Patent ID: 8131002

Claim:
An electro-acoustic transducer, comprising: a diaphragm; a casing which is formed with an opening in a part thereof for supporting therein the diaphragm so as to allow the diaphragm to vibrate; a first magnet which is provided on a side of the opening with respect to the diaphragm and polarized in a vibration direction of the diaphragm; a second magnet which is provided on a side of an inner bottom surface of the casing with respect to the diaphragm so that the diaphragm is interposed between the first magnet and the second magnet and which is polarized in a direction opposite to a polarization direction of the first magnet; a drive coil which is provided on the diaphragm so as to be located in a magnetic gap formed by a pair of the first and second magnets for generating a driving force which causes the diaphragm to vibrate; and a first yoke for forming a magnetic path in at least a portion around the first magnet, and a second yoke for forming a magnetic path in at least a portion around the second magnet, wherein when the first and second magnets are projected on the diaphragm, a first projected area of the first magnet is smaller than a second projected area of the second magnet, the diaphragm has an elongated shape in which a width in a lateral direction thereof is smaller than a width in a longitudinal direction thereof, the drive coil has an elongated shape in which a width in a lateral direction thereof is smaller than a width in a longitudinal direction thereof, and is provided on the diaphragm so that the longitudinal direction of the drive coil is parallel to the longitudinal direction of the diaphragm, the drive coil has a length in the longitudinal direction thereof, which is 60% or more of a length of the diaphragm in the longitudinal direction thereof, the drive coil and the diaphragm are disposed so that a central axis of the drive coil in a vibration direction thereof substantially coincides with a central axis of the diaphragm in the vibration direction thereof, and the diaphragm is driven over an entire surface thereof with respect to the longitudinal direction thereof, and wherein the first yoke is provided only on a surface of the first magnet opposite to a surface of the first magnet which faces the diaphragm, the second yoke is provided so as to surround surfaces of the second magnet other than a surface of the second magnet which faces the diaphragm, each of the first and second magnets is an elongated rectangular parallelepiped shape having long sides parallel to a long side portion of the drive coil, the first magnet having a same width in a long side direction thereof as a width of the second magnet in a long side direction thereof, and the first magnet has a width in a short side direction thereof, which is smaller than a width of the second magnet in a short side direction thereof, and a width of a first yoke in a short side direction thereof is the same as a width of the first magnet in a short side direction thereof.