In a method or digital printer to print a recording material with toner particles applied with a liquid developer, toner concentration of the liquid developer is determined via measurement with aid of ultrasonic pulses. At least one ultrasonic pulse is generated by discharging an ultrasound emitter charged with a charge voltage. The ultrasonic pulse permeates the liquid developer and is received by an ultrasound receiver which generates a reception signal corresponding to the received ultrasonic pulse. With a control device, the charge voltage is regulated to a value such that a representative value of the reception signal corresponds to a predetermined desired value. The toner concentration is determined from the regulated charge voltage.