Ultrasound refers to sound with a frequency above 20,000 Hertz, whereas infrasound refers to sound with a frequency below 20 Hz, according to Dr. Colin France. Ultrasound is used for scanning, and range and direction finding, while infrasound is used by animals to communicate.
Ultrasound scanning is a safe way to view images of a developing baby inside a pregnant mother's uterus, Dr. France explains. This method is called "pre-natal scanning" or "fetal imaging," and it helps to confirm the health of a baby. Ultrasound scanning works by directing ultrasound at the body, allowing the surfaces of various tissues inside the body to reflect the ultrasound. Depending on the depth of the tissue surfaces, a detector receives ultrasound echoes at varying times and sends electrical signals to a computer. An image is then produced and displayed on a screen.
Infrasound emanates from natural and man-made sources, according to the Department of Physics and Astronomy of the University of Western Ontario. Infrasonic waves can propagate over great distances, as they propagate with extremely small attenuation. Large animals, such as elephants, giraffes and whales, communicate over long distances through infrasound. Infrasonic waves also come from meteors, ocean waves, earthquakes, volcanoes, avalanches and waterfalls. Man-made sources of infrasound include engines, airplanes, and nuclear and chemical explosions.