• Ultrasound - Sound waves with a frequency above the human hearing range (above 20,000 Hz)
• Infrasound - Sound waves below the human hearing range (below 20 Hz)

Uses of Ultrasound and Infrasound:

• Sonar - ultrasound
• Fetal scanning - ultrasound
• Exploration of the Earth's core

Sonar

• Sonar uses ultrasound to detect objects underwater
• The sound wave is reflected off the ocean bottom
• The time it takes for the sound wave to return is used to calculate the depth of the water
• The distance the wave travels is twice the depth of the ocean
  • This is the distance to the ocean floor plus the distance for the wave to return

Foetal scanning

• In medicine, ultrasound can be used to construct images of a fetus in the womb