Two layers of a plurality of rod-like permanent magnets each having a width Wm, a thickness Tm and a predetermined length are aligned on a plane in such a way that they have opposite magnetic pole orientations alternately and are aligned at a fixed pole pitch τp are arranged to be opposed to each other with the magnetic pole orientation of each magnet in one of the layers being identical to that of the opposing magnet in the other layer. The opposing surfaces of the magnets are spaced a distance 2×lg from each other, and a vibrating membrane on which coils each having a conductive pattern are arranged is placed in a gap between any two adjacent rod-like permanent magnets in each of the two layers, where lg is a distance from the vibrating membrane to the surface of a magnet. The arrangement of the rod-like permanent magnets is optimized by using Wm, Tm, τp, and lg.