Electromagnetic transducer of improved efficiency

An electromagnetic transducer is disclosed wherein separate elongated strips of magnetic material are secured on both sides of a plane containing a flat flexible insulating thin-film diaphragm. Conductors attached to the diaphram extend primarily parallel to the magnetic strips and cause movement of the diaphram when excited electrically. The conductors can also deliver current induced by diaphram movement within the field. The magnetic strips are fastened to a frame that clamps the perimeter of the diaphram. The sub-assemblies containing the magnetic strips provide maximum available area for the propagation of acoustic waves to and from the environment and provide increased flux density.

BACKGROUND OF THE INVENTION 
1. Field of the Invention. 
The invention relates to a planar electromagnetic transducer that can be 
used in loudspeakers, headphones, microphones, or devices of a similar 
nature. 
Planar magnetic loudspeakers have enjoyed distinct advantages over dynamic 
cone direct radiator type loudspeakers. They exhibit wider bandwidth and 
phase characteristics than typical cone units. Planar magnetics are at a 
disadvantage when compared to electrostatic type planar loudspeakers which 
exhibit better linearity. They are also at a disadvantage when compared to 
direct radiator cone loudspeakers with respect to efficiency. 
The disadvantages of planar magnetics when compared to electrostatic type 
planar loudspeakers are the result of several factors. The electrostatic 
loudspeakers have the advantage of being able to drive a diaphragm 
uniformly over its entire area, which prevents a diaphragm breakup, as 
well as to drive it linearly by means of a constant charge operation. 
An inherent problem with planar magnetic loudspeakers is driving the 
diaphragm uniformly across the entire area without becoming non-linear. 
See discussion in the patent to Torgeson, U.S. Pat. No. 4,468,530. The 
planar magnetic designs which have magnets on only one side of the 
diaphragm are inherently non-linear as indicated in the patent to Winey 
U.S. Pat. No. 3,674,946. The planar magnetic loudspeakers with like poles 
opposed on either side of the diaphragm are efficient, but have been 
demonstrated to have a linear magnetic field only within a narrow region 
between the magnets as indicated in the Torgeson patent. If conductors are 
placed across the entire area of the diaphragm, non-linearity results. The 
only known prior art type of planar magnetic loudspeaker that can be 
linear and have uniform drive area is one where the magnets are located on 
both sides of the diaphragm with alternate poles and with the magnets 
staggered so that flux lines run diagonally across the diaphragm. This 
approach in order to be linear must have conductors substantially covering 
the entire diaphragm as indicated in the patent to McKay U.S. Pat. No. 
3,939,312. Because of the diagonal flux lines, the transfer efficiency is 
lower than that of other planar magnetics, electrostatics, and cone type 
loudspeakers. 
2. Description of the Prior Art 
The Patent to McKay U.S. Pat. No. 3,939,312 discloses a transducer having 
sheets of magnets in which the poles are alternately spaced so that the 
flux lines are disposed in a zig-zag fashion across the space between the 
two lines of magnets. Conductors are positioned in the space to intercept 
the magnetic flux that is present in the zones between the magnets. The 
plates which carry the magnets are pierced with a pattern of holes to 
permit passage of the sound vibrations from the diaphram on which the 
conductors are mounted. 
The Patent to High U.S. Pat. No. 1,815,564 also discloses the use of 
magnetic poles alternately arranged providing a flux field between which 
the diaphram carrying conductors is positioned. 
The Patent to Winey U.S. Pat. No. 3,674,946 discloses the use of a 
plurality of vibratable diaphrams formed to vibrate independently of each 
other and which are electrically isolated in an electromagnetic transducer 
having magnetic material located on one side of the diaphram. 
Other Patents to Winey U.S. Pat. Nos. 4,471,172; 4,471,173; and 4,480,155 
disclose elongated magnet strips carried on a sheet of magnetic material 
or connected by narrow bridges and mounted in pairs so that the flux field 
passes between adjacent magnets and through the conductors on a diaphram 
in a space between oppositely positioned magnets. Various arrangements of 
magnets and conductors are disclosed. The conductors may be formed in 
various ways, such as with round wire, foil, or metal film deposited on 
the film and etched away. A multiplicity of parallel conductors in a band 
is also disclosed. 
The Patent to Whelan U.S. Pat. No. 4,156,801 discloses a diaphram having 
conductors thereon in a space between opposed pairs of magnets of like 
polarity and in which undriven areas of the diaphram are baffled to 
minimize the effects of vibrations. 
The Patent to Torgeson U.S. Pat. No. 4,468,530 discloses a diaphram having 
conductors thereon in a space between opposed pairs of magnets of like 
polarity, the diaphram being divided into sections by means of rubber 
separator strips, and the arrangement of magnets within the various 
sections differing in order to provide zones of different frequency. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved 
electromagnetic transducer. 
A further object is to provide an electromagnetic transducer having an 
improved permanent magnetic field structure, resulting, among other 
advantages, in an increase in its efficiency. 
A further object is to provide an electromagnetic transducer in which the 
positioning of the conductors upon the diaphragm within the permanent 
magnetic fields results in an increase in its linearity. 
It is a further object to provide an electromagnetic transducer in which, 
due to the arrangement of a plurality of conductors, the diaphram is 
driven uniformaly over its entire area and various parts of the diaphram 
can be controlled or driven separately. 
A further object is to provide an electromagnetic transducer used as a 
loudspeaker in which, the arrangement of conductors permits low, 
mid-range, and high frequency drivers to co-exist on the same diaphram. 
A still further object is to provide an electromagnetic transducer used as 
a loudspeaker in which the arrangement of conductors can be used to permit 
portions of a single diaphram to produce separate phase differentiated and 
or time delayed signals to enhance the sound stage imaging of a stereo 
signal. 
To accomplish these objectives, the present invention includes separate 
elongated strips of permanently magnetic material alternately spaced and 
secured on both sides of a plane containing a flat flexible insulating 
thin-film diaphram. The use of separate strips of magnetic material makes 
it possible to control the amount of open area available for the coupling 
of the acoustic wave. The shape and magnetic permeability of the magnetic 
strip directs the flux towards the diaphragm and yields an increase in 
efficiency and band width over previous transducers of this type. The 
strips need not be the same distance from the diaphram nor from each 
other. It is important that they do not hinder the movement of the 
diaphram and that they allow a sufficient space for acoustic wave 
communiation. 
Attached to the diaphram are conductors shaped into thin oblong coils. The 
conductors are arranged so that a large percentage of the total conductor 
length is parallel to the elongated magnetic strips and therefore within a 
static magnetic flux field and so that the diaphragm is substantially 
covered, as shown. 
A group of conductors may be within a static flux field from a group of two 
or three magnetic strips. Further, any given magnetic strip may be used in 
association with more than one conductor group. 
Any conductor group or element within a flux field created by the magnetic 
strips can be used to independently drive the portion of the diaphram that 
it covers. Time and frequency altered signals from different parts of the 
spectrum can be direct to selected areas of the diaphram thereby 
permitting distinct drivers to co-exist on the same diaphram. Further, a 
small amount of an inverted signal from a cooperating stereo or 
quadraphonic channel transmitted by some of the conductor groups in a 
loudpeaker of this design, would serve to attenuate, for a given listening 
ear, the effects of interaural crosstalk from the main driver of the 
cooperating loudspeaker, thereby aiding perception of localization in the 
reproduced sound stage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the drawings, a planar electromagnetic transducer in the 
form of a unit loudspeaker is illustrated. It has a flat flexible 
insulating thin-film diaphragm 10 in a rectangular shape with upper side 
and end frame members 11, 12 and lower side and end members 13 and 14 
holding the perimeter edges. Attached to the diaphragm are conductors 19 
which may be arranged in elements 20 in various configurations, thin 
oblong coil patterns, as shown, being preferred. Unitary elongated 
permanent magnetic strips 22 alternately spaced and secured to both sides 
of the diaphragm, conductors, and frame. 
The diaphragm may comprise any suitable material that is rugged, yet 
flexible, has shape memory, and is electrically insulating. These 
characteristics may be found in many different kinds of thin-films such as 
a polyester, Mylar made by E. I. Du Pont de Nemours & Co. Inc., 
Circleville, Ohio. A thin layer of adhesive material is used to attach the 
diaphragm 10 to the frame. A suitable adhesive is the Joining Systems with 
Isotac P.S. available in a roll of 5 mil thick tape from the 3M Co., St. 
Paul, Minn. 55101. The tensioning of the diaphragm prior to adhering it to 
the frame may be done in any desired manner well known in the art. 
The frame members are preferably of a rigid material such as aluminum of 
sufficient strength to hold the diaphragm under tension and may be 
anodized to provide good cosmetic appearance and electrical insulation 
from the conductors 19. Frame end members 12, 14 are extended to provide a 
pocket or space 23 to receive an insulating terminal board 24 that is 
connected thereto by suitable means such as adhesive. Screws 27 and 28 
(see FIG. 4) are connected to pads 29, 30 at each end of the elements and 
extend through the terminal board for retention by nuts 31, 32, providing 
terminals for the elements. 
Conductor elements 20 (see FIG. 2) can be produced by etching the desired 
configuration on a thin sheet of aluminum foil that has been laminated to 
the Mylar. The foil/polyester laminate is available from Lamotite, Inc. of 
Cleveland, Ohio. The first step in the etching process is to "freeze" the 
laminate to glass or another smooth flat material. This is done by using a 
squeegee to spread a thin layer of water between the laminate and the flat 
material. The result is a smooth working surface of foil upon which a 
design can be silk screened. After the ink dries the material is etched in 
a Ferric Chloride solution. The ink is then removed and only the elements 
remain adhered to the diaphragm substrate. 
FIG. 2 is a detail of a corner portion of the plan view of FIG. 1, the 
insulating terminal board 24 being partially broken away. Conductor 
elements with 3 turns are illustrated, although other arrangements may be 
used. FIG. 3 shows a diaphragm 10 with conductor elements 20 mounted on 
both sides of the diaphrapm, although in certain situations one side 
attachment may be preferred. Such mounting on both sides provides 
flexibility in selecting the total impedance of the transducer by the 
series or parallel wiring of the elements. 
The assembly includes the magnetic strips 22 that are alternately spaced 
across the diaphragm 30 and secured by spaced upper and lower cross bars 
35 and 36. Bars 36 are extended and may support baffles 37 and 38 beyond 
the sides of the unit loudspeaker to prevent attenuation of low 
frequencies. The baffles may be made of wood or other suitable sound 
damping material. The width of the loudspeaker may be increased or 
decreased by adding or removing magnetic strips and elements on either 
side. 
The elongated strips 22 of magnetic material may be formed by short 
rectangular ceramic magnets 40 laid end-to-end in an elongated U-shaped 
channel 42 (see FIG. 5). A suitable magnet is the Genox 5 available from 
General Magnetic Co., Dallas, Tex. 75211. The channels are fastened by 
suitable means, such as welding, to the upper and lower cross-bars 35 and 
36 in proper position with respect to each other and to the diaphram. The 
cross-bars are connected by fastening means 41 extending through the frame 
members and spacers 44. 
The magnet channels or strips 42 provide increased efficiency necessary for 
wide band operation. In a planar magnetic loudspeaker, magnetic field 
strenght over the area of the diaphragm is required. Therefore, a large 
surface area of magnetic material is necessary. 
In some prior magnetic transducers, such as Torgeson U.S. Pat. No. 
4,468,530, the lines of magnetic flux run at right angles to the 
conductors on the diaphragm. Thus, the conversion efficiency (electrical 
input to acoustical power output) is good. In the present system, the 
lines of flux pass through the conductors 19 at an angle (see FIG. 3) 
dependent upon the magnetic spacing. The driving force decreases by the 
cosine of that angle which results in lower conversion efficiency. FIG. 6 
indicates the use of a combination of alternative strip arrangement 
patterns. One pattern is seen on the left side of FIG. 6, and is similar 
to that of FIG. 3. The other pattern on the right side of FIG. 6, is used 
to drive two of the elements on one side of the transducer. The two 
elements on the right side are used in high frequency reproduction. 
When the transducer is used as a loudspeaker it is desirable to reduce the 
higher frequency portions to a narrow strip to increase the sound 
dispersion characteristics at those frequencies. Since the mass of the 
diaphragm and air mass begin to have a greater effect on the output of a 
speaker at very high frequencies, more magnetic force is required to 
maintain constant output. The altered pattern includes three sets of 
relatively narrow magnetic strips 52, 52', 53, 53', and 54, 54' facing 
each other with like poles across the diaphram. The three sets are 
separated by two magnetic strips 55, 56 that are on opposite sides of the 
diaphram with no magnetic strip directly opposite them. The magnetic poles 
of the strips 55, 56 are opposite to that of the corresponding poles on 
the three sets of magnetic strips. This arrangement provides efficient 
driving of the high frequency portion of the diaphram. In addition, the 
structural support of the channels carrying the strips may be aided by the 
magnetic repulsion of the opposing sets of magnetic strips. 
FIG. 7 illustrates a sound and vibration damper 60 in the form of a 
silicone glue or other suitable material in a thin layer between the 
magnetic material and the U-shaped channel. This prevents buzzing noises 
caused by the vibration of the magnetic material.