Infrasonic sound is of frequency:

Infrasonic sound refers to sound waves with frequencies below the lower limit of human hearing, which is typically considered to be around 20 hertz (Hz). Infrasound can have frequencies as low as 1 Hz or even lower. These low-frequency sound waves are often produced by natural phenomena such as earthquakes, volcanic eruptions, and atmospheric events, as well as by some man-made sources like industrial machinery. While humans cannot hear infrasound, some animals are sensitive to these low-frequency vibrations and use them for communication or navigation purposes.