Ultrasonic frequencies refer to sounds or waves that are too high-pitched for humans to hear. They vibrate very quickly and have a frequency above the range of our hearing abilities. Full definition
These humidifiers create a fine mist by exciting and evaporating water using a small diaphragm that vibrates at ultrasonic frequencies. (wetheparents.org)
(The sounds should be inaudible to human ears though because they're sent using ultrasonic frequencies, according to the Financial Times.) (theverge.com)
These pest control units broadcast variable ultrasonic frequencies from 22 KHz to 65 KHz to repel pests and keep them from adjusting to the uncomfortable noise. (geekwrapped.com)