An ultrasonic humidifier is a very simple device. It includes a reservoir of water and a diaphragm or other type of vibrating element. The diaphragm vibrates at an extremely high frequency, so high that it is above the range of human hearing (which is why they’re called ultrasonic humidifiers). The vibrations propel microscopic water droplets into the air. Once in the air, the droplets evaporate, adding humidity to the air in the room. There is no heating of the water at any point, so these humidifiers are sometimes known as “cool mist” humidifiers. However, note that many evaporative humidifiers also accomplish evaporation without heating and are also referred to as “cool mist” humidifiers in marketing materials. The important terms to differentiate humidifiers are ultrasonic and evaporative.

As mentioned before, ultrasonic humidifiers use vibrations to send water droplets into the air. Evaporative humidifiers, however, evaporate the water inside the humidifier and send water vapor into the air. Some accomplish this by heating the water into steam.