At what frequency is sound classified as ultrasound?

Ultrasound refers to sound waves that have frequencies above the range of human hearing. The upper limit of human hearing is typically considered to be around 20 kHz. Therefore, any frequency that exceeds this limit is classified as ultrasound. As a result, sound that is characterized as ultrasound is recognized at frequencies above 20 kHz. This classification is crucial in medical imaging and therapeutic applications of ultrasound, as it allows the clear distinction between audible sound and the frequencies used for sonographic purposes.

In this context, the other choices refer to ranges of sound that do not fall under the definition of ultrasound. Frequencies below 10 kHz and those between 0 Hz and 20 kHz are considered infrasonic and audible sound, respectively. The range of 1 MHz to 5 MHz, while it does fall under ultrasound, is too specific and does not encompass the entire definition of ultrasound as it overlooks the broader spectrum of frequencies above 20 kHz. Therefore, the classification of ultrasound is accurately described by the option indicating frequencies above 20 kHz.