Possibilities for control of slag foam height by sound waves were investigated using cold models. The foaming and defoaming rates were measured under various frequencies and intensities of sound. Nitrogen gas was injected into a liquid containing a surfactant to produce the foam layer. Simultaneously, the foam surface was exposed to sound waves propagated through the gas phase to the surface. The sound waves were generated by a loudspeaker. Two liquids of different viscosities were used: water and a water-glycerin solution. It was found that sound waves can control the defoaming rate, while the foaming rate does not vary practically under the application of sound. The latter finding is associated with the fact that the height of foam, produced in the experiments, was much smaller than the steady height. The defoaming rate is essentially reduced by the sound of lower frequency (200-1 000 Hz). This effect is explained in terms of a radiation force arising when the sound waves are propagated inside the foam layer. Sound waves of intermediate frequencies (1 000-8 000 Hz) cause the defoaming rate to increase compared with that in the absence of a sound field. The defoaming mechanism is assumed to be associated with a rupturing of the foam film accelerated by the sound waves. On the whole, the effect of sound on the defoaming rate increases as sound intensity and liquid viscosity become higher.