Ultrasonic humidifiers are famed for their low noise. They use high-frequency sound waves to break down moisture particles and disperse a fine mist, which can look like a fog.
If you use hard water with this type of humidifier, you can end up with a fine white dust on your furniture as a result of the minerals in your water. If that does happen, use distilled water in your ultrasonic machine.
These ultrasonic machines are usually cool mist-producing, not warm mist-producing. They are considered safer than typical warm-mist machines because they do not boil water to emit a vapor, thus if they are knocked over, no boiling water can run out.