Christopher Andrews, age 13, of Fargo, N.D., for his question:

WHAT IS ULTRASOUND?

Ultrasound is a sound that is too high pitched to be heard by human beings.

The pitch of a sound is called its frequency. It is the number of sound waves produced per second. The higher the frequency, the higher the pitch of the sound produced.

Most people can hear sounds of frequencies between about 20 and 20,000 vibrations per second. Ultrasound has more than 20,000 vibrations per second.

Ultrasound waves are produced by scientists with a device called a transducer. An ultrasonic transducer changes electric waves into mechanical vibrations and vice versa.

Ultrasound has many uses in science and industry. The waves can clean tiny watch parts and other delicate objects by shaking loose the dirt. They can also mix liquids that do not dissolve by emulsification, which is the separation of a liquid into tiny droplets.

Ultrasound is also used for measurement and detection because it can be focused like a beam of light at a small object. Ultrasound produces echoes which are sound waves reflected off objects. Doctors use ultrasound echoes to detect brain tumors and other conditions. In industry, ultrasound is used to test metals and plastics for defects.