Ultrasound is sound that is too high-pitched for human beings to hear. Many animals can hear ultrasound, however. Some animals use it to locate obstacles and to detect the movement of animals they are hunting. Scientists and engineers have invented ultrasound devices, especially for medical and industrial uses. The pitch of a sound depends upon the frequency of vibration of the sound wave.
Frequency is the number of cycles of vibration per second.
Scientists measure frequencies in hertz. One hertz is one cycle per second. The highest frequency that a human being can hear depends upon the person's age and other factors. However, scientists commonly use a figure of 20,000 hertz. Sound with frequencies higher than 20,000 hertz is considered ultrasound.