[PROBLEMS] To accurately tune the frequency of vibration by means of a suspension during assembly. [MEANS FOR SOLVING PROBLEMS] Securing planned positions (3e) where a magnetic circuit portion (2) is secured to a suspension (3) are provided near to each other. From among the securing planned positions (3e), a securing planned position (3e) corresponding to the characteristics of the suspension (3) to be mounted or the weight of the magnetic circuit portion (2) is selected, and they are secured to each other. Thus, the length from the securing position to the vibration center position of the suspension (3) and a housing (1) is varies so that the frequency may be tuned to a desired value.