By having multiple air passage holes 10c in the housing 1 and by creating a clearance G2 between the outer surface of the yoke 20 and the inner surface of the housing 1, the measurement of the clearance being confined to a range exceeding 0% but not exceeding 2.5% of the inner radius of the housing, the amount of air movement is limited by the clearance G2, and by this means the frequency range within which the desired acceleration can be attained is expanded.