Ultrasonic means involving sound waves that are too high-pitched for human ears to hear. The prefix ultra- means “beyond,” so ultrasonic sounds are beyond the range of what we can detect.

Humans can hear sounds up to about 20,000 vibrations per second, but ultrasonic waves vibrate even faster than that. Dogs can hear some ultrasonic frequencies, which is why a dog whistle seems silent to us but gets a dog's attention immediately. Bats use ultrasonic calls to navigate in complete darkness, sending out high-pitched squeaks and listening for the echoes that bounce back from insects and obstacles.

Ultrasonic technology has many practical uses. Doctors use ultrasound machines to see images of babies before they're born: the machine sends ultrasonic waves into the body and creates pictures from the echoes. Dentists use ultrasonic cleaners to remove plaque from teeth. Some pest control devices claim to repel mice and insects with ultrasonic noise, though scientists debate how well these work.

The key idea is that ultrasonic describes a frequency of sound, not a volume. These sound waves aren't necessarily loud or soft: they're simply pitched too high for human ears to perceive at all.