Electromagnetic Sonic Generator for an Alarm

This invention relates to an electro-acoustic transducer of electromagnetic type and more particularly to an electro-acoustic transducer having the property of being not influenced by circumstances.

In an electronic alarm watch, an electro-acoustic transducer of electromagnetic type, which is used as an alarm generator, has an exciting part and a vibrating plate excited by it.

A conventional electro-acoustic transducer of electromagnetic type is shown in FIG. 1, as an example. An exciting part 1 is composed of a plate 2 which has a magnetic core 3 and is made of magnetic material, e.g., pure iron, an exciting coil 4 wound on the magnetic core 3 and a cylindrical permanent magnet 5 which is provided on the plate 2 to surround the exciting coil 4. This is received in a supporting body 6 which is made of non-magnetic material, e.g., brass. A vibrating plate 7, which is a flexible metal plate, the circumference of which is simply supported on the circular end of the supporting body 6, is provided oppositely to the magnetic core of the exciting part 1 at an interval. A thin plate 8, which is a ferromagnetic substance and is formed smaller than the vibrating plate 7, is connected to the vibrating plate 7. In this case, the thin plate 8 is connected to the surface of the vibrating plate 7 opposite to that on the side of the magnetic core 3, as shown in FIG. 2, at one point "a" in center by means of spot welding or the like. As the thin plate 8 is attracted to the side of the exciting part 1 by the magnetic attraction of the permanent magnet 5 of the exciting part 1, the circular end of the vibrating plate 7 placed merely on the top end of the supporting body 6 is pressed so as to be held to the supporting body 6. The vibrating plate 7 is bent, as shown in FIG. 3, as the thin plate 8 is attracted to the side of the exciting part 1. Accordingly, the circular end of the thin plate 8 is pressed to the vibrating plate 7 and the thin plate 8 itself is also bent, so that the one point connection at only the point "a" makes a sufficient connection between the thin plate 8 and the vibrating plate 7. The thin plate 8 does not operate independently as a rigid body but vibrates together with the vibrating plate 7 in the direction of X. In accordance with the foregoing, even if the connection between the vibrating plate 7 and the thin plate 8 is a spot one, it offers no problem for the function of the electro-acoustic transducer of electromagnetic type. As the thin plate 8 is connected to the surface of the vibrating plate 7 on the side opposite to the side of the exciting part 1, however, in case that the electro-acoustic transducer of electromagnetic type is used for an electronic alarm watch or the like, the thin plate 8 is directly exposed to the open air through sound releasing holes formed through a watch casing or the like. The thin plate 8 is generally made of pure iron. Since such a ferromagnetic substance offers a problem of corrosion resistance, it is undesirable to use a conventional electro-acoustic transducer of electromagnetic type for an electronic alarm watch or the like.

It is an object of the invention to solve the problem of corrosion resistance of the thin plate which is connected to the vibrating plate and is made of ferromagnetic material.

It is another object of the invention to provide an electro-acoustic transducer of electromagnetic type which has the property of being not influenced by circumstances so that it may be used under any environment.

It is another object of the invention to provide a transducer which may restrain the undesirable vibration of the thin plate.

These and other objects, features and advantages of the invention will become more apparent upon a reading of the following detailed specification and drawing, in which:

FIG. 1 is a vertical section showing a structure of a conventional electro-acoustic transducer of electromagnetic type.

FIG. 2 is a plan view showing the connecting condition between the vibrating plate and the thin plate shown in FIG. 1.

FIG. 3 is a view showing the static transformation state and the vibrating direction at the time of exciting of the vibrating plate shown in FIG. 1.

FIG. 4 is a vertical section showing an embodiment of the electro-acoustic transducer of electromagnetic type according to the invention.

FIG. 5 is a view for explaining the connecting condition between the vibrating plate and the thin plate, the static transformation state of the vibrating plate and the vibrating direction at the time of exciting, in a case of one point connection.

FIG. 6 is a plan view showing another connecting condition between the vibrating plate and the thin plate.

FIG. 7 is a view for explaining the static transformation state of the vibrating plate and the vibrating direction at the time of exciting, in the connecting condition shown in FIG. 6.

FIG. 8 is a plan view showing another connecting condition between the vibrating plate and the thin plate, in the electro-acoustic transducer of electromagnetic type according to the invention.

FIG. 9 is a plan view showing another connecting condition between the vibrating plate and the thin plate similar to that.

FIG. 10 is a plan view showing another connecting condition between the vibrating plate and the thin plate similar to that.

Referring to FIG. 4 which shows an embodiment of the electro-acoustic transducer of electromagnetic type according of this invention, numeral 9 is an exciting part. This is composed of a plate 10 which has a magnetic core 11 in the center and is made of magnetic material, an exciting coil 12 wound on the magnetic core 11 and a cylindrical permanent magnet 13 which is provided on the plate 10 and surrounds the exciting coil 12. The exciting part 9 is received in a cylindrical supporting body 14. A vibrating plate 15, which is made of flexible metal, the circumference of which is simply or fixedly supported on the circular end of the supporting body 14, is provided oppositely to the magnetic core 11 of the exciting part 9. A thin plate 16, which is made of ferromagnetic substance and is formed smaller than the vibrating plate 15, is provided on the surface of the vibrating plate 15 on the side of the magnetic core 11.

Since the electro-acoustic transducer of electromagnetic type having such composition has the thin plate 16 which is provided on the surface of the vibrating plate 15 on the side of the magnetic core 11, when it is used for an electronic alarm watch or the like, for example, the thin plate 16 is prevented from being directly exposed to the open air, so that it is possible to prevent corrosion of the thin plate 16 being caused by the open air.

The connection between the vibrating plate 15 and the thin plate 16 may be performed at one point "b" by means of spot welding, as shown in FIG. 5, for example, similarly to the case of conventional electro-acoustic transducer of electromagnetic type. In this case, however, as the thin plate 16 is attracted by the exciting part 9, the vibrating plate 15 is bent, so that a gap is formed between the part except the center of the thin plate 16 and the vibrating plate 15. At the time of exciting, the thin plate 16 vibrates in the direction of X together with the vibrating plate 15. On the other hand, the thin plate 16 operates as a rigid body independently of the vibrating plate 15 and vibrates in the direction of Y with a point "b" being a supporting point. The undesirable vibration in the direction of Y generated by the thin plate 16 interferes in the main vibration of the vibrating plate 15 in the direction of X and has a bad influence upon it.

In a severe case of that, the connecting part on the point "b" between the vibrating plate 15 and the thin plate 16 becomes weak. Consequently, it is possible that the connection between the vibrating plate 15 and the thin plate 16 may be separated.

In order to avoid that, the thin plate 16 is spot-welded to the vibrating plate 15 at plural points "c" , " d" , " e" , " f" and "g" , as shown in FIG. 6. In such a case of connecting at plural points, the static transformation of the vibrating plate 15 has a bent state, as shown in FIG. 7, since the thin plate 16 is attracted by magnetic force of the exciting part 9. As the thin plate 16 is connected to the vibrating plate 15 at plural points, the relative position to the vibrating plate 15 is immovable, that is, unlike the case of connecting at one point, the thin plate 16 operates as not a rigid body, so that it is possible to restrain the undesirable oscillation mode in the direction of Y. Consequently, the thin plate 16 vibrates with the vibrating plate 15 in the direction of only X.

In the embodiment shown in FIG. 6, the thin plate 16 is connected to the vibrating plate 15 at five points in a spot state. However, even if the connection is performed at three points "h" , " i" and "j", as shown in FIG. 8, similar effects may be expected. But, in a case of connecting at two points in a spot state, the thin plate vibrates undesirably with a supporting axis, which is a line connecting the two spots, and operates as a rigid body. In a case of connecting the vibrating plate 15 and the thin plate 16 in a spot state, therefore, it is necessary to perform at plural points more than three.

In the embodiments shown in FIG. 6 or FIG. 8, the thin plate 16 is connected to the vibrating plate 15 in a spot state. However, even if the connection is performed on a circular band line k which surrounds the center of the vibrating plate 15 and of the thin plate 16, using a bonding agent or the like, as shown in FIG. 9, similar effects may be obtained. This line "k" is not limited to a circle and it does not matter if it is another shape, e.g., a loop shape, a shape cut to pieces at plural position. It is possible, furthermore, to connect on a surface "l" using a bonding agent or the like, as shown in FIG. 10. When the thin plate 16 is connected to the vibrating plate 15 in not a spot state, i.e., in a state having some area, shown in FIG. 9 and FIG. 10, the thin plate 16 operates as not a rigid body independently of the vibrating plate 15 and the undesirable oscillation mode of the thin plate 16 is restrained, so that the main oscillation mode of the vibrating plate 15 becomes good.

In accordance with the foregoing, in the electro-acoustic transducer of electromagnetic type according to the invention, as the thin plate is provided on the surface of the vibrating plate on the side of the magnetic core of the exciting part, the thin plate made of ferromagnetic substance is covered to the open air by the vibrating plate, so that it is possible to prevent corrosion of the thin plate through the open air, to give the transducer the property of being not influenced by circumstances and to use the transducer anywhere.

By connecting the thin plate with the vibrating plate at plural points more than three or at the part which is not a spot and has some area, the undesirable vibration of the thin plate may be restrained, the main oscillation mode of the vibrating plate may not be interfered with by the undesirable vibration of the thin plate, the main oscillation mode becomes good, and the condition between the vibrating plate and the thin plate becomes firm, so that there is no risk of being disconnected.

As described hereinbefore, the electro-acoustic transducer of electromagnetic type has been explained in detail on the basis of the shown embodiments. The present invention, however, is not limited to the shown embodiment, and then various modifications and improvements may be made.