
of the mid-range diaphragm 3B and extends perpendicularly to the 
respective sides of the inner frame 12. Reference numeral 33 designates a 
damper and 34 a damper base supporting the damper 33. 
As is clear from the above description, the connection arms 13 forming the 
frame assembly of the loudspeaker are provided with grooves 14 into which 
the magnetic circuit units 2 are fitted so that the magnetic circuit units 
2 can be mounted so as to leave accurate air gaps with respect to the 
voice coil 32. With this construction, the driving force of the voice coil 
is positively transmitted to the entire surface of the diaphragm. 
FIG. 16 shows connection arms 13 which extend in the form of the character 
"V" to the outer frame 11 of the loudspeaker. However, it should be noted 
that the technical concept of the invention is applicable to connection 
arms which have the form of parallel crosses as shown in FIG. 1. 
As is apparent from the above description, according to the invention, the 
magnetic circuit of the loudspeaker is formed with a plurality of straight 
magnetic circuit units each having a number of small magnets and air 
flowing holes. The straight magnetic circuit units are mechanically and 
magnetically connected to one another at the corners or ends thereof to 
form a complete magnetic circuit as a result of which the effective length 
of the voice coil is increased and the driving force at the corners of the 
square voice coil can be effectively utilized. Thus, a small magnetic 
circuit of high efficiency and which is simple to manufacture and maintain 
is provided with the invention. Since the straight magnetic circuit units 
are connected at the corners, they can easily be assembled and there is no 
problem that dust such as iron powder may stick to the magnet in the 
magnetic circuit.