Question

Ultrasound is of frequency
(A) below 20Hz
(B) above 20Hz
(C) below 20kHz
(D) above 20kHz

Answer 1

Hint Ultrasound frequencies are not audible to human ears which can hear frequencies in the range of 20-20000 Hz. Ultrasound frequencies have very high penetrating power i.e. very high energies.

Complete step by step answer
Ultrasound waves are sound waves with frequencies higher than the upper audible limit of human hearing which is 20000 Hz or 20 kHz. Hence waves with frequency higher than 20 kHz frequency are ultrasound waves that correspond to option (D).
Ultrasound waves are not different from normal audible sound waves in their physical properties, i.e. both are longitudinal waves. But the only difference is that ultrasound waves are not audible to humans.

Additional Information
Ultrasound waves are used in many different fields. Ultrasonic devices are used to detect objects and measure distances that cannot be measured using measuring scales which are typically very long or far away. Ultrasound imaging or sonography is often used in medicine to view the images of a baby in a pregnant mother. In the non-destructive testing of products and structures, ultrasound is used to detect invisible flaws that are not at the surface but are inside the material like in airplanes, steel beams, or buildings, or any kind of structure. Industrially, ultrasound is used for cleaning, mixing the different surfaces of ingrained impurities and dust. Animals such as bats use ultrasound for locating prey and obstacles. Submarines use a similar mechanism to detect obstacles underwater as well as to track other objects in the sky.

Note
The only reason waves with a frequency above 20 kHz are called ultrasounds waves is because of our incapability of hearing them otherwise they are just normal sound waves with a higher frequency which can be heard by many animals such as bats or dogs.