What is Ultrasound and infrasound?

The human ear hears sound between about $20\,\mathrm{Hz}$ and $20\,\mathrm{kHz}$ (the ceiling drops with age). Sound above $20\,\mathrm{kHz}$ is ultrasound — bats hunt at $50\,\mathrm{kHz}$, medical scanners image the body at a few megahertz. Sound below $20\,\mathrm{Hz}$ is infrasound, felt by elephants and seismometers.