Abstract: A cleaning head is adapted to rest on and be moved over the material to be cleaned. The cleaning head has a central chamber, open at its bottom, and at least one relatively narrow chamber, open at its bottom, peripherally surrounding the central chamber. Ultrasonic transducer means are provided in the central chamber and means are provided for supplying a cleaning solution to the central chamber and for maintaining the level of the solution in the chamber such that at least a portion of the transducer means is immersed in the solution so that vibrations of the transducer means are imparted to the solution. Means are provided for adjusting, i.e. controlling, the pressure of the cleaning solution in the central chamber and the pressure in the at least one peripheral chamber to substantially eliminate the flow of air into the solution in the central chamber and to prevent escape of substantial amounts of solution from the cleaning head.