A vibration element using giant magnetostrictive material (VEG) was developed as of a unit of an acoustic source for an ocean acoustic tomography system. This acoustic source was required to have low frequency and high power output. The magnetostriction characteristics of the giant magnetostrictive material were greatly altered by prestress. A VEG with changeable prestress was constructed, and its characteristics were measured for various values of prestress. This paper describes the effects prestress and changes in the magnetomechanical coupling factor on the operation of the VEG. Measurement of the static magnetostriction and impedance characteristics of the VEG showed that there were optimum values of the prestress and magnetic bias of the operating point on the static magnetostriction characteristics. Correlation between static magnetostriction and magnetomechanical coupling factor was indicated.
