Electro-acoustic Magnetic Reed Type Transducer Having Box-shaped Pole Piece

Abstract

An electro-acoustic transducer with an open ended box-shaped inner housing which supports one leg of a U-shaped armature within a main frame. The other leg of the U-shaped armature extends through the open end of the box shaped housing and is free to vibrate in an air gap formed between two permanent magnets which are fixedly mounted on opposite inner wall portions of the box-shaped inner housing. The vibratable leg of the U-shaped armature is surrounded by a coil and is connected by a drive pin to a diaphragm. The box-shaped inner housing forms the closing portion of d.c. and a.c. flux paths.

Description

The invention relates to an electro-acoustic transducer.

A transducer known from U.S. Pat. No. 3,347,991 comprises a U-shaped armature which is so supported as to permit free movement of one leg in an air gap defined by two permanent magnets and the other leg with the permanent magnets is included in a magnetic circuit.

In these prior constructions the armature is supported owing to the fact that the relevant end is clamped in an assembly of component parts, which involves that in the magnetic circuit of which the permanent magnets form part, parasitic magnetic resistances are introduced, namely owing to irregularities in the bonds of the stacked component parts. Especially when the transducer is mass produced, this may give rise to relatively high magnetic contact resistances. This does not only result in undesirable loss of field strength in the active air gap, but undesirable asymmetries in the magnetic field can also be caused.

Moreover it is necessary in this prior construction that for centering the free armature leg in the effective air gap a relatively cumbersome operation is necessary, which is less attractive from a manufacturing and technical point of view.

The object of the invention is to remove the above drawbacks and to provide an electro-acoustic transducer which can be produced in an easy manner and wherein the magnetic field in the active air gap is optimized. Moreover minimization of the liability of losses and asymmetries in the magnetic field has been aimed at.

According to the present invention there is provided an electro-acoustic transducer comprising a U-shaped armature which is so supported as to permit free movement of one leg thereof in an air gap defined between two permanent magnets and the other leg with said permanent magnets is included in a magnetic circuit, wherein the two magnets are abutted against a corresponding inner wall portion of a box-shaped housing manufactured of magnetically conductive material, which housing has an opening exclusively on the side facing the armature.

To ensure that also the symmetry of the magnetic circuit for the alternating flux is optimal, the leg of the U-shaped armature which is not positioned in the air gap, is abutted against a corresponding outer wall portion of the box-shaped housing. Owing to the symmetrical configuration of said box-shaped housing the symmetry of the magnetic circuit for the alternating flux is now also ensured.

In this connection it is observed that with a construction proposed by the invention a better magnetic shielding is obtained as compared with constructions known for instance from the above U.S. patent, wherein the air gap is open at two or more sides. Owing to the fact that in a transducer constructed according to the invention said box-shaped housing is provided with an opening solely on the side facing the armature disturbances caused by radiation, and sensitivity to extraneous effects are considerably reduced. These properties can be utilized advantageously if the transducer according to the invention must be used in hearing aid devices of the smallest possible dimensions and in which a microphone and a telephone are positioned relatively close to each other. In such devices it is important that the magnetic coupling between micophone and telephone is as weak as possible, because if the gain is made to exceed a certain limit, singing may occur. With a transducer constructed in accordance with the invention, wherein such a magnetic coupling is minimized, a relatively high gain is therefore still allowable.

Although the above U.S. patent mentions steps for reducing the susceptibility to shocks of an electro-acoustic transducer, the drawback of this device is that part of the external housing of the transducer is included in the magnetic circuit for the d-c flux, while a mechanical coupling exists between the armature and a magnet on the one hand and said housing on the other. This involves that the centering of the free end of the armature in the active air gap can be disturbed if the transducer is exposed to shocks and impacts, as a result of which the housing may be slightly deformed. As the centering of the free armature end in the active air gap is particularly critical (the allowable tolerances have an order of magnitude of some microns), slight distortions of the transducer may render it unusable.

To remove these objections a tranducer according to the invention is further characterized in that the coil and the box-shaped housing are secured to one side of a mounting plate, on the other side of which a diaphragm sheet is spring-mounted, the drive pin for said diaphragm sheet extending through an opening in said mounting plate, and being attached in close proximity to the box-shaped housing to the movable leg of the armature.

To further reduce the susceptibility to shocks the transducer according to the invention is characterized in that said mounting plate with the motor attached thereto are so positioned in a housing of the device formed by a cover and a container that said box-shaped housing and the armature are so spaced from said housing that even in case of slight distortion or indentation thereof there will be no contact.

One embodiment of the invention will be explained in greater detail with reference to the accompanying drawings, in which:

FIG. 1 shows a perspective embodiment of a transducer according to the invention; and

FIG. 2 shows a cross-section on the line II--II of the construction shown in FIG. 1.

In the embodiment shown in FIG. 1 a container indicated by 1 with a cover 2 fitting thereon forms a housing for the transducer. A mounting plate 3 is fittingly mounted in said container, which mounting plate forms a base for on the one hand the electro-mechanical part, also referred to as motor, of the transducer, and on the otherhand for a diaphragm sheet 9. As is also schematically shown in FIG. 2 said motor comprises a box-shaped housing 4 manufactured of magnetically conductive material and serving as a housing for two permanent magnets 5 and 6 which define an air gap between which the free end of a U-shaped armature 7 can move. These two permanent magnets 5 and 6 can easily be attached to a corresponding inner wall portion of the box-shaped housing 4, whereby the correct position of these magnets is ensured. The armature 7 is a U-shaped bent strip of magnetically conductive metal, for instance .mu.-metal. One end of said armature 7 is attached to the lower side wall of the box-shaped housing 4, whereas the other end of said armature extends via the single opening in the housing 4 into the air gap between the magnets 5 and 6. In said air gap there prevails a strong, constant magnetic field caused by magnetization of the magnets 5 and 6. Around one of the legs of the armature 7 there is provided a coil 8 in such a manner that said coil nowhere contacts the armature. Said coil 8 and the housing 4 are attached to one side of the mounting plate 3. To the other side of said mounting plate there is attached the diaphragm 9 in such a manner that said diaphragm is spring-supported on said mounting plate, which has been achieved by means of the rib 10. This diaphragm is formed of a thin and relatively rigid sheet, for instance light metal, said rib 10 also ensuring that the diaphragm rests on the mounting plate and seals hermetically. By producing the rib 10 from thin foil it is achieved that the diaphragm sheet 9 can move in vertical direction with slight rigidity relative to the mounting plate 3. A drive pin or strip 11 extends through an opening made in the mounting plate 3 to the free end of the armature 7, and is attached to said armature end there and in close proximity of the housing 4, for instance by a butt joint. The other end of the drive pin 11 is attached to the diaphragm.

The depth of the above container 1 is selected in such a manner that the bottom thereof is spaced from those parts which are mounted on the lower side of the mounting plate 3.

The space above the diaphragm sheet 9 is limited by the cover 2, which is drawn in FIG. 1 in detached condition. In the ready condition of the transducer, said cover 2 is mounted with its lower edge on the upper edge of the container 1 in a hermetically sealed fashion, while at the same time a mechanically strong connection is obtained. Said cover 2 has an opening 12.

With such a construction both the armature and the magnets are solely connected to the box-shaped housing 4 so that this assembly is spaced from the housing of the transducer formed by the container 1 and the cover 2, which reduces the susceptibility of shocks considerably, while slight distortions or damages of said housing formed by the cover and the container, do not affect the proper operation of the transducer.

If an electric current flows through the coil 8, a magnetic flux is caused to flow in the armature 7, so that the free end 14 (FIG. 2) of said armature is magnetically polarized with respect to the stationary end 13 (FIG. 2) of said armature and the box-shaped housing 4 magnetically conductively connected to said stationary end. As a result of said polarization said free end of the armature experiences a force in the field caused by the permanent magnets, so that a displacement results in vertical direction. In this way electrical vibrations can be converted into mechanical vibrations which are transferred via the drive pin 11 to the diaphragm and can subsequently leave the opening 12 as acoustic vibrations. In this situation the transducer acts as a receiver. The transducer is also useful to convert acoustic vibrations into corresponding electric vibrations. These acoustic vibrations can actuate the free end of the armature via the diaphragm and the drive pin, as a result of which an alternating flux is caused to flow in the armature which in turn induces a current in the coil.

An electro-acoustic transducer constructed according to the invention has a high output, as well as a simple and attractive construction from a manufacturing point of view.

Claims

I claim

1. An electro-acoustic transducer comprising a case and an associated cover member;

a mounting plate connected intermediate said case and said cover member;

a motor unit including, a box-shaped housing of magnetically conductive material having at least five closed sides and a single opening including top, bottom, side and an end wall portions;

two permanent magnets secured to the interior surfaces of said top and bottom wall portions and being spaced apart forming an airgap therebetween in said housing;

the exterior of said top wall portion connecting said box-shaped housing to said mounting plate;

a coil connected to said mounting plate in spaced relation to said housing and adjacent the single opening thereof;

a substantially U-shaped armature having one leg thereof connected to said housing and the other leg thereof extending through said coil and the single opening of said housing and having a free end positioned in the airgap between said permanent magnets and spaced from said end wall portion of said housing.

2. An electro-acoustic transducer as set forth in claim 1 in which said one leg of said U-shaped armature is connected for support to the exterior of said bottom wall portion of said box-shaped housing.

3. An electro-acoustic transducer as set forth in claim 1, in which said coil is connected throughout one exterior longitudinal surface thereof to the same surface of said mounting plate as said box-shaped housing.

4. An electro-acoustic transducer as set forth in claim 1 in which said end wall of said box-shaped housing is positioned in spaced relation adjacent said case and is disposed between said case and the single opening of said housing.