It is shown theoretically that plane longitudinal sound waves traveling in an electrically conducting plate will generate complex electric currents provided there is a magnetic induction perpendicular to the particle displacement. The currents react with the induction to produce a complex mechanical force which opposes the particle motion. The impedance associated with this force effectively modifies the impedance of the medium by adding a resistance and a stiffness reactance. The induced resistance attenuates the waves while the induced stiffness increases Young's modulus and therefore increases the sound velocity.Of all the elements, it appears that this effect is a maximum in lithium, i.e., at a given frequency and induction.