Electromagnetic audible signal devices, commonly referred to as buzzers, or horns, are known to the art. Examples of such devices are generally disclosed in U.S. Pat. Nos. 3,784,944 (Schantz, et al), 3,653,039 (Hoare) and 3,656,155 (Parkes).
The typical prior art electromagnetic horn includes an electromagnetic assembly (including a coil and an associated core) and a movable spring-like armature positioned thereabove. The assembly and the armature are housed in a single container having a diaphragm across its top. The diaphragm and the armature carry cooperating electrical contacts which are in series, electrically, with the coil.
Operation of the horn is conventional. When the coil is energized, its core magnetically attracts the armature. As the armature moves toward the core, the contacts open, breaking the circuit to the coil. This releases the armature which resiliently returns, its contact impacting on the diaphragm contact and reconnecting the circuit causing the cycle to repeat. The repeated impacts produce the desired audible signal.
The intensity, pitch, and quality of the tone produced by the device depend on many different factors including the type of material from which the diaphragm is made, the shape of the diaphragm, the shape of the armature, the resiliency of the armature, and the distance between the armature and the magnetic assembly.
With the exception of the distance between the armature and the magnetic core, all of the aforementioned factors are normally predetermined by the design of the device and can be adjusted or varied only with difficulty. However, the prior art devices generally include means for factory adjustment of the gap between the magnetic assembly and the armature.
The reason the distance between the armature and the magnetic assembly affects the intensity of the noise produced by the horn is that the more the armature is bent towards the core, the greater is the resilient force stored therein and the greater is its impact against the diaphragm when released.
The means provided by the prior art for adjusting the distance between the armature and the core include a rotatable screw received by a threaded opening in the bottom of the housing below one side of the coil. When the screw is turned it rises, and lifts one side of the magnetic assembly. Because the other side remains stationary, the assembly tilts, thus decreasing the gap between the armature and the upwardly inclined portion of the magnetic core.
This approach is not completely satisfactory for many reasons. Because the coil is tilted when adjusted, its magnetic force is not utilized efficiently. It is difficult to make a precise gap adjustment since there is no uniform distance between the armature and the core. The slanting magnetic core subjects the system to misalignment and loss of magnetic and mechanical efficiency.
Another disadvantage of the prior art approach is dissipation of the magnetic field of the energized coil because it is not isolated from the other components of the horn. Also, the known devices do not provide means for restoring mechanical energy transferred to the housing back into the vibrating system.
Furthermore, the magnetic efficiency of the prior art horns is diminished because the conducting means through which the magnetic field is applied to the armature includes the resilient armature support means, thereby increasing the path traveled by the field.
It is a primary object of the present invention to provide an improved audible signal device having simple and economical means for linearly adjusting the gap between the magnetic assembly and the armature such that the top of the coil and the armature always remain in substantially parallel relationship.