An Idylis humidifier is an ultrasonic humidifier that doesn't emit as much noise as regular humidifiers. It utilizes high frequency sound waves to dissolve particles of moisture in the air while giving off a particulate mist.
The mist given off by an Idylis humidifier can look more like a fog instead of a steam. One downside of the particulate mist is that the particles are so small that they can potentially gather on furniture and other surfaces in the form of a white powder. This usually happens when hard water is used in the humidifier rather than distilled water. The reason distilled water doesn't leave a residue is that all of the minerals and bacterial have been eliminated.