Ultrasound refers to sound waves with frequencies higher than the upper limit of human hearing, which is typically considered to be around 20,000 Hz (20 kHz). Therefore, ultrasound waves have frequencies greater than 20 kHz.
In medical imaging and diagnostic applications, ultrasound waves are commonly used in the frequency range of 1 to 20 megahertz (MHz), which is 1 million to 20 million cycles per second. These frequencies are well above the audible range and allow for detailed imaging of internal body structures.
It's worth noting that the specific frequency used in ultrasound applications depends on the particular requirements of the imaging task. Lower frequencies provide deeper penetration but lower resolution, while higher frequencies offer better resolution but limited depth penetration. Therefore, the choice of ultrasound frequency depends on factors such as the target organ or tissue, imaging depth, and desired image quality.