Final answer:
A sound frequency that falls outside the human audible range, which is 20 Hz to 20,000 Hz, is classified as infrasound if it is below 20 Hz and as ultrasound if it is above 20,000 Hz. These frequencies are used in various applications but are not heard by humans, although infrasound can be felt as vibrations.
Step-by-step explanation:
A sound frequency that lies outside the audible range of human hearing can either be infrasound or ultrasound. The normal range of human hearing is from 20 Hz to 20,000 Hz. Therefore, sounds with frequencies lower than 20 Hz are referred to as infrasound, and those that have frequencies higher than 20,000 Hz are known as ultrasound. Infrasound can sometimes be felt as vibrations, whereas ultrasound is not perceived by the ear but can be used for various applications such as medical imaging. Different animals have different ranges of hearing; for example, dogs can hear up to 30,000 Hz, and bats and dolphins can detect frequencies as high as 100,000 Hz.
If a source of sound moves faster than the speed of sound, the resultant sound waves have a frequency exceeding the upper limit of human hearing, and thus, produce a sonic boom, which cannot be heard but can be experienced as a shock wave. This phenomenon occurs because the object is traveling at a speed faster than the sound waves it produces, leading to a pile-up of the waves which then creates a powerful pressure wave.