Normal sound occupies the frequency of about 20 vibrations a second (a vibration per second is a Hertz
through to 20,000 Hertz
. Frequencies beyond 20,000Hz are called ultrasound. Infrasound is the opposite; it's frequencies from about 1 Hz to 20 Hz.
Infrasound has many interesting properties. For starters, infrasound has longer range than sound. Animals that can hear infrasound, like elephants can use it easily over distances exceeding one kilometer to communicate with other elephants. Seagulls use infrasound generated by ocean waves to tell them which way the water is.
Another interesting feature is that the body cavity has a natural frequency of 3-7 Hertz. This means that if you are near an infrasound generator at this frequency, then your own internal organs will begin to resonate, which will at least make you feel very sick, and in one case caused haemorrhaging occasioning death. Your anal sphincter apparently also has a natural frequency of 10-15 Hz, and apparently there are a new wave of crowd control devices that vibrate in this range, causing people to want to defecate. These devices literally give you the shits.