The key difference between infrasonic and ultrasonic waves is that infrasonic waves include sounds emitted at levels below frequencies of noise that can be heard by humans while ultrasonic waves are those that exceed 20 kilohertz, which is the upper limit of human noise perception. Ultrasonic waves are waves that have high amplitudes. These waves are also called micro sound, and exist primarily as longitudinal waves.
In addition to varying in physical description, infrasonic and ultrasonic sounds come from different sources. Infrasonic sounds are most frequently produced by large reactions within the surface of the Earth, such as earthquakes and volcanoes. Avalanches and meteorites also produce infrasonic waves, which are perceived as noise by some creatures. Other animals, such as elephants, whales and rhinos, perceive infrasonic sounds as vibrations, and even communicate with each other at this level of sound. Since humans cannot hear infrasonic waves, they rely on technology, such as sound monitors, to detect movements within the Earth that signal an impending volcanic eruption or earthquake. While some animals perceive low-frequency sounds that humans cannot hear, others can hear ultrasonic sounds. Cats, bats and rodents are all types of land animals that can hear ultrasonic sounds while whales and dolphins are water-dwelling animals that can detect high sound levels.