U.S. patent number 3,742,156 [Application Number 05/177,643] was granted by the patent office on 1973-06-26 for electro-acoustic magnetic reed type transducer having box-shaped pole piece.
This patent grant is currently assigned to Microtel N.V.. Invention is credited to Harmen Broersma.
United States Patent |
3,742,156 |
Broersma |
June 26, 1973 |
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.
Inventors: |
Broersma; Harmen (Amsterdam,
NL) |
Assignee: |
Microtel N.V. (Amsterdam,
NL)
|
Family
ID: |
19813623 |
Appl.
No.: |
05/177,643 |
Filed: |
September 3, 1971 |
Foreign Application Priority Data
Current U.S.
Class: |
381/386;
381/417 |
Current CPC
Class: |
H04R
11/00 (20130101); H04R 25/604 (20130101) |
Current International
Class: |
H04R
11/00 (20060101); H04R 25/00 (20060101); H04r
011/00 () |
Field of
Search: |
;179/114R,114A,115R,115A,117,119R,119A ;335/231 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kundert; Thomas L.
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.
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.
* * * * *