Ultrasonic humidifiers are the quietest humidifiers around. They are so quiet, they are often described as being nearly silent. An ultrasonic humidifier uses the power of high frequency sound waves to vibrate a metal diaphragm at an "ultrasonic" frequency, thereby breaking the water in the humidifier down into an ultra-fine vapor mist. This vapor mist, which looks like a fog, is released into the air using a very small and quiet fan. Any minerals or impurities that are present in the water are released into the
air with the mist. Many ultrasonic humidifiers use demineralization cartridges, which serve to trap these minerals before they are released into the air, although the demineralization cartridges are usually not 100% effective. You may want to use distilled water if you experience a problem with "white dust" settling on surfaces near the humidifier. Some ultrasonic humidifiers have a heating option as well, which heats up the water before releasing it into the air. This helps eliminate bacteria growth in the water, and also keeps the room from feeling too cold.
Ultra quiet, near silent operation.
No wick filters to replace.
Low power consumption (when using cool mist).
Unless using distilled water, impurities can sometimes enter the air in the form of "white dust" (this can sometimes be controlled with a replaceable demineralization cartridge).
Most units are limited to small rooms only.