An object of the present invention is to provide a voltage control apparatus enabling reduction in an angle formed by first and second circumferential end surfaces of a general fan shape centered around a slinger. 4 or higher in the sound absorbing coefficient even in the case where a perforated panel is a thin plate such as aluminum foil within a range where a reduction in the weight of a vehicle body is not impaired and through holes have a large hole diameter in order to make the processing of providing a large number of through holes in the perforated panel. The method also includes determining, in case of an unsuccessful transmission of the frames, whether the unsuccessful transmission is attributed to a frame collision.