Ultrasound.

The sounds that humans can hear are limited to the audible range of frequencies , between twenty and twenty thousand hertz. Within this range, higher frequencies are heard as higher pitched, and lower frequencies are heard as lower pitched. Anything above the audible range is considered ultrasonic, and any sound that falls below it is called infrasound. Some animals use ultrasound to navigate their environment, a technique known as "echolocation." The principle behind echolocation is one that scientists have co-opted to great effect.Ultrasonic acoustics follows the same principles as all other acoustics. The same physics is used to describe all waves, whether they be sound, light, or mechanical waves in the ocean. What people hear as sounds are vibrations caused by waves traveling through air, water, or some other medium.