Abstract: Method and apparatus in which moving metal strip is treated in a treating solution in an elongated tank having a granite bottom sloping upwardly on each side of the longitudinal center line of the tank and ultrasonic energy is directed downwardly through the moving metal strip to be reflected back and forth between the sloping granite bottom and the surface of the treating solution in a path that extends across the width of the tank and repeatedly intersects the moving metal strip. The minimum number of ultrasonic generating units are possible in an arrangement in which the ultrasonic generating units extend longitudinally of the tank along a line above a marginal portion of the moving metal strip. The sonic generators can be supported in liquid-containing receptacles which extend below the liquid level of the treating solution in the tank. The liquid-holding receptacles can be supported on cover members which extend across the tank and enclose the top of the tank.