WO1999030533A1

WO1999030533A1 - Electrodynamic acoustic transducer with reduced equivalent inductance of the moving parts

Info

Publication number: WO1999030533A1
Application number: PCT/IT1998/000315
Authority: WO
Inventors: Alessandro Pancani; Roberto Coppini
Original assignee: B & C Speakers S.P.A.
Priority date: 1997-12-05
Filing date: 1998-11-11
Publication date: 1999-06-17
Also published as: IT1297378B1; ITFI970266A0; ITFI970266A1

Abstract

An electrodynamic acoustic transducer with reduced equivalent inductance of the moving equipment, comprising a magnetic circuit structure including a central core (21), a magnet assembly and a polar expansion (23) defining an annular air gap in which a voice coil (16) carrying an acoustic-frequency current is placed; and at least one stationary coil (30, 31) in the annular air gap arranged to carry acoustic-frequency current. The winding of the voice coil and the stationary coil are arranged in such a way to reduce the equivalent impedance of the moving equipment constituted by the voice coil and the associated elements.

Description

TITLE

ELECTRODYNAMIC ACOUSTIC TRANSDUCER WITH RESUCED EQUIVALENT INDUCTANCE OF THE MOVING PARTS

DESCRIPTION Field of the Invention

The present invention relates to an electrodynamic acoustic transducer with reduced equivalent inductance of the moving equipment .

Description of the prior art Electrodynamic acoustic transducers used as loudspeakers, drive units (compression drivers) for exponential horns and the like are well known in the specific technical field. This type of transducer comprises a moving equipment or voice coil fastened to a cone or a diaphragm in the case of a loudspeaker and, respectively, of a compression driver for exponential horns .

The mobile coil is immersed in a static magnetic field that is currently produced by means of permanent magnets in different configurations and arrangements.

The voice coil is movable in an annular air gap comprising a cylindrical central core surrounded by an annular member, both made of ferromagnetic material and associated with permanent magnets which produce static magnetic field.

When the voice coil carries an acoustic frequency current, forces tending to "draw" or "move away" the voice coil are generated with respect to the static magnetic field generated in the air gap, this resulting in a mechanical effect on the above mentioned cone or diaphragm at the acoustic frequency of the drive current.

In the following, reference will be made for simplicity to a voice coil mechanically associated to a cone, fed by a signal at acoustic frequency, and immersed in a magnetic field produced by an annular pole piece of construction well-known for a person skilled in the field.' Summary of the Invention

According to the present invention at least a stationary coil is provided, associated to the central core of the pole piece, or to the annular pole portion, defining an air gap within which the voice coil can move. The stationary coil/s is/are electrically connected in series, in parallel or in "compound" to the voice coil, optionally with the interposition of active- or passive- type "cross-over" circuits. The stationary coil/s is/are fed by the same acoustic- frequency signal used to drive the voice coil and arranged in such a way to reduce equivalent inductance or impedance of the moving equipment .

Brief description of the drawings

The electrodynamic acoustic transducer according to the present invention will be now described on the bases of presently preferred embodiments given as non limiting, illustrative examples and with reference to the attached drawings, in which:

-Figure 1 is a simplified partial view of an electrodynamic loudspeaker of the prior art;

-Figure 2 is a typical arrangement of the magnetic circuit of a loudspeaker or electrodynamic transducer as used both in the prior art loudspeakers and in the present invention; -Figure 3 is a structure of an electrodynamic loudspeaker according to the present invention;

-Figure 4 is the frequency response (2,83 V at 8 ohm/lm, standard box) of a 15in loudspeaker with and without the device according to the invention;

-Figure 5 shows the behaviour of the impedance of the mobile equipment according to the prior art and according to the invention as a function of the driving frequency on a logarithmic scale; and

-Figures 6A, 6B, 6C are possible arrangements of windings of the voice coil and auxiliary windings according to the present invention. Description of preferred embodiments

With reference to the drawings and in particular to figure 1, a typical electrodynamic loudspeaker, generally indicated at 10, comprises a basket 11 fixed to a block 12 supporting magnets and pole pieces. A cone 15 is suspended to the edge 13 of basket 11 by means of a first diaphragm member 14, while a voice coil 16 is mounted at the apex of cone 15. A second diaphragm member 17, extending between the apex of cone 15 and the other edge of basket 11, together with first diaphragm member 14, allows cone 15 to move along its axis when mobile coil 16 is driven by acoustic-frequency currents provided by the output stage of an amplifier (not shown) .

Block 12 typically comprises a plate 20 made of a ferromagnetic material with a circular column-shaped core 21 placed in correspondence to a hole 22 of a ferromagnetic annular member 23. One or more magnets 24 for generating, in a known way, a magnetic flux in the air gap between hole 22 and core 21 are arranged between plate 20 and annular member 23. In Figure 3 a spacing between elements 11, 23, 24 and 12 is shown for sake of clarity only. As a matter of fact, in practice these elements are fixedly connected to each other by mechanical means, adhesives or the like, as well known from the prior art.

Voice coil 16 is free to move in a perpendicular direction with respect to the magnetic flux lines in the above mentioned air gap, thus forming a moving equipment integral to cone 15. It is usual to maintain the magnetic flux in the air gap as high as possible, as known. The voice coil 16 has a specific resistance and inductance as a function of the winding parameters.

While the resistive component of the winding of mobile coil 16 is constant, its impedance varies with the driving frequency and other parameters of the loudspeakers or other transducer.

A typical impedance behaviour of an electrodynamic loudspeaker of the subject kind is represented by the curve A of figure 5, in which it is noted that, starting from a nominal value of 5 ohm (D.C.), with the increase of the frequency a peak Al of about 45 ohm at the resonance frequency of about 50 Hz in encountered. Then, the impedance decreases, as the frequency increases, to a minimum A2 at about 200 Hz, from which it increases again to high figures (A3) at frequencies of the order of 10.000 Hz.

Each loudspeaker of this type has its own "signature" which is characteristic of the impedance/frequency curve. Furthermore, even apparently identical loudspeakers can exhibit different curves caused by both production differences and different conditions pf use (acoustic boxes, exponential horns, etc.) .

According to the present invention there is provided an arrangement directed to make uniform the impedance read at the ends of the voice coil as a function of the frequency, with the addition of one or two stationary coils, placed in the air gap of the static magnetic structure of the loudspeaker or other electrodynamic transducer and driven by the same acoustic-frequency current that drives the mobile coil . The arrangement according to the invention is shown in figure 3, wherein the same reference numbers of figures 1 and 2 are used to indicate the same components.

In the embodiment of figure 3 a stationary coil 30 and/or 31 is provided, in the sense that only one coil 30 or 31 can be provided. Stationary coils 30, 31 and mobile coil 16 can be interconnected as shown in figures 6A, 6B, 6C.

In figures 6A, 6B, 6C stationary coils 30, 31 are shown as directly connected together with voice coil 16. It is included in the scope of the present invention the arrangement in which stationary coils 30, 31 are connected to the acoustic source driving voice coil 16 through transformers or active or passive cross-over filters.

The interaction between voice coil 16 and stationary coil/s 30, 31, being part of the same electromagnetic circuit and connected according to the arrangement of figures 6A, 6B, 6C, results, as a primary effect, in the cancellation or_ reduction of the inductive effect of the mobile coil. Furthermore, thanks to the addition of the magnetic fields generated by stationary coil/s 30, 31 and by the transducer magnetic assembly, the force acting on the voice coil is increased. It is possible to have an experimental evidence of this effect from figures 4 and 5, in which, to an exemplifying ' frequency of 4 kHz (fig. 5), corresponds an impedance of 20 ohm for the conventional transducer (point B) and an impedance of 5 ohm (point B') for the arrangement according to the invention. This corresponds to a theoretical increase of 6 dB on the response curve of figure 4 (curve K) : really, an output increase of 10 dB can be observed due to the interaction of the combined forces of the stationary coil/s and the voice coil .

The impedance ratio between voice coil and stationary coil/s can be decided by the designer as a function of the intended use of the transducer.

The results obtained are exemplified in figure 4, wherein there are shown the response curves of a 15 in. woofer taken as a reference. A stationary coil (D.C. Resistance 16 ohm) , connected in parallel and push-pull to the voice coil (D.C. Resistance 3,4 ohm), is connected to the central core of the woofer. The winding of both the voice coil and the stationary coil/s can be made by any of the methods currently in use in the construction of loudspeakers or electrodynamic transducers .

It has also to be pointed out that the arrangement according to the present invention can be used, besides with loudspeakers, also with electrodynamic transducers used with exponential horns.

Variations and/or modifications can be brought to the electrodynamic acoustic transducer with reduced equivalent inductance of the mobile equipment according to the present invention without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS 1. An electrodynamic acoustic transducer with reduced equivalent inductance of the mobile equipment characterized in that it comprises : a magnetic circuit structure including a central core (21) a magnet assembly (24) and a polar expansion (23) defining an annular air gap in which a voice coil (16) carrying acoustic-frequency current is placed; and at least one stationary coil (30, 31) in said annular air gap arranged to carry said acoustic- frequency current, the winding of said voice coil (16) and said at least one stationary coil (30, 31) being arranged in such a way to reduce the equivalent impedance of the moving equipment consisting of said voice coil (16) and associated members.

2. An electrodynamic acoustic transducer according to claim 1, wherein said stationary coil (31) is wound around said central core (21) .

3. An electrodynamic acoustic transducer according to claim 1, wherein said stationary coil (31) is wound around said polar expansion (23) .

4. An electrodynamic acoustic transducer according to claim 1, wherein said stationary coil (30, 31) is constituted of two coils, a first one (31) being wound around said central core (21) and a. second one (30) around said polar expansion (23) .

5. An electrodynamic acoustic transducer according to claim 4, wherein said stationary coils (30, 31) are connected in series to one another and in parallel to said voice coil (16) .

6. An electrodynamic acoustic transducer according to claim 4, wherein _. said stationary coils (30, 31) are connected in parallel to one another and in parallel to said voice coil (16)