Patent ID: 8222984

Claim:
An electromagnetic transducer comprising: a first magnet arrangement layer in which a plurality of rod-like permanent magnets each having a width Wm, a thickness Tm and a predetermined length are aligned in parallel with one another on a plane such that they have opposite magnetic pole orientations alternately and are aligned at a fixed pole pitch τp, a second magnet arrangement layer in which a plurality of rod-like permanent magnets are aligned in a same way as the rod-like permanent magnets of the first magnet arrangement layer are aligned, and are arranged to be perpendicularly opposed to those in the first magnet arrangement layer with a magnetic pole orientation of each of the plurality of rod-like permanent magnets in the second magnet layer being identical to that of an opposing rod-like permanent magnet in the first magnet arrangement layer, and opposing surfaces of any two permanent magnets facing each other in the first and second magnet arrangement layers are spaced a distance 2×lg apart from each other, and a vibrating membrane on which coils each having a serpentine conductive pattern are arranged to be opposed to each other in such a way as to be placed in a gap between any two adjacent rod-like permanent magnets in each of said first and second magnet arrangement layers, such that the coils extend along a surface corresponding to each of said magnet arrangement layers at an intermediate position between said opposing surfaces of any two permanent magnets facing each other in the first and second magnet arrangement layers, wherein: when α=τp/lg, β=Wm/τp, and γ=Tm/lg, said rod-like permanent magnets are arranged in such a way that β<=0.15α+0.1 is satisfied, and lg is a distance from a surface of each of the magnets to the vibrating membrane, respectively.