What type of sound is referred to as infrasound?

Infrasound refers to sound waves that are below the frequency range of human hearing, specifically frequencies lower than about 20 Hertz (Hz). Humans typically can hear sounds ranging from approximately 20 Hz to 20,000 Hz (20 kHz). Sounds classified as infrasound occur at frequencies below this threshold and can be generated by natural phenomena, such as earthquakes, volcanic eruptions, or even man-made sources like machinery.

The concept of infrasound is significant because, while it is not audible to the human ear, it can still have physical effects on the body and can be detected by specialized equipment. Understanding infrasound is important in various fields, including environmental science, engineering, and even in monitoring natural disasters.

In contrast, the other options refer to different frequencies or descriptors of sound. Sounds above the human hearing range are known as ultrasound, sounds within the range may include those we perceive as normal sounds, and harsh sounds could refer to noise pollution or similar concepts but do not accurately define infrasound.