Minimization of distortion due to a voice coil displacement in a speaker unit

摘要

A speaker of a type having a voice coil wound to a greater length than the length of a magnetic gap into which it extends formed between a pole of a yoke and an annular plate. A magnetic flux density setting structure is provided for establishing a constant value of Bl, where B represents the density of the magnetic flux capable of linking the voice coil and l represents the effective length of the voice coil. The magnetic flux density setting structure may be formed as a tapered surface or surfaces on the pole of the yoke or by a separate tapered plate or plates attached to the annular plate. Further, a metal cap is mounted on the pole covering an upwardly facing cup-shaped body of magnetic material.