PURPOSE: To increase the reproduced sound pressure at the medium and low pitched sound bands, by transmitting the vibration of a piezoelectric oscillator to a mobile member via a sealed-up liquid and as a result driving a diaphragm connected to the mobile member with a large amplitude.

CONSTITUTION: A disk-shape piezoelectric oscillator 5 generates the inflexive vibration while forming concentric node circles with application of an electric signal. A cylinder 6 has an equal diameter to that of the node circle and touches close to the position of the node circle of the oscillator 5 through the lower edge aperture part. A bowl-shape frame 7 is fitted to the upper edge aperture part of the cylinder 6 at a bottom part 8, and a dome-shape mobile member 9 is attached to a through-hole at the center of the part 8. Then a liquid is filled into a closet formed by the oscillator 5, the cylinder 67, the bottom part 8 of the frame 7, and the member 9. The vibration of the oscillator 5 is amplified through the liquid and transmitted to the member 9 to drive a conical cone paper 11 with a large amplitude.