The output level of vibration of a portion below the spring of a vehicle detected by a sensor, and its frequency is converted to obtain the frequency spectrum of the vibration level. An inner sheet of the stack has formed therein a pair of bifurcated projections, wherein at least one first apex faces in one direction and at least one second apex faces in the opposite direction. The piece and the pockets are each formed to a thickness to extend beyond the plane of the corresponding surface of the device captured therein. A gate electrode is disposed between the first and second floating bodies. The other chamber is partially filled with ink.