This document compares and contrasts infrasonic and ultrasonic sound. It defines infrasonic sound as having a frequency below 20 Hz, which is inaudible to humans but can be detected by elephants and whales. Ultrasonic sound is defined as having a frequency above 20,000 Hz, which is also inaudible to humans but can be detected by dogs, bats and mice. It then provides examples of applications of ultrasound like tracking fetal growth and using it for medical imaging.