Ultrasonic humidifiers have no fan, making them quieter than the evaporative models. The ultrasonic humidifier creates a fine water mist using high-frequency sound vibrations. That mist is expelled into the room to add moisture to the air.
Evaporative humidifiers are more common and less expensive. Most models use wick-style filters that sit in front of a fan. The filter draws up the water and the fan, which has various speeds, expels the water vapor into the room. On many models, the "low" setting on the fan is barely noticeable by most people. Staying away from the higher fan settings can help keep the noise down.