The frequency of ultrasound waves is

Sound waves are categorized based on their frequency relative to the human hearing range. The standard audible spectrum for humans is approximately 20 Hz to 20,000 Hz (20 kHz) [2]. Sound waves with frequencies below 20 Hz are classified as infrasound [1]. Ultrasound is defined as acoustic or mechanical waves with frequencies greater than 20 kHz, which is above the upper threshold of human hearing [4]. While the general definition starts at 20 kHz, practical applications such as diagnostic medical ultrasound typically utilize much higher frequencies, often ranging from 2 MHz to 15 MHz or more to achieve better image resolution [3]. Therefore, the fundamental characteristic of ultrasound is that its frequency exceeds the 20 kHz limit.

Sources

1. [2] https://www.ncbi.nlm.nih.gov/books/NBK10924/
2. [4] https://www.sciencedirect.com/science/article/pii/S0079610706000812
3. [3] https://www.nibib.nih.gov/science-education/science-topics/ultrasound