PURPOSE: To carry out operation, such as sepn. of particles suspended in a fluid medium within an ultrasonic standing wave system in a constant state under controlling of an external influence factor by maintaining a sweep cycle time of a frequency so as to be made substantially longer than the travel time of acoustic energy output over the entire length of a route.

CONSTITUTION: This ultrasonic system is equipped with a conversion means 4 which is disposed to propagate the acoustic energy along the route extending into a fluid medium and a drive means 8 of the conversion means 4 which includes a device for changing the output frequency of the acoustic energy from the conversion means 4. The means for sweeping the range corresponding to the difference in at least one half wavelength at the average sound wave frequency over the entire length of the rout passing the inside of the fluid medium includes a means 14 which is a means for inputting a series of digital signals and generates ramps from the digital signals for controlling the conversion means 4.