What frequency range defines ultrasound?

Ultrasound is typically defined as sound waves that have frequencies above the range of human hearing, which is generally considered to start at 20 kHz. The correct choice reflects this understanding, as it defines the frequency range for ultrasound as 20 kHz and above, extending to frequencies that can exceed 50 kHz and even reach into the megahertz range for various applications.

Ultrasound is widely used in various fields such as medical imaging, industrial nondestructive testing, and cleaning processes, which often utilize frequencies well beyond the initial 20 kHz threshold. This capability allows for greater precision and the ability to detect smaller flaws or to generate clearer images.

The other options fail to accurately encompass the full spectrum of ultrasound frequencies as they either start below the human hearing threshold or do not extend sufficiently beyond it to cover the high-frequency applications associated with ultrasound. Thus, the selected range effectively includes the comprehensive use cases for ultrasound technology, underscoring its importance in practical applications.