Note: Descriptions are shown in the official language in which they were submitted.
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PERCUSSION AIR MOTOR
This invention relates to a percusison air motor. More
particularly, this invention relates to a percussion air
motor, such as a loudspeaker, for converting electrical
signals into corresponding percussive air movements.
It is well known that transducer systems, especially of
the type which convert electrical energy into mechanical
energy, are inefficient in operation. Generally, only a
small fraction of the total electrical energy supplied
to 5uch transducer systems is actuaLly converted and
made available as mechanical energy.
It is also well known that when the electrical energy
supplied to a transducer system is ;n the form of a
lS desired signal, the resulting signal in the converted
energy form will not accurately correspond to the input
signal. The difference in signal content between the
input and output ~ignals is a form of distortion which
is a function of the construction of the transducer
system. Further, as transducers are usually operated as
components in large systems, an inefficiency in
converting from one form of energy to the other will
increase the distortion of the larger system. For
example, an inefficient audio speaker which converts
electrical energy from an amplifier into a mechanical
energy for creating percussive air movements, i.e.,
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sound pressure waves, requires such large amounts of
electrical energy to be delivered from an amplifier in
order to achieve acceptable sound pressure levels that
the distortion introduced into the overall system by the
amplifier will increase.
As is known, transducers which are used as loudspeakers
generally have a driving assembly for receiving an
electrical signal and a diaphragm which is connected to
the driving assembly to vibrate in accordance with the
received signal. The driving assembly, e.g. a voice
coil, usually has a coil of wire and a magnet core
mounted within the coil to move in response to the
magnetic field caused by the coil upon energization. In
these cases, the coil is secured to the diaphragm, e.g.
at a central point, while the diaphragm is hinged or the
like at the outer periphery. In other cases, it has been
known to havle the diaphragm driven by peripheral forces.
On~ such system is described in U.S. Patent 3,979,566.
However, in this system, the diaphragm is driven in an
unbalanced manner such that distortion may well result.
Other similar systems are described in U.S. Patent
3,012,107; 2,535,757 and 2,520,646. It must be noted
that any improvement in the mechanic~l advantage of a
transducer system will affect only the particular form
of the available mechanical energy. In other words, in
a transducer system which produces reciprocatiny axial
mechanical motion in response to an electrical input, a
variation in the mechanical advantage will cause short
and forceful strokes to be exchanged for longer, but
weaker ones, or vice versa. The actual work available,
defined by the product of the mechanical force and the
distance over which the force operates, will remain the
same, signifying that a fundamental improvement in the
conversion ef~iciency has not been achieved.
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In many cases, the construction of loudspeakers have
generally focused upon the diaphragm, being the element
which directly couples to the air medium, as the mass of
primary interest. Although the development of an
effective low mass diaphragm is a worthwhile design
goal, advances in materials sciences and manufacturing
methods have enabled the production of diaphragms which
are low in weight with respect to the driving assembly.
Since it is known that efficiency is related to the
ratio of voice coil weight to diaphragm weight, as
diaphragm weight is reduced, so must the weight of the
driving coil, if efficient operation is to be retained.
This is especially significant in peripherally driven
transducer systems wherein the driving coils are
situated at the periphery of the diaphragmr thereby
having decreased the mechanical advantage with respect
to the coupling of low frequency signals which are
center-loaded in the diaphragm.
It becomes apparent from this analysis, that although
the mechanical advantage of an electromagnetic
transducer system operating at high frequencies is
improved by peripheral drive, the same advantage does
not hold true for low frequencies which produce maximum
air pressure at a region near the center of the
diaphragm, which region is furthest from the peripheral
drive means. Thus, the operating efficiency of
electromagnetic acoustic transducer systems at high
audio frequencies which radiate along the surface of the
transducer diaphragm, can be improved by producing a low
mass peripheral drive system. Moreover, such reduced
driving means mass will ~lso improve the efficiency of
the transducer system at low frequencies because
instantaneous low frequency sound pressure waves are
produced essen~ially by causing the diaphragm to push
the air axially with the entire width of the diaphragm,
the drive coils appear to be at one end of a radial
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moment arm, while the air load is at the other end.
Accordingly, it is an object of this invention to
provide an electromechanical transducer system with
improved energy conversion efficiency.
It is another object of this invention to provide an
electromechanical transducer system which retains a high
correspondence between the signal content of the
respective input and output signals.
It is a further object of ~his invention to provide a
transducer system which is economical to manufacture and
simple to assemble.
It is another object of the invention to eliminate
distortion in a transducer.
It is another object of the invention to eliminate
distortion in a loudspeaker.
It is another object of this invention to provide equal
but separate electro-magnetic circuits for each
direction of alternating current flow.
Briefly, the invention provides a transducer, such as a
loudspeaker, which is formed, in part, of a sleeve, a
diaphragm secured to and within the sleeve in a central
plane and electromagnetic means disposed in symmetric
relation to said sleeve for reciprocating the sleeve and
diaphragm in a linear manner to produce a balanced
distortion free sound.
The means for reciprocating the sleeve and diaphragm
includes a pair of coils wound about the sleeve in
symmetrical relation about the central plane and means
defining a pair of stationary magnetic circuits disposed
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in symmetrical relation about the central plane. The
coils are equally wound to effect equal and co-
directional electromagnetic forces therein in response
to an alternating current supplied to the coils while
each magnetic circuit is disposed about a respective one
of the coils.
The arrangement of the coils and magnetic circuits
relative to the sleeve and diaphragm improves low and
high fre~uency operation of acoustic transducers by
providing a peripheral drive system which contains less
mass and is more efficient in converting to mechanical
energy than prior art systems.
By winding the coils in symmetric relation, and
particularly, as mirror images of each other, an
isomagnetic equilibrium is obtained at the plane of the
diaphragm. The magnetic circuits are each of the type
which produces magnetic lines of flux radially across an
air gap in which is disposed a portion of the sleeve
which contains the respectively associated coil.
Energization of the coils by application of an
electrical signal at the coil ends or terminals results
in corresponding mechanical motion of the sleeve in a
direction parallel to the central axis of the magnetic
circuits.
In this construction, the transducer, i.e., loudspeaker,
is able to produce sound which is distortion ~ree as the
electromotive forces imposed on the sleeve are applied
symmetrically, i.e., in a balanced manner. Thus, the
diaphragm is able to reciprocate, i.e., vibrate, in a
balanced manner to produce a true sound wave
corresponding to the electrical signal received.
Further, in this comstruction, th~ combined mass of the
mirror image coil is less than the mass of a single coil
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required in previously known transducer systems for
producing the same energy conversion efficiency. Thus,
conversion efficiency is improved over prior art single
coil systems having equivalent mass.
Still further, the reduced effective equivalent mass of
the peripheral drive system elevates the mechanical
resonant frequency of the sleeve thereby improving the
ability of the sleeve to cause high frequency signals
to radiate across the surface of the diaphragm.
Additionally, reduced sleeve mass, with respect to a
given conversion efficiency, reduces the effective
moment arm at low frequencies thereby further improving
the mechnical advantage and eEficiency of low frequency
operations.
These and other objects and advantages of the invention
will become more apparent from the following detailed
description and accompanying drawings in which:
Fig. 1 is a perspective view of the transducer
constructed in accordance with the invention;
Fig. 2 is a cross-sectional view of the transducer of
Fig. l;
Fig. 3 is an enlarged view of the sleeve, coils and
associated parts of the magnetic circuits of the
transducer of Fig. l; and
Fig. 4 is an illustrative arrangement for winding the
coils on the sleeve in accordance with the invention.
Referring to Fig. 1, the electroacoustic transducer
apparatus is constructed to convert electrical energy
to mechanical energy. As illustrated, the transducer
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10 has a generally cylindrical configuration with open
ends.
As shown in Fig. 2, the transducer 10 includes a central
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mounting frame 11 by which the transducer 10 can be
mounted in a suitable housing or on a suitable support.
The frame 11 is of annular shape and has a pair of
inwardly directed spaced apart flanges 12. In addition,
a thin-walled flangeless sleeve 13 is housed within the
frame 11 and a flat disc-shaped diaphragm 14 is secured
to and within the sleeve 13 in a central plane of the
sleeve 13. The sleeve 13 is made of any suitable
material, such as brass, while the diaphragm 14 is
likewise made of a suitable material, such as metal,
e.g. brass.
As shown in Figs. 2 and 3, an electromagnetic means is
provided in the transducer 10 for reciprocating the
sleeve 13 and diaphragm 14 in a linear manner in a
direction transverse to the central plane of the sleeve
13 to prodllce a balanced sound. This means includes a
pair of coils 15, 16 equally wound about and on the
sleeve 13 in symmetrical relation to the diaphragm 14
and a pair of stationary magnetic circuits 17, 18
disposed in symmetrical relation about the diaphragm 14.
The coils 15, 16 are equally wound on the sleeve 13 in
fixed relation to effect equal and codirectional
electromotive forces therein in response to an
alternating current supplied to the ~oils 15, 16. As
shown in Fig. 4, the equally wound ~oils 15, 16 are
wound to achieve a mirror image symmetry with respect to
one another. Only two windings are shown in each coil
for purposes of clarifying the winding directions. Each
coil 15, 16 has a pair of ends or leads 13, 19', ~0, 20'
0 which extend away from the sleeve 13 to a suitable
common tap ~not shown) in the frame 11. In addition,
the current entering lead 19', 20' of each coil is
located under the wound coil. The coils 15, 16 are
energized in parallel by applying an electrical signal
from a suitable common source (not shown~ to the
terminals 19, 19', 20, 20' at the tap (not shown). The
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symmetrical arrangement shown provides an equal
distribution of incoming electrical current throughout
each coil 15, 16.
In this embodiment, the current leads for the two coils
are illustrated as being next to one another.
Variations in the placement of leads 19, 19' with
respect to leads 20, 20' will establish manufacturing
methods and signal transformation requirements.
Referring to Fig 2, the magnetic circuits 17, 18 are
mounted in the frame 11 in spaced apart relation and
are abutted against the flange 12. Each circuit 17, 18
includes a pair of magnetic flux translative elements
in the form of concentric spaced apart pole rings 21,
22 disposed about a respective coil 15, 16. One pole
ring 21 acts, e.g. as a South pole, while the other
pole ring 22 acts as a North pole. In addition, each
circuit 17, 18 has an annular magnet 23 secured to the
outer pole ring 22, a transfer ring 24 secured to the
inner pole ring 21 and an annular transfer plate 25
secured to the magnet 23 and transfer ring 24. As
indicated, the transfer plate 25 forms a circular
opening in communication with the diaphragm 14.
The pole rings 21, 22 define air gaps in which the
sleeve 13 and respective coils 15, 16 are located and
across which magnetic lines of flux flow. The magnetic
flux thus flows via the magnet 23, transfer plate 25
and transfer ring 24 between the pole rings 21, 22. In
this construction, the magnets 23, pole rings 21, 22
and flux transfer plates 25 are made of low loss
magnetic material. The transfer rings 24 are made of
cast iron.
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A pair of resilient pads 26 t for example of compressible
fiber, are mounted between the pole rings 21 to center
the diaphragm 14. These pads 26 are sufficiently
resilient to avoid imparting a damping effect on the
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_ _ _ . . _
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g
reciprocating motion of the diaphragm 14 during
operation.
In operation, an electrical signal in the form of an
alternating current is supplied to the terminals 19,
l9', 20, 20' of ~he two coils 15, 16. The signal is
then divided into two balanced currents which are passed
through the two coils 15, 16 and interact with the
stationary lines of flux generated across the pole rings
~l, 22. As a result, an electromotive force is created
to move each coil 15, 16 and, thus, the sleeve 13. As
these forces are co-directional, i.e. directed in the
same direction, for example to the right as viewed in
Fig. 2, the sleeve 13 and diaphragm 14 move to the
right. As the current alternates, so does the direction
of electromotive forces. Thus, the sleeve 13 and
diaphragm 14 move to the left as viewed in Fig. 2. A
linear reciprocating motion is thus effected in the
diaphragm 14.
It is noted that the stationary magnetic field width and
flux density is dependent on the size and width of each
respective wound coil 15, 16.
In one embodiment, ~he inner pole ring 21 has an outside
diameter of 3.593 inches while the outer pole ring 22
has an inner diameter of 3.6~5 inches to form an annular
air gap of 0.032 inches. The sleeve 13 has an inner
diameter of 3.607 inches while the coil 15, 16 has an
outer diameter of 3.643 inches to give a clearance of
0.032 inches from the outer pole ring 22. The coils may
be formed of wires of a width of 0.003 inches and The
use of two coils 15, 16 and two magnetic circuits 17, 18
for driving the diaphragm 14 results in a balanced
movement. If a single magnetic circuit were used with
the two coils 15, 16 or if two magnetic circuits 17, 18
were used with one symmetrically placed coil, there
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would be an imbalance of forces. This imbalance of
forces, in turn, restricts the electrical current from
generating motion to maximum potential. Further, the
length of two equal and parallel coils 15, 16 compared
to a single series conductor of equal total length
reduces the time required for a current to travel their
lengths. That is, the time is reduced by one-half.
Thus, a transducer employing this parallel arrangement
generates an increased efficiency relative to time.
The invention thus provides a transducer which, when
used as a loudspeaker, can reciprocate a diaphragm in a
balanced manner and, as such, can produce sound waves
without distortion. As a result, less power is required
to produce sound of a given intensity.
The transducer 10 may be provided with suitable gridlike
diffuser covers at the open ends for diffusion of the
produced sound waves. These covers can be secured in
place in any known manner.
