PURPOSE: To prevent discrete vibration from occurring and to flatten the frequency characteristic by depositing a powder resin for powder coating reinforced chiefly by a carbon fiber on the surface of a diaphragm substrate of pulp material by means of electrostatic powder blasting, fluidized immersion, or electrostatic immersion.

CONSTITUTION: The diaphragm substrate is made by dispersing a pulp material in water and paper making it, on whose surface a thermo-setting liquid acryl resin mixed with 20 percent by weight of carbon fiber and 10 percent by weight of graphite is sprayed and dried, and made as paint for powder coating. Then this powder paint is charged with a high voltage, and attracted and adhered on the surface of the substrate. If the temperature of the substrate is higher than the melting point of the powder resin, the paint attached o the surface melts and is fused and adhered. When a speaker is assembled using the diaphragm made of the substrate which is powder- coated on both surfaces, its sound-pressure frequency characteristic is made as a curve 2. The reproduction band of the curve 2 extends wider than a curve 1, and the rigidity goes to large, preventing the discrete vibration of the diaphragm from occurring. I.e. the sound pressure frequency characteristic is made flat and the diaphragm is provided with excellent features.