What is the frequency range defined as ultrasound?

Ultrasound refers to sound waves that are above the audible range for humans. The upper limit of the human hearing range is generally accepted to be around 20,000 Hz (or 20 kHz). Any frequency that exceeds this threshold is categorized as ultrasound. This classification allows ultrasound to be differentiated from other types of sound, such as infrasound (below 20 Hz) and the audible range (20 Hz to 20,000 Hz). Ultrasound is commonly utilized in various applications, notably in medical imaging and industrial testing, due to its ability to penetrate materials and its high frequency which results in better resolution.

In contrast, the other options refer to frequencies that do not fall under the definition of ultrasound, either being within the range of human hearing or below it.