PURPOSE:To form an acoustic diaphragm having superior characteristics by performing glow discharge in vacuum including gas consisting of carbon and hydrogen atoms at least and forming a thin film of amorphous carbon including hydrogen on a negative plate. CONSTITUTION:A plate-like cathode 2 is installed in a vacuum case 1 as 0.5mm. copper plate e.g. After exhausting from an exhausting port 8 by an vacuum pump not shown, inert gas Ar is fed through a pipe 5 and a valve 7 and the negative plate 2 is ion-bombarded for ten minutues while applying DC high voltage from a power supply 3 to clean up the negative plate 2. After stopping the feeding of Ar, gaseous C2H2 are fed through a pipe 4 and a valve 6, -5kV DC voltage is applied to the negative plate 2 to generate glow discharge to deposite the carbon to form 20mum carbon film. Subsequently, the negative plate 2 is taken out, resolved with acid solution to remove copper and to wash with water. Consequently a diaphragm with high specific elastic modulus and high tensile strength is formed.
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