Electric sound generator for battery alarm clock - has variable gap between ferromagnetic cylindrical core and diaphragm to set tone quality

摘要

Electric sound generator for battery-powered alarm clock comprises a rotationally symmetrical hollow body (1) which is closed airtightly by a ferromagnetic diaphragm (2). Inside, in the manner of a bell-shaped magnet, are disposed a ferromagnetic cylindrical core (4, 5) spaced apart from the diaphragm, and an energising coil (3) surrounding the core. A magnetic d.c. field is superimposed on the electromagnetic a.c. field produced by the coil. On the outer surface of the diaphragm, coaxial with the air gap between core and diaphragm, is a permanent magnet (13). Means are provided for varying the gap between core and diaphragm. The core may be provided at its lower end with a fine thread which engages with a threaded hole in the bottom of the hollow body.