Question

What is the term used for the sound with a frequency below 20 Hz?

Answer 1

1. Infrasonic sound: Infrasonic sounds (noises) are those with a too lower frequency to somehow be perceived by the human ear.
2. Infrasonic noises are those with a frequency of less than 20 hertz.
3. Example: Lightning, volcanoes, earthquakes, etc.

Therefore, a sound of the frequency below $20\mathrm{Hz}$ is known as Infrasonic sound.