Loudspeaker with commutated coil drive

An improvement is provided in a loudspeaker having a current carrying coil disposed in the flux gap of a permanent magnet and carrying current to drive the coil and an associated sound diaphragm. The coil is substantially longer than the thickness of the flux gap and has an exposed conductive surface. Contacts are mounted near each side of the flux gap and slidably engage the coil surface to supply current to a limited portion of the coil situated in the flux gap, irrespective of movement of the coil.

BACKGROUND OF THE INVENTION 
This invention relates to improvements in so-called dynamic loudspeakers 
wherein a coil is provided with electrical current and is disposed in a 
magnetic field to produce an axial driving force on the coil and on a 
sound radiator attached to the coil. 
Most conventional loudspeakers in use today comprise a cylindrical voice 
coil that is disposed in a radial magnetic field created by the pole 
pieces of a magnet. The pole pieces are arranged to provide a circular 
gap, and the coil, having one or more layers of turns of wire, is disposed 
in the gap. The ends of the coil are connected to a source of signal 
current, such as an amplifier. The current passing through the length of 
the coil creates a field that links with the magnetic field of the magnet 
to produce a driving force on the coil. The force (F) on the coil is equal 
to BLI, where B is the flux density, L is the length of wire immersed in 
the magnetic flux, and I is the current. 
In many types of speaker drives, the length of the cylindrical coil is 
fabricated to be approximately equal to the fixed thickness or depth of 
the magnetic poles at the gap. In this manner, substantially the entire 
useful length of the coil is immersed in the magnetic field, enhancing 
total efficiency. 
The production of low frequency sound presents special problems for a 
woofer having a moving coil drive. In order to produce low frequency sound 
of sufficient intensity, it is desirable for the available peak to peak 
excursion of the voice coil be as large as possible, in order for the 
sound diaphragm to excite large volumes of air. Such large excursions, 
however, cause the voice coil to move either partially or entirely out of 
the region of constant magnetic flux, producing distortions, since the 
force per unit current decreases as the active length of the voice coil is 
reduced. The term "active length" is defined herein as only that part of 
the coil which remains in the magnetic field in the gap. 
In order to overcome the low frequency distortion problem and increase 
available excursion, longer or overhanging voice coils have been employed, 
in which the length of the coil is greater than the thickness of the 
magnetic gap. The use of a longer coil, however, results in a substantial 
loss of efficiency, since only the active portion of the coil in the 
magnetic field contributes to the driving force. The use of a longer coil 
also creates greater total resistance to the flow of current and places a 
limit on power handling capacity due to the production of heat, which is 
proportional to the resistance. 
From the foregoing, it may be seen that if large peak to peak excursions 
are required in a movable coil drive for a speaker, problems are 
encountered that result in loss of linearity, loss of efficiency, or both, 
particularly in lowermost frequencies of human hearing, i.e., below 75 Hz. 
Thus, it would be desirable to devise a moving-coil drive having features 
that would overcome or avoid these problems. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a moving-coil speaker drive is 
provided with a cylindrical coil disposed and supported for axial movement 
in an annular magnetic field having a given thickness, in order to produce 
linear axial forces on the coil. The length of the coil is substantially 
greater than the thickness of the magnetic field at the gap. Unlike prior 
art versions, however, current is not passed through the entire length of 
the coil to effect the driving force. Instead, current is passed through 
only the limited portion of the coil that is immersed in the magnetic 
field. This is accomplished by a stationary electrical contact on both 
sides of the magnetic pole that is in sliding electrical contact with the 
coil, with the contacts being connected to the source of signal current. 
This communication results in a situation where only a fraction of the 
length of the coil is active, and such active fraction is always disposed 
in the region of magnetic flux irrespective of the movement or axial 
position of the coil. 
Since only a portion of the coil carries current at any given time, the 
remaining portions can absorb and dissipate heat generated by the active 
portion, thereby greatly increasing power handling capacity and improving 
electrical efficiency. Moreover, since the coil is commutated, the drive 
force on the coil is constant per unit current, whereby the drive force is 
linear over the entire length of available excursion, thereby greatly 
reducing distortion.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The improvement of the present invention will be described in conjunction 
with a speaker having an arrangement of parts similar to those used in a 
conventional moving-coil loudspeaker. A rigid cone-shaped support, called 
a basket 10, is used to support the various components of the speaker, 
including the outer perimeter of a cone-shaped sound diaphragm 12, and a 
permanent magnet assembly, generally indicated at 14. The magnet assembly 
comprises an annular outer pole 16 and a central inner pole 18 of opposite 
polarity, with the inner and outer poles being spaced to define an annular 
gap 20. The distance between the inner and outer poles is referred to as 
the gap width, and the other dimension, indicated at 22, is referred as 
the gap thickness or depth. The permanent magnet assembly creates a zone 
of constant magnetic flux in the annular gap 20 which extends 
substantially entirely between the opposite pole faces that define the 
gap. 
An elongate coil 24 is resiliently suspended in the gap 20 such that the 
sides of the coil are spaced from the opposed pole faces. The length of 
the coil 22 is substantially longer than the thickness 22 of the gap 20, 
and, in fact, may be of any desired length, depending on the amount of 
excursion required. The coil is made up from a continuous or unterminated 
layer of windings of conductive wire, and the wire, such as copper, may 
have a rectangular cross section to increase efficiency. Adjacent or 
contiguous wire strands are insulated from one another, such as by using 
coated wire. Either one or both of the inner and outer cylindrical 
surfaces of the coil, however, are bare and electrically conductive along 
successive individual exposed wires in the coil. The exterior surfaces of 
the coil are preferably smooth and uninterrupted. 
One end of the coil 24 projects outwardly from the magnetic gap 20 and is 
secured directly or via a cylindrical extension 26 to the apex of the cone 
diaphragm 12, similar to prior art constructions. An annular flexible web 
or spider 28 extends between the support basket 10 and the coil extension 
26 to support the coil and to confine the movement thereof in an axial 
linear direction. 
In prior art loudspeaker systems, the coil would be formed of a single 
length of wire in which the ends of the wire would be connected by 
flexible wires to terminals on the basket and thence to the leads from the 
source of signal current. In this fashion, current would pass through the 
entire length of the coil, although only a portion of the coil would be 
disposed within the magnetic gap. As a result, the entire length of the 
coil would generate resistance losses and would impose a power handling 
limit, which if exceeded, would cause thermal destruction of the coil. 
Since only that portion of the coil within the magnetic gap is active, a 
substantial loss in electrical efficiency would be experienced. 
In accordance with the present invention, means are provided to limit the 
current carrying portion of the coil substantially to the region of the 
coil located with the magnetic gap 20, irrespective of the axial position 
of the coil. This is accomplished by placing a stationary contact on both 
sides of the thickness of the gap, with the contacts being in electrical 
sliding contact with the individual successive wires on the surface of the 
coil. In this manner, the flow of electrical current is confined to that 
limited portion of the coil situated in the magnetic gap, regardless of 
movement of the coil. 
As shown, the contacts may comprise a first circular conductive member 30 
attached to the center pole 18 of the magnet and having a plurality of 
conductive resilient fingers 32 to slidably engage the inner cylindrical 
surface of the coil 24 at a location very near one end of the magnetic gap 
28. A second conductive member 34 may be supported from the outer magnet 
pole 16 and may have resilient fingers 36 in slidable engagement with the 
outer surface of the coil at or near the other end of the magnetic gap 28. 
The members 30 and 34 are electrically isolated from each other and are 
connected to respective leads 31 and 32 from a source of signal current, 
such as an amplifier. 
It will be appreciated that the contacts may engage either the inner or 
outer surface of the coil to define a limited conductive path therewith. 
Other types of contacts to accomplish the same general result may be 
employed. 
In operation, it will be understood that the coil can move in either axial 
direction, as indicated by the arrow, depending on the direction of flow 
of current through the coil. As current is applied across contacts 30 and 
34, the coil moves axially in one direction or the other. In moving to a 
new position, however, the exposed coil wires traverse across the 
contacts, such that only a given fraction of the total coil length is 
activated, such active length substantially corresponding to the thickness 
or depth 22 of the gap located between the contacts. Thus, irrespective of 
the axial movement or position of the coil, only that portion within the 
magnetic gap 20 will carry current. In such fashion, the coil is 
commutated, in that the current is switched in the coil as the coil moves 
to keep the active portion in the region of constant magnetic flux. 
From the foregoing, it may be seen that the improved moving-coil speaker 
drive of the present invention offers several distinct advantages over 
similar linear drives in the prior art. Since only the active portion of 
the coil is within the magnetic flux, substantial electrical efficiencies 
are realized. The inactive portions of the coil do not generate heat and 
can absorb and dissipate heat from the active portion, thereby increasing 
power handling capacity. The excursion limit can be any desired length, 
whereas the force on the coil will remain constant per unit of current 
irrespective of coil position, thereby minimizing sound distortion, 
particularly at high power, low frequency applications. 
In addition, the improved efficiency of the drive enables the use of larger 
moving masses in the coil and diaphragm, thereby allowing the use of a 
smaller speaker enclosure for the same intensity of sound production, per 
watt of input. Finally, the improvement does not substantially increase 
the cost over a conventional speaker, since many conventional speaker 
parts may be used. 
In viewing the FIGURE of the drawing, it is possible to compare the 
operating properties of two identical speakers having overhanging coils of 
the same length, with only one of the drives having the commutating 
contacts. For example, if the total length of the coil is twice the 
thickness of the flux gap, the coil resistance in a prior art speaker is 
double that of the coil described herein. This means that the power 
efficiency of the present drive would be double that of the prior art 
drive, and the heat losses would be one-half or less, allowing a potential 
increase of as much as four times in sound output.