LOUDSPEAKER
TECHNICAL FIELD The present invention relates to a loudspeaker. BACKGROUND ART
As is known, currently marketed loudspeakers comprise: a frame or basket having two coaxial portions, one truncated-cone-shaped and the other cylindrical; a magnetic core coaxial with the frame axis and housed inside the cylindrical portion of the frame; a cone or diaphragm housed axially ovably inside the truncated- cone-shaped portion of the frame, with the smaller ■peripheral edge fitted in sliding manner to a portion of the magnetic core, and the larger peripheral edge connected to the frame via an elastic annular membrane; and a moving coil fitted to the smaller peripheral edge of the cone so as to slide on the magnetic core.
Known loudspeakers also comprise an elastic locating and damping membrane connecting the moving coil to the cylindrical portion of the frame to keep the coil coaxial with the magnetic core as the cone moves axially; and a guard cap fitted to the cone to cover and protect the
portion of the magnetic core projecting inside the cone.
In some types of currently marketed loudspeakers, the magnetic core is cylindrical, and is defined by two stacked button-shaped permanent magnets, and by a spacer washer made of ferromagnetic material and interposed between the two permanent magnets to keep them a given distance apart. The permanent magnets are magnetized in a direction parallel to the frame axis and are oriented with similar poles facing the spacer washer. Unfortunately, performance of the magnetic core so formed is unsatisfactory when the size of the loudspeaker exceeds a given diameter, typically 165 mm.
Tests conducted on loudspeakers of over 165 mm in diameter, in fact, have shown that often, during operation, the air flowing through the gap between the moving coil and the magnetic core is subject to local turbulence which tends to deform the moving coil so that it knocks against the magnetic core; and the sliding movement of the moving coil against the portion of the magnetic core projecting inside the cone impairs the response curve and long-term reliability of the loudspeaker .
DISCLOSURE OF INVENTION
It is an object of the present invention to provide a loudspeaker designed to minimize disturbance of the air flowing in the gap between the moving coil and the magnetic core, and so ensure optimum performance at all times.
According to the present invention, there is provided a loudspeaker comprising a substantially cup- shaped frame; a magnetic core fitted integrally to and inside the frame; a diaphragm fitted movably inside the frame so as to be fitted in sliding manner to said magnetic core; and a moving coil fixed to the diaphragm and fitted in sliding manner to said magnetic core; the magnetic core comprising at least two stacked permanent magnets, and at least one spacer washer made of ferromagnetic material and interposed between said two permanent magnets; and said loudspeaker being characterized in that said magnetic core comprises a jacket fitted to the stack defined by the two permanent magnets and by the spacer washer; said moving coil being fitted in sliding manner directly to said jacket. BRIEF DESCRIPTION OF THE DRAWINGS
A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a section, with parts removed for clarity, of a loudspeaker in accordance with the teachings of the present invention;
Figure 2 shows a section, with parts removed for clarity, of a variation of the Figure 1 loudspeaker. BEST MODE FOR CARRYING OUT THE INVENTION
Number 1 in Figure 1 indicates as a whole a loudspeaker which is especially suitable for assembly to motor vehicles and similar.
Loudspeaker 1 comprises a substantially cup-shaped frame 2; a magnetic core 3 fixed to the bottom of frame 2 and coaxial with the axis A of frame 2; and a cone or diaphragm 4 housed inside frame 2, coaxially with axis A, with the smaller peripheral edge 4a facing and coaxial with magnetic core 3, and with the larger peripheral edge 4b located at the opening of f ame 2. Loudspeaker 1 also comprises an elastic annular supporting membrane 5 connecting the larger peripheral edge 4b of cone 4 to the edge defining the opening of frame 2 ; a moving coil 6 fixed to the smaller peripheral edge 4a of cone 4 so as to fitted in sliding manner to magnetic core 3; and an elastic locating membrane 7 connecting coil 6 and/or cone 4 to frame 2 to keep coil 6 coaxial with magnetic core 3 as cone 4 moves axially.
Loudspeaker 1 also comprises a guard cap 8 fitted to the inner surface of cone 4 to cover and protect the portion of magnetic core 3 projecting inside cone 4.
More specifically, elastic annular supporting membrane 5 is so formed as to only permit axial displacement of cone 4, and coil 6 is immersed in the magnetic field generated by magnetic core 3 so that interaction between the magnetic field and any electric current circulating in coil 6 produces forces capable of moving cone 4 axially inside frame 2. Electric current is supplied to coil 6 by two terminals of electrically conducting material (not shown) .
In the example shown, coil 6 is defined by four
layers of electrically conductive wire wound about a cylindrical tubular element fitted in sliding manner to magnetic core 3.
With reference to Figure 1, in the example shown, frame 2 comprises two coaxial portions, one truncated- cone-shaped and the other substantially cylindrical, and is preferably, though not necessarily, made of nonferromagnetic material.
Elastic locating membrane 7 connects coil 6 to the lateral wall of frame 2 , and is so formed as to only permit and to simultaneously dampen axial displacement of cone 4 and coil 6. Together with guard cap 8, elastic locating membrane 7 also provides for preventing dirt from depositing in the gap between magnetic core 3 and coil 6 and so impairing operation of loudspeaker 1.
The various component parts of loudspeaker 1 must, obviously, be so sized that the guard cap 8 fixed to cone 4 does not knock against the top end of the portion of magnetic core 3 projecting inside cone 4 when cone 4 is withdrawn inside frame 2 by the forces generated in coil 6 by the interaction of the magnetic field and the electric current circulating in coil 6.
In the Figure 2 variation, loudspeaker 1 is provided, in lieu of guard cap 8, with a second elastic locating membrane 9 connecting the inner surface of cone 4 to the top end of the portion of magnetic core 3 projecting inside cone 4; and both locating membranes 7 and 9 are so formed as to only permit and to
simultaneously dampen axial displacement of cone 4 and coil 6.
With reference to Figures 1 and 2, magnetic core 3 is substantially cylindrical and, in the example shown, comprises two substantially button-shaped permanent magnets 10, and a spacer washer 11 made of ferromagnetic material and interposed between the two permanent magnets 10 to form a stack coaxial with axis A. Permanent magnets 10 are magnetized in a direction parallel to axis A, and are stacked so that similar poles face spacer washer 11. Magnetic core 3 also comprises a cylindrical tubular jacket 12 fitted to the stack defined by the two permanent magnets 10 and by spacer washer 11.
More specifically, with reference to Figure 1, jacket 12 has an inside diameter approximately equal to but no smaller than the largest of the outside diameters of the two permanent magnets 10 and spacer washer 11, is fixed by adhesive 13 or similar fastening means to the stack defined by the two permanent magnets 10 and spacer washer 11, and is made of material preferably, though not necessarily, having nonferromagnetic properties .
More specifically, jacket 12 is preferably, though not necessarily, made of an electrically conductive material with nonferromagnetic properties, such as copper, brass, aluminium and alloys of these.
In an alternative embodiment, jacket 12 may be made of plastic or composite material with nonferromagnetic properties, but which is not electrically conductive.
In the example shown, magnetic core 3 is fixed to frame 2 by a bolt 14 comprising a threaded screw extending coaxially with axis A through the bottom wall of frame 2 and through magnets 10 and spacer washer 11, and a nut screwed to the end of the screw.
Besides fitting magnetic core 3 integrally to frame 2, bolt 14 also prevents the stack defined by the two magnets 10 and by spacer washer 11 from parting owing to the tendency of the two permanent magnets 10 to repel each other.
Operation of loudspeaker 1 can easily be deduced from the foregoing description with no further explanation required.
To appreciate the advantages afforded by jacket 12, it should be explained that tests have shown the turbulence in the air flowing through the gap between the moving coil and the magnetic core to be caused directly by a slight unevenness of the surface of the magnetic core over which the air flows, and which is substantially due to the outside diameters of the magnets and the spacer washer not coinciding perfectly.
Since, in loudspeaker 1, magnets 10 and spacer washer 11 are encased in jacket 12 and coil 6 is fitted in sliding manner directly to jacket 12, the surface over which the air flows is perfectly smooth, thus eliminating any disturbance and so ensuring an optimum response curve at all times and improved overall reliability of loudspeaker 1.
Loudspeaker 1 also has the advantage of having a better response curve as compared with currently marketed loudspeakers, and of reducing induced currents which may interfere with the amplifier supplying loudspeaker 1. The presence of a jacket 12 of nonferromagnetic, electrically conductive material, in fact, modifies the impedance modulus curve as a function of frequency, making it more linear as compared with currently marketed loudspeakers.
Clearly, changes may be made to loudspeaker 1 as described and illustrated herein without, however, departing from the scope of the present invention.
More specifically, in a variation not shown, jacket 12 may be formed in a number of connected pieces .