Ultrasound

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. Higher-pitched sounds have higher frequencies than do lower-pitched sounds. 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.

Animal uses of ultrasound. Bats, dolphins, and other animals use ultrasound to locate objects in the dark and to determine the motion of prey. In a technique called echolocation, the animal sends out pulses (short bursts) of ultrasound. The pulses reflect from objects, producing echoes. The animal hears the echoes, and uses them to determine the direction and distance to the reflecting object.

To determine motion, animals use echolocation and a change in the frequency of waves known as the Doppler effect. This change occurs due to relative motion between the source of the waves and the object that reflects the waves--in this case, the hunted animal.

Some hunted animals--for example, the mantid, or praying mantis--can also hear ultrasound. Thus, these animals can use their hearing to avoid capture by animals that are using echolocation to try to catch them.

Ultrasound devices. People have invented various devices that produce ultrasound. One simple device, an ultrasonic whistle, can produce ultrasound in air. Dogs can hear ultrasound, and so people use such whistles to call their dogs.

Complex devices called ultrasonic transducers send ultrasound into liquids and solids, and receive echoes of the ultrasound. These devices convert electric energy into ultrasound and vice versa. Ultrasonic transducers are made of materials, such as quartz and some ceramics, that are piezoelectric (pronounced pee ay zoh ih LEHK trihk). This kind of material vibrates when an electric voltage is applied to it, and produces a voltage when a sound wave causes it to vibrate.

Medical uses of ultrasonic transducers include machines that can detect the heartbeat of fetuses (unborn, developing babies). Other machines produce images of normal and diseased tissue inside the body, and images of fetuses. Physicians can use these machines to detect and evaluate cancer, fetal abnormalities, and other conditions. Instruments that use the Doppler effect can measure the flow of blood in the heart and blood vessels. A lithotripter uses pulses of ultrasound to break up gallstones or kidney stones.

Other uses of ultrasound include burglar alarms, automatic door openers, instruments that detect flaws in metal parts, machines that weld plastic, and tools that cut metal. Underwater sonar devices operate much like radar to measure distances to the ocean floor, detect submarines and other vessels, and even locate schools of fish. Ultrasonic cleaning instruments use sound waves to loosen dust and other contaminants from small, delicate products such as watches and electronic components.

Contributor: Frederick W. Kremkau, Ph.D., Prof. and Director, Center for Medical Ultrasound, Bowman Gray School of Medicine, Wake Forest Univ.

Ultrasound

Master Index