An experimental setup, based on a properly designed torsion balance has been built, which allows to produce ultrasonic standing waves and to control, through the radiation pressure, the energy density of the field during the time of its application. The control of the energy density is usually difficult, when employing transducers rigidly connected with reflectors, in particular at high energies and when cavitation occurs. The standing waves produced by this device have been used to investigate the darkening of photographic plates in water and the coloration of starch plates in dilute iodine solutions. These processes have been studied with reference to their dependence on the energy density, frequency, time, cavitation, and hydrodynamic flow within the liquid. A comparison is made between the experimental results obtained with standing waves and with progressive waves at the same frequency and energy.