An ultrasonic humidifier is one that uses high-frequency sound vibrations to produce an extra fine water mist that is then expelled to add moisture to the room . Some prefer this type of humidifier because it is considered safer in that there is no hot water present in the unit and therefore no risk of scalding as the unit does not heat the water in any way.  Since there is no boiling water process, an ultrasonic humidifier is quieter to operate.