Ann, I found this article on evaporative humidifiers vs ultrasonic ones.
Humidifier Types - Ultrasonic versus Evaporative Humidifiers
Ultrasonics are quieter, but more expensive, easier to clean (no filters) but leaves more white dust (minerals in the water) on your stuff.
Evaps are cheaper, but you have to buy and clean filters more often, and have a fan so are generally louder, but less white dust bc of the filters.
We have a soft water system here at the house, so I think I will look into an ultrasonic one.