Elgin Burtrum, age 12, of Decatur, I11., 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 per  second.   A  device called a transducer is used to produce ultrasound waves.

Ultrasound has many uses in science and industry. Ultrasound 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.