PURPOSE: To suppress the resonance with a peak and to improve acoustic characteristic by using the skin manufactured to sheets and provided with ruggedness having thin and dense material quality concentrically to the skin adhered to both faces of a core member.

CONSTITUTION: The skin 12 on the surface adhered on one face of the core member by means of an adhesives is formed by manufacturing to sheets a pulp fiber with press forming. Lots of projections 1 are formed on circumferences on the surface of the skin 12 are formed and the projected parts and recessed parts 14 between the projections form alternately annuraly. The press pressure of the projections 13 is large in the hardness of the skin 12 because the projected parts 13 and the recessed parts 14 are formed by applying a press pressure to the paper manufactured to sheets, the composition is made thin and soft, and the recessed parts 14 has a large press pressure and the composition is made dense and formed hard. In incorporating a voice coil to the inner circumference of the honey-comb diaphragm thus formed and vibrating the assembly, the mechanical internal loss is larger at the thin part formed with lots of the projected parts 13 of the skin 12, the resonance with a peak at a high frequency is suppressed so as to obtain a flat sound pressure versus frequency characteristic.