A sound with a frequency below 20 Hz is called:

Sound waves are vibrations that travel through a medium like air, water, or solids, and they are characterized by their frequency, which is measured in Hertz (Hz). The frequency of a sound determines whether it is audible to the human ear or not. Sounds with frequencies below 20 Hz are called infrasonic waves.

Infrasonic waves have frequencies lower than the audible range of human hearing, which is typically 20 Hz to 20,000 Hz. Humans usually cannot hear infrasonic waves, but some animals, such as elephants, whales, and giraffes, use them for long-distance communication because these low-frequency waves can travel large distances without significant loss of energy.

Other types of sound waves include ultrasonic waves, which have frequencies above 20,000 Hz, used in applications like medical imaging (ultrasound), cleaning, and industrial testing. Audiosonic is a general term for sound waves within the audible range. Supersonic refers to speeds exceeding the speed of sound in a medium, not a type of frequency.

Infrasonic waves are also naturally produced by earthquakes, volcanic eruptions, ocean waves, and even strong wind. Scientists use infrasonic detection to monitor natural disasters, predict volcanic activity, and study atmospheric phenomena. These waves can travel hundreds of kilometers due to their long wavelengths and low energy loss.

Understanding infrasonic waves is important in physics and everyday science because it explains how sound behaves at extreme frequencies, how animals communicate beyond human perception, and how technology can detect events imperceptible to the human ear. Infrasonic waves illustrate the wide range of the sound spectrum and the practical applications of physics in real life.