Ultrasonic vs. evaporative what is the difference?

Ultrasonic humidifiers use high frequency vibrations to create a fine mist they are whisper quiet, energy efficient, and compact. Evaporative humidifiers blow air through a wet wick filter they are self regulating (output decreases as humidity rises) and do not produce white dust. Ultrasonic models can leave white mineral dust on surfaces if used with hard water; using distilled water or a demineralization cartridge solves this.