Patent ID: 7792318
Filing Date: 2010-09-07
Classification: H04R

Abstract:
1. A speaker comprising: a center pole; a voice coil bobbin which comprises a nonmetallic pipe body; and a first and a second electrodes which are provided on an inner peripheral face of the nonmetallic pipe body for detecting an electrostatic capacity that is formed between the electrodes and the center pole, and the first and the second electrodes being disposed on the inner peripheral face and separated from each other with a predetermined space along an axial direction of the center pole; a stepped portion which is formed in a side circumferential face of the center pole; and a subtracter which subtracts a second electrical signal obtained from the second electrode from a first electrical signal obtained from the first electrode and outputs a third electrical signal corresponding to a result of the subtraction; wherein, when the voice coil bobbin is operated, a first facing area of the first electrode to the side circumferential face defined by the stepped portion of the center pole increases while a second facing area of the second electrode to the side circumferential face defined by the stepped portion of the center pole decreases by a same amount as an increased amount of the first facing area.