Infrasonic and ultrasonic vibrations can’t be heard by humans because they’re outside of the range of frequencies that can be detected. Infrasonic vibrations have a lower frequency than can be heard by the human ear; ultrasonic vibrations have a higher frequency than can be heard by the human ear. Infrasonic vibrations can be felt if the amplitude is great enough. Ultrasonic vibrations are utilized in sonar equipment and in applications to detect flaws in steel castings and to remove grease or foreign material from machine-part surfaces.
