U.S. patent number 3,686,446 [Application Number 04/885,658] was granted by the patent office on 1972-08-22 for push-pull moving coil loudspeaker having electromagnetic centering means.
Invention is credited to Josef W. Manger.
United States Patent |
3,686,446 |
Manger |
August 22, 1972 |
PUSH-PULL MOVING COIL LOUDSPEAKER HAVING ELECTROMAGNETIC CENTERING
MEANS
Abstract
An electrodynamic acoustic transducer such as a loudspeaker or
microphone in which the datum or center position of the diaphragm
is determined by two coils. The coils are fixed to one another and
to the diaphragm and cut magnetic fields. Datum currents are
provided for the coils and the coils are of non-uniform density of
winding and/or the magnetic fields are non-uniform. The coils and
the datum currents are arranged to provide balanced forces on the
coils only when the diaphragm is in the datum position.
Inventors: |
Manger; Josef W. (8725
Arnstein, DT) |
Family
ID: |
5716741 |
Appl.
No.: |
04/885,658 |
Filed: |
December 16, 1969 |
Foreign Application Priority Data
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Dec 19, 1968 [DT] |
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P 18 15 694.0 |
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Current U.S.
Class: |
381/117; 381/401;
381/402 |
Current CPC
Class: |
H04R
7/02 (20130101); H04R 9/063 (20130101); H04R
9/041 (20130101) |
Current International
Class: |
H04R
7/02 (20060101); H04R 9/00 (20060101); H04R
7/00 (20060101); H04R 9/04 (20060101); H04R
9/06 (20060101); H04r 009/02 () |
Field of
Search: |
;179/115.5R,115.5PS,115.5VC,115.5DV,180 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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312,950 |
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Jun 1929 |
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GB |
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37,272 |
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Nov 1930 |
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FR |
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Other References
Symmetrical Properties of Transistors and Their Applications,
Sziklai, Proceeding Inst. Radio Engr., June 1953, pp. 717,
719..
|
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kundert; Thomas L.
Claims
What is claimed is:
1. In an electrodynamic acoustic transducer with a vibratory
diaphragm, the provision of centering means for returning the
diaphragm to a datum position and including
a. two moving coils,
b. means for providing magnetic fields cutting said coils,
c. mechanical means connecting said coils to one another and to
said diaphragm,
c'. said vibratory diaphragm being arranged to vibrate without any
mechanical restraining element affixed between said mechanical
means and said means for providing magnetic fields cutting said
coils, and
d. means for providing two datum currents in said coils
respectively to interact with said fields and produce balanced
forces thereon solely at said datum position,
said diaphragm being returned to said datum position solely by
electromagnetic forces,
said two moving coils having a common axis disposed perpendicular
to lines of flux of said magnetic fields cutting said coils,
and
said two moving coils being spaced apart from each other along the
direction of said common axis.
2. A transducer according to claim 1, wherein said fields are
inhomogeneous.
3. A transducer according to claim 1, wherein said coils are wound
with non-uniform density of winding.
4. A transducer according to claim 1, wherein said magnetic fields
are provided by pot magnets.
5. A transducer according to claim 4, wherein said pot magnets open
towards one another.
6. An electrodynamic acoustic transducer with a vibratory diaphragm
and centering means for returning said diaphragm to a datum
position, said centering means including:
two moving coils;
means for providing magnetic fields cutting said coils;
mechanical means connecting said coils to one another and to said
diaphragm;
means for providing two datum currents in said coils respectively
to interact with said fields and produce balanced forces thereon
solely at said datum position;
said coils being coaxial;
said two moving coils having a common axis disposed perpendicular
to lines of flux of said magnetic fields cutting said coils;
said two moving coils being spaced apart from each other along the
direction of said common axis;
said magnetic fields being provided by pot magnets; and
wherein said pot magnets open away from one another.
7. A transducer according to claim 6, wherein said forces acting
oppositely on said coils act away from one another.
8. An electrodynamic acoustic transducer with a vibratory diaphragm
and centering means for returning said diaphragm to a datum
position, said centering means including:
two moving coils;
means for providing magnetic fields cutting said coils;
mechanical means connecting said coils to one another and to said
diaphragm;
means for providing two datum currents in said coils respectively
to interact with said fields and produce balanced forces thereon
solely at said datum position;
said coils being coaxial;
said diaphragm being returned to said datum position solely by
electromagnetic forces;
said two moving coils having a common axis disposed perpendicular
to lines of flux of said magnetic fields cutting said coils;
said two moving coils being spaced apart from each other along the
direction of said common axis;
said magnetic fields being provided by pot magnets;
said pot magnets open towards one another; and
wherein said forces acting oppositely on said coils act towards one
another.
9. An electrodynamic acoustic transducer with a vibratory diaphragm
and centering means for returning said diaphragm to a datum
position, said centering means including:
two moving coils;
means for providing magnetic fields cutting said coils;
mechanical means connecting said coils to one another and to said
diaphragm;
said diaphragm being returned to said datum position solely by
electromagnetic forces;
said two moving coils having a common axis disposed perpendicular
to lines of flux of said magnetic fields cutting said coils;
said two moving coils being spaced apart from each other along the
direction of said common axis;
means for providing two datum currents in said coils respectively
to interact with said fields and produce balanced forces thereon
solely at said datum position; and
wherein said coils are coaxial and said fields are provided by a
single pot magnet.
10. A transducer according to claim 1, wherein said coils of the
centering means are also operating coils of the transducer and said
datum currents are superimposed upon operating currents and produce
out-of-balance forces on said coils.
11. A transducer according to claim 1, wherein said coils, said
magnetic-field providing means, and said mechanical means are
surrounded by a hollow body rigidly connected to said coils.
12. A transducer according to claim 11, wherein said hollow body
carries said diaphragm.
13. A transducer according to claim 11, wherein said diaphragm is
mounted upon said hollow body.
14. An electrodynamic acoustic transducer with a vibratory
diaphragm and centering means for returning said diaphragm to a
datum position, said centering means including:
two coils;
means for providing magnetic fields cutting said coils;
mechanical means connecting said coils to one another and to said
diaphragm;
means to provide two datum currents in said coils respectively to
interact with said fields and produce balanced forces thereon
solely at said datum position;
wherein said means to provide said datum currents comprise a
circuit arrangement with two batteries to supply said two datum
currents respectively in the same sense, and
means are provided to vary said currents differentially.
15. A transducer according to claim 14, wherein said batteries are
connected to said coils through two transistors respectively and
means are provided to vary the base currents of said transistors
differentially.
16. An electrodynamic acoustic transducer with a vibratory
diaphragm and centering means for returning said diaphragm to a
datum position, said centering means including:
two coils;
means for providing magnetic fields cutting said coils;
mechanical means connecting said coils to one another and to said
diaphragm;
means for providing two datum currents in said coils respectively
to interact with said fields and produce balanced forces thereon
solely at said datum position;
said coils, said magnetic-field providing means, and said
mechanical means are surrounded by a hollow body rigidly connected
to said coils; and
wherein said magnetic fields are provided by pot-magnet means and
means for coaxial centering are provided in the form of rubber
stockings fixed to the hollow body and during axial movement of the
hollow body rolling in the axial direction with sliding seating in
the hollow body.
Description
The present invention relates to electro-dynamic transducers. The
most commonly used loudspeakers and microphones comprise a
cylindrical moving coil in the air-gap of a permanent magnet which
provides a radial field through the coil. The electric power
supplied to the moving coil is converted into acoustic power by a
cone diaphragm attached to the coil.
It is known that the moving parts, that is to say, the moving coil
and cone diaphragm, form a vibrating system having a pronounced
resonance frequency or frequencies, mainly dependent on the mass of
the vibrating system and the return force or spring force. Below
the resonance frequency, the power radiated depends substantially
on the spring force or compliance, and above the resonance
frequency it depends substantially on the mass.
A disadvantage of conventional electro-dynamic loudspeaker of this
form is that the resonance frequency of the mass-and-spring
vibrating system is generally between 100 and 200 c/s. At lower
frequencies, the mechanical vibration amplitude diminishes
considerably. For this reason, endeavors are made to reduce the
resonance frequency of the mechanical vibrating system as much as
possible, which however involves much expenditure and, due to the
requirement D<m (D = spring force, m = mass) is accompanied by
other disadvantages. It is true that nowadays low frequencies may
be generated with sufficient amplitude by using expensive, large
loudspeakers, but the amplitude variation of the original sound is
strongly distorted by the processes of vibration decay and
build-up.
For improving conventional loudspeaker systems, it is known to use
two moving coils and two magnet units, one moving coil, which with
its magnet unit is located in front of the diaphragm, supplying a
feedback voltage. The intention is to control the amplification by
means of this voltage and hence achieve linearization of the
frequency variation. It is not possible, however, to compensate the
spring-induced drop in vibration amplitude in this way.
In another known form of loudspeaker, the moving coil is coated
with iron powder to replace the spring return by a permanent magnet
return. This system has not proved satisfactory, however, since
leakage of the magnetic field at the edges of the permanent magnet
results in a considerable reduction in the return forces, so that
return at large amplitudes cannot be ensured.
The present invention is based on a solution to the problem of
providing an electro-dynamic transducer which, by simple means, can
ensure that even at very low frequencies sufficient amplitude is
radiated, and that in the entire audio-frequency range for constant
rapidity (deflection amplitude x frequency), there is good
proportionality between deflection amplitude and control-current
amplitude.
This problem is solved according to the invention in that, in an
electro-dynamic transducer system, the inoperative position of the
diaphragm is fixed by electromagnetic forces with the aid of two
mechanically coupled coils.
In an electromagnetic return or centering of the diaphragm in the
inoperative position, considerable advantages are obtained compared
with known loudspeaker systems. On account of the absence of a
spring and the resulting low inertia of the system, a very much
higher amplitude fidelity is obtained, that is to say, the
diaphragm vibration is only very slightly retarded relative to the
energizing low-frequency oscillation. A further advantage is that,
owing to electromagnetic self-damping, excessive vibration no
longer occurs. In conventional loudspeaker systems, for example, in
mechanical deflection of the diaphragm, followed by its release, it
is impossible to prevent the diaphragm vibrating about the
inoperative position. Such disadvantages are prevented in
electromagnetic return. Due to the electromagnetic return according
to the invention, not only is good proportionality obtained between
the amplitude of the energizing low-frequency current and the
mechanical vibration amplitude, but also a substantial increase in
the vibration amplitude at low frequencies compared with known
loudspeaker systems.
Another substantial advantage of the invention is that the known
equation for the vibration amplitude (amplitude equal to electric
power supplied divided by the product of the mass and square of the
angular frequency) is satisfied almost ideally. Down to at least 15
c/s, no drop in amplitude can be detected in the measured
curve.
A transducer according to the invention, when used in loudspeakers,
is suitable particularly for high-fidelity reproduction of
music.
Apart from the above-mentioned known transducer systems, two other
transducers having two moving coils are known. For example, French
Patent Specification 970,283 shows a transducer having a double
cone with a moving coil at each apex, the double cone not being
suspended at the center but being guided slidingly in a cylinder,
for increasing reproduction fidelity, while in U.S. Pat. No.
1,992,300 a loudspeaker arrangement is described, whose two moving
coils are intended to prevent excessive heating. In both
arrangements, however, the usual means, for example springs, are
necessary for middle centering. These known arrangements, despite
the two moving coils, also have no electromagnetic middle
centering, so that the advantages according to the invention cannot
be obtained in their case.
For returning the loudspeaker diaphragm to the inoperative
position, according to a preferred embodiment example of the
invention, a coil and permanent magnet arrangement is used, the
direction of the return force being fixed by the direction of a
constant direct current flowing through the coils, as well as by
inhomogeneity of the magnetic field and/or non-uniform winding
density of the coils.
An embodiment in which two mechanically coupled coils having the
same axis and arranged in pot magnets, and in which the
electromagnetic forces exerted on the coils in the inoperative
position are equal in magnitude but opposite in direction, in all
other positions of the coils are also opposite in direction but are
different in magnitude owing to inhomogeneous field distribution in
the pot magnets, affords the advantage that the distribution of the
magnetic field, necessarily present in the pot magnets, can be
utilized, without the need to vary the width of the air-gap by
means of suitably constructed pole pieces or the winding density of
the coils, that is to say, the number of turns per unit length.
With the use of one or more pot magnets combined with moving coils,
it is possible either to provide, for the low-frequency current a
further moving coil, connected to the diaphragm, arranged separate
from the centering device and movable in the field of a further pot
magnet, or to superimpose on the low-frequency current the direct
current effecting return to the inoperative position, such that the
forces impressed on the two coils by the low-frequency current
result in a vibration of the coils in rhythm with the low
frequency.
In the embodiments described, the diaphragm may be secured either
directly to the coils or to a hollow body surrounding the
coil-permanent magnet arrangements, which hollow body is in turn
rigidly connected to the coils, or the diaphragm may form such a
hollow body. The free ends of the diaphragms are separated in the
direction of movement by narrow tolerances with respect to the
surrounding form (for example a housing or baffle), or by sealing
protuberances without any spring effect. Preference is particularly
given to the use of elliptical or spherical hollow bodies, giving a
substantially less pronounced sound-beaming effect compared with
conical funnels.
The invention will now also be described by way of example with
reference to the accompanying drawings, in which:
FIGS. 1, 2, 4 and 6 show embodiments of coil and permanent magnet
arrangements according to the invention, and
FIGS. 3 and 5 show circuit arrangements for use with the
loudspeaker systems shown in FIGS. 1 to 5;
FIG. 7 shows the dependence of the vibration amplitude on the
frequency.
In FIG. 1, a loudspeaker arrangement comprises two, symmetrical,
moving-coil and permanent-magnet arrangements 1 and 1a. The
permanent magnets consist of pot magnets having the same axis,
adjoining each other by their base faces or spaced apart so that
they both open outwardly. The magnets are secured to centering
spindles 2, 2a, rigidly clamped at their free ends. Two moving
coils 5,6 are movable in the annular air-gaps L.sub.1 and L.sub.2
between the yokes and cores of the two pot magnets respectively and
have a common axis disposed perpendicular to the lines of flux of
magnetic fields cutting said coils. The moving coils 5 and 6 are
rigidly secured together by a hollow body 7, which is for example
elliptical or spherical. The hollow body 7, which surrounds the
moving-coil and permanent-magnet arrangements 1,1a is slidable on
the centering spindles 2,2a and consequently moves to-and-fro in
rhythm with the moving coils. The plain bearings must be
substantially frictionless and without spring effect. It is
particularly favorable to arrange rubber stockings on the hollow
body which, during the movement of the centering spindle, roll in
the longitudinal axis of the latter. Secured to the hollow body 7
in any desired manner is the loudspeaker diaphragm, not show, which
may have any form, suitable to the individual case. Preferably, the
hollow body may itself act as a diaphragm.
In the embodiment shown as example in FIG. 2, two moving coil and
permanent magnet arrangements 8,9 are used, are connected by a rod
10 and are so arranged that they have the same axis of symmetry and
are open towards the center of symmetry. A diaphragm 11 is slidable
on the rod 10. Two moving coils 12, 13 are movable in the annular
air-gaps L.sub.1, L.sub.2 of the permanent magnets, and are
connected rigidly together by rods 16,17 secured to the diaphragm
11, or by a hollow cylinder or the like. The diaphragm 11
consequently moves to-and-fro with the same rhythm as the moving
coils 12,13.
Since, in the embodiments described, the loudspeaker diaphragm is
fixed only to the moving coils, and vibrates together with them,
the pg,8 return produced by spring force, as in conventional
loudspeakers, is not possible. Contrary to the known transducers,
therefore, return of the diaphragm or moving coils to the starting
position is produced by electromagnetic forces.
FIG. 3 shows the basic diagram of a circuit arrangement whereby
both the mechanical return of the diaphragm and deflection of the
diaphragm are produced in rhythm with the low-frequency current.
The circuit arrangement may also be used in combination with the
embodiment examples shown in FIGS. 1 and 2, the moving-coil
connections denoted by E.sub.1,2 or A.sub.1,2 being connected to
the moving-coil connections also denoted by E.sub.1,2 and
A.sub.1,2, respectively. According to FIG. 3, the connections
E.sub.1,2 are short-circuited and are connected to the negative
terminal of a battery. The connections A.sub.1 and A.sub.2 of the
two coils are connected respectively to the collector of a
transistor pair 20,21, whose emitters are connected to the positive
terminal of the battery. The base electrodes of the transistors
20,21 are connected together by the secondary winding of a
transformer 22, supplying the low frequency. The center tap of the
secondary coil is connected in A.C. fashion with a capacitor C to
the positive terminal of the battery. The inoperative current
adjustment is done by adjusting the base biases by means of two
resistors R.sub.1 and R.sub.2.
In the embodiments shown in FIGS. 1 and 2 the moving coils in the
inoperative condition lie symmetrically in the air-gaps L.sub.1,
L.sub.2 of the pot magnets. The coil length is preferably rather
greater than the length of the pole faces of the pot magnets, so
that the coil ends of each coil are already located to some extent
in the inhomogeneous magnetic field of the pot magnets.
When the loudspeaker arrangement described is operating, the direct
current supplied by the battery must flow through the moving coils
in such a manner that one moving coil is deflected in one direction
(in FIG. 1 for example the right-hand coil to the right) and the
other moving coil in the opposite direction (in FIG. 1 for example
the left hand coil to the left). In this case, as long as the two
moving coils are in the central position, the direct current has no
effect, that is to say an increase or decrease in the direct
current or switching on or off of the battery does not result in
any deflection of the mechanically connected coils. If, however,
the two coils are deflected in the direction of their axes by
mechanical vibrations or the like, they are returned automatically
to the central position again by the action of the direct current.
In the embodiment shown as example in FIG. 1, this is produced
because the inhomogeneity of the magnetic fields increases more
slowly towards the center of symmetry, that is to say, towards the
closed end of the pot magnets or their air-gap L.sub.1, L.sub.2
than in the opposite direction. In other words, starting from the
embodiment example according to FIG. 1 and assuming that the two
moving coils 5,6 are deflected to the left, the force exerted on
the right-hand moving coil and resulting from the magnetic field
and the direct current, decreases, while the force exerted on the
left-hand coil increases. The resultant difference of these two
forces pulls the pair of coils back to the central position.
It was assumed in the foregoing analysis that direct current alone
flowed through the two coils. It may now be furthermore assumed
that by means of the circuit arrangement shown in FIG. 3, the
low-frequency is supplied in push-pull operation. By "push-pull" is
here understood phase opposition, that is to say, the direct
current flowing through one of the moving coils is weakened, while
the direct current flowing through the other moving coil is
strengthened, or vice versa. Since the same applies to the forces
acting on the moving coils, both coils when energized by low
frequency are deflected in the same direction, so that the hollow
body 7 (FIG. 1) or diaphragm 11 (FIG. 2) together with the moving
coils are forced to-and-fro in rhythm with the low frequency.
Independently of this, the moving coils are returned to the central
position again, due to the return forces producing the
deflection.
It is obvious that the invention is not limited to the described
examples of FIGS. 1 to 3, but that many modifications are possible.
Whereas, for example, in the embodiment according to FIG. 1, by
suitable choice of the magnetic field, winding direction of the
moving coils and the direction of the direct current flowing
through the moving coils, it is necessary to ensure that the
right-hand moving coil is deflected by the direct current to the
right, and the left-hand moving coil is deflected to the left, in
the example according to FIG. 2 it is necessary by suitable choice
of the said parameters to ensure that the right-hand moving coil is
deflected by the direct current to the left and the left-hand
moving coil is deflected to the right, since in this embodiment
example the pot magnets are open in the opposite direction. It is
also possible, however, to provide moving coil and permanent magnet
arrangements in which the difference in the return forces occuring
in a deflection is not due to the different inhomogeneity of the
pot magnets inwardly or outwardly. For example, different
inhomogeneity could be obtained or improved by suitable variation
in the gap width of the magnets, by non-uniform winding of the
moving coils or by suitable construction of the pole faces at the
ends. A form will always be selected such that the transmission
ratios are optimum, that is to say, good proportionality will be
obtained between the amplitude of the energizing low frequency
current and the amplitude of the mechanical vibration on the one
hand and an almost constant rapidity (angular frequency x
amplitude) for all frequencies to be transmitted on the other.
According to FIG. 4, it is furthermore possible to use instead of
two pot magnets only one pot magnet 23 and to mount in it two
moving coils one behind the other, as shown in FIG. 4. The arrows
indicate the directions in which the two coils are deflected by the
direct current. This embodiment example has the advantage of
permitting cheaper and more compact loudspeaker systems but
nevertheless with electromagnetic centering.
Finally, it is also possible to separate completely the centering
device from the moving coil for the low frequency. Such an
arrangement is shown diagrammatically in FIG. 6 in which the arrows
again indicate the directions in which the individual coils are
deflected by the direct current. Such an arrangement has the
special advantage that any inhomogeneous magnetic fields may be
used for the return without having to accept distortions with
regard to sound transmission. In the embodiment example according
to FIG. 6, on the contrary, the moving coil for the low frequency
may be arranged in a completely homogeneous magnetic field.
The circuit arrangement shown in FIG. 5 concerns a simplification
of the circuit arrangement shown in FIG. 3. The connections shown
at E.sub.1,2 and A.sub.1,2 are to be connected correspondingly to
the connections E.sub.1,2 and A.sub.1,2 of the embodiment examples
shown in FIGS. 1, 2, 4 and 6. In this case, as shown in FIG. 5,
reversal of polarity is necessary, compared with the circuit
according to FIG. 3, since the direct current flows through the
coils in series.
The use of an n-p-n transistor 24 and a p-n-p transistor 25
provides the possibility of like-phase control of the circuit by
the low-frequency current instead of push-pull. The use of two
batteries B.sub.1 and B.sub.2 is necessary for this purpose. The
additional advantage in this circuit arrangement is that in the
deflection of the moving coils, the voltages induced in them (Lenz
law) are in phase opposition in the circuit path 26. Much better
matching with the low-frequency source is thereby made possible,
since the input impedance of A.sub.1 to E.sub.2 of the coil
arrangement, which is normally subject to relatively large
fluctuations (up to about 1:20), due to the back-emf induced by the
movement of the coils, behaves almost completely linearly in this
circuit arrangement.
Finally, FIG. 7 shows the dependence of the vibration amplitude on
the frequency. Curve 27 is typical of a conventional
electro-dynamic loudspeaker, while curve 28 shows the behavior of a
loudspeaker according to the invention. Owing to the absence of the
mechanical natural frequency f.sub.o (curve 27), caused by the
mass/spring system, the amplitude in loudspeakers according to the
invention is exceptionally large, even at the lowest frequencies.
The relationship of amplitude equal to the electrical energy
divided by the product of the mass and the square of the angular
frequency (A = (E)/(M .sup.. .alpha..sup.2) is satisfied almost
ideally.
The configuration of the electromechanical transducer according to
the invention permits of absolutely new body forms for the
diaphragms. As shown by FIGS. 1 and 2, the diaphragm may be set
vibrating in many ways, so that the problem of its clamping no
longer arises. In particular, it is no longer necessary to use cone
diaphragms, so that the limitations which these impose, in
particular with regard to sound-beaming effects, are
eliminated.
The transducer according to the invention may be used not only in
loudspeaker systems, but also wherever high demands are laid on the
quality of the transducer.
* * * * *