Abstract: A holographic-based directional sound device is provided, the device including: a sound wave generating means generating a sound wave; and a flat plate configured to have the sound wave generating means installed at the center so as to radiate the sound wave to the outside through a surface, and to be composed of a plurality of unit cells, in which at least one groove is formed on a surface of the unit cell, and a radiation angle of the sound wave is determined according to a depth of the groove with respect to the unit cell, wherein the depth of the groove is determined by an individual surface admittance calculated by a cosine function or a sine function of the sum of a first value and a second value on the basis of a preset radiation angle of the sound wave and a preset frequency of the sound wave.

Abstract: Provided is a metal ball fabricating apparatus for fabricating a metal ball by melting a material. The metal ball fabricating apparatus includes: a fabricating unit configured to fabricate a metal ball; and a collecting unit configured to collect the metal ball. The fabricating unit includes: a chamber configured to receive and store a material; a heating unit configured to apply heat to melt the material in the chamber; an orifice disposed at a lower portion of the chamber to which a metal ball droplet drops; a piston disposed over the orifice to generate a metal ball droplet; and a purifying system configured to remove a foreign substance from the material.