what is the use of infrasonic sound and why can't we hear ultrasound our infrasound

Infrasound is sound that is lower in frequency than 20 Hz (Hertz) or cycles per second, the "normal" limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. The ear is the primary organ for sensing infrasound, but at higher levels it is possible to feel infrasound vibrations in various parts of the body. Animals have been known to perceive the infrasonic waves going through the earth by natural disasters and can use these as an early warning. Some film soundtracks make use of infrasound to produce unease or disorientation in the audience.