Magnet assembly

A loudspeaker design for use in transmitting and receiving sound waves is disclosed. The loudspeaker consists of a basket (12) and magnet assembly (14). The magnet assembly consists of a sandwich of front (16) and back (20) plates on either side of a ring-shaped ceramic magnet (18). The back plate (20) has a pole piece (22) projecting outward therefrom. The ceramic magnet (18) has a center aperture. The magnet (18) is affixed to back plate (20) concentrically around the projecting pole (22). The front plate (16) is circular in shape and also has a center aperture (31). A centering device and shield (24) is placed concentrically around the projecting pole (22) and is adjacent to the inside surface of the back plate (20), the inner surface of the aperture (30) within the ceramic magnet (18), and the inner surface of front plate (16) when it is affixed to the ceramic magnet, thereby creating the above-mentioned sandwich. The front plate (16) has a lip (34) on its outer surface and a recess (42) in its inner surface. Lip (34) has an outer diameter (38) slightly less than that of the center hole in the speaker basket (12). The speaker basket is attached to the magnet assembly by placing lip (34) within the hole in the speaker basket and staking the two together. The inner recess (42) of the front plate (16) is designed to accept the vertical wall of the magnet centering device and shield (24) so as to prevent the entry of particles of magnet and other foreign materials into the air gap (46) in which the speaker voice coil must move.

TECHNICAL FIELD 
The present invention relates to sound transducer assemblies, more 
particularly the attachment of a ceramic magnet assembly to a loudspeaker 
basket. 
BACKGROUND OF THE PRIOR ART 
As is well known, a loudspeaker comprises a frame or "basket" which 
supports the rim of a speaker cone of light, fairly rigid material such as 
stiff paper, the cone terminating near its apex by a cylindrical "voice 
coil", through which the audio frequency energization to the speaker 
passes. The basket also supports a magnet assembly to create a fixed 
radial magnetic field in which the voice coil can move axially because of 
the resilience of the speaker cone. Magnet assemblies typically employ 
ceramic magnets although some use alnico magnets. Many magnet assemblies 
employ a flat front plate which forms part of the assembly and serves as 
the surface to which the basket is attached, as well as completing the 
magnetic path outside the voice coil, which it is apertured to pass. In 
the prior art, numerous methods were employed to secure the front plate to 
the basket, including welding or bolting the two parts together or forming 
staking studs in the front plate and piercing the basket with 
corresponding holes for accepting the studs, and then staking the studs 
thereby, bonding the two parts to each other. 
All of these prior methods had significant drawbacks which resulted in 
increased cost of production. Bolting, for example, requires expensive 
drilling and tapping of the front plate. Stud staking requires specially 
designed front plates and the extra step of staking. In the welding 
process, alignment of the basket and front plate is difficult and quality 
control is a problem. These methods have also proved unsatisfactory for 
use in attaching a large magnet assembly to a small basket due to the lack 
of available surface area on the basket necessary to weld, bolt or stud 
stake. 
SUMMARY OF THE INVENTION 
The present invention comprises an improved loudspeaker structure which 
facilitates the connection of a magnet assembly to a basket without 
introducing the problems encountered in known methods. To accomplish this, 
the front plate of the magnet assembly is provided with a raised lip 
around its voice coil aperture, and the basket has a central aperture into 
which the lip fits. By staking the lip into the rim of the basket aperture 
at sites therearound, the front plate and basket are securely connected 
together without any space between them. 
In a preferred form of the invention, a magnet centering device and shield 
is located between the front and back plates of the magnet assembly, in 
peripheral contact with the inner surface of the annular magnet, and this 
device extends axially beyond the front surface of the magnet into an 
annular recess formed in the back surface of the front plate. The annular 
recess may be dispensed with the axial extent of the centering device and 
shield being the same as that of the magnet, so that the shield preferably 
contacts the side of the front plate which faces the magnet. 
Various advantages and features of novelty which characterize the invention 
are pointed out with particularity in the claims annexed hereto and 
forming a part hereof. However, for a better understanding of the 
invention, its advantages, and objects attained by its use, reference 
should be had to the drawings which form a further part hereof, and to the 
accompanying descriptive matter, in which there are illustrated and 
described preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings in detail, there is shown in FIG. 1 a loudspeaker 
assembly generally designated by the numeral 10 having a speaker cone 11, 
a basket 12, and a magnet assembly 14 including a front plate 16, a 
ring-shaped ceramic magnet 18 and a back plate 20. It should be noted that 
although magnet assembly 14 is shown as circular in shape, it is not 
limited to that form and may be square, etc. 
An exploded view of magnet assembly 14 is shown in FIG. 2 of the drawings. 
Attached to the back plate 20 is a projecting pole piece 22. Both plate 20 
and pole 22 are made of steel. Placed concentrically around the pole piece 
22 is a magnet centering device and shield 24. A ceramic magnet 18 has 
flat pole faces and a circular aperture having an inner periphery 30 which 
is larger than the diameter of the pole 22 to define an annular space 
therearound. The front plate 16 has a central aperture 31 larger than pole 
piece 22, a front face 32, and a lip 34 having generally vertical walls 
projecting substantially perpendicularly from the front face and 
surrounding aperture 31. The lip 34 also has an outer diameter 38. The 
back face 40 of front plate 16 is shown in FIG. 3 of the drawings. Back 
face 40 contains an annular recess 42 of a radius larger than aperture 31 
and extending thereto. 
The centering device and shield 24 is made of a non-magnetic material and 
has a cylindrical wall 26 and a bottom wall 28 which has an aperture 
therein. The centering device and shield is more completely described in 
my U.S. Pat. No. 3,898,393. 
An axial section of the magnet assembly 14 is shown in FIG. 4 of the 
drawings. In this drawing, the cylindrical wall 26 of the centering device 
and shield 24 can be seen to have snug axial engagement with the recess 42 
in the front plate 16. This is the preferred construction although 
manufacturing tolerance may produce some small space therebetween. It is, 
of course, also possible to use a centering device and shield with a wall 
of a height less than would be necessary to engage recess 42. 
Additionally, cylindrical wall 26 is preferably intimately disposed 
against the inner periphery 30. 
The magnet assembly is typically held together as shown in FIG. 4 in a 
sandwich-like fashion where the ceramic magnet 18 is adhesively held to 
the front plate 16 and back plate 20 to define a voice coil air gap 46. A 
speaker basket 12 having a center hole is now placed over the lip 34 and 
then the lip may be staked to the basket to mount the speaker basket 
securely to the front plate and thus to the entire magnet assembly. This 
method of manufacture is completely sufficient in itself to hold the 
magnet assembly to the speaker basket. However, if desired, it is possible 
to weld, bolt or stud stake the basket to the front plate in addition to 
staking lip 34 for added rigidity if required. My staking method has 
numerous advantages over prior methods of mounting as discussed above. 
With a completely round and airtight seal between the speaker basket and 
front plate there is virtually no possibility of foreign particles 
entering the air gap 46 through the basket to front plate interface 60. A 
gasket (not shown) can be placed between the front plate and the speaker 
basket before staking in order to reduce resonance of the two parts should 
it be desired. 
Foreign particles, including chips of ceramic magnet, are prevented from 
entering the air gap 46 from the interface 60 between the basket and front 
plate, or along the interfaces 52 and 54 between the magnet and front and 
back plates, by the magnet centering device and shield 24 and its 
cooperation with recess 42 in front plate 16. Ceramic chips from magnet 18 
are completely blocked from entering gap 46 by the snug or close fit of 
shield 24 in the recess 42 and also against the plate 20. In the prior 
art, the back side of the plate 16 lacked recess 42 and thus it was 
possible even with shield 24, for particles to find their way past shield 
24 to the air gap 46. 
Preliminary investigation has indicated that the use of this front plate 
with the lip 34 and recess 42 may improve the linearity and efficiency of 
the magnetic field in the air gap 46. Linearity is a significant advantage 
in loudspeakers as non-linearity in the field causes a non-linear movment 
of the voice coil and in turn the speaker cone, creating a distorted audio 
response. Increased efficiency is a significant advantage in loudspeaker 
magnet circuits since increase in magnetic energy can result in increased 
sound pressure level and/or increased power handling ability. Comparisons 
were made of the total energy of the magnetic circuits by measuring the 
air flux (in maxwells) using a search coil and electronic integrator. It 
may be possible to further modify the magnetic field distribution in the 
air gap by altering the shape of recess 42 from the configuration shown to 
a non-flat configuration. 
It is understood that although the invention is described in detail with 
respect to a ceramic magnet speaker assembly, the improved front plate as 
disclosed is equally applicable to other magnet assemblies having a flat 
front plate. 
Numerous characteristics and advantages of the invention have been set 
forth in the foregoing description, together with details of the structure 
and function of the invention, and the novel features thereof are pointed 
out in the appended claims. The disclosure, however, is illustrative only, 
and changes may be made in detail, especially in matters of shape, size 
and arrangement of parts, within the principle of the invention, to the 
full extent extended by the broad general meaning of the terms in which 
the appended claims are expressed.