PURPOSE: To secure the easy impregnation for a proper amount of the damping agent by providing the layer, which functions to impregnate the damping agent, onto the sheet-type surface of the outer layer of the diaphragm featuring the snadwich structure.

CONSTITUTION: The diaphragm is formed by sandwiching the honeycomb core with sheet substance 30. Substance 30 contains 2nd layer 32 of the Japanese paper, the cloth or the like featuring the impregnating properties to 1st layer 31 composed of the film of the aluminum alloy component, the beryllium alloy component or the titanium alloy. And furthermore, 3rd layer 33 of the same material as the 1st layer is provided over the 2nd layer. The damping agent is impregnated previously into 2nd layer 32 or impregnated later with 3rd layer 33 formed meshy.