Kelly Crandall, age 11, of Sioux City, Iowa, for her question:

IS IT TRUE THERE ARE SOUNDS WE CANNOT HEAR?

Yes, this is true. There are sounds going around that are either too high or .too low for the human ear to hear. The range of high notes and low notes is called pitch, which is caused by the pulsing vibrations of sound energy. The number of vibrations per second are called frequencies. Human ears can hear frequencies ranging from 20 to 20,000. Sounds with higher or lower frequencies are outside the range of the average human ear. However, dogs, bats and certain other animals can hear some of these silent sounds.