ABSTRACT
One defi nition of sound is mechanical vibrations transmitted by an elastic medium. We generally think of sound as what we can hear, from 15 or 20 Hz or cycles/second up to as high as 20,000 Hz (20 kHz) for normal young people. Th e upper limit tends to decrease with age, more for men than for women. A middle-aged person may have an upper limit of 12 to 14 kHz. “Ultrasound,” above the frequency of human hearing, is generally considered to be anything above about 20 kHz.
