Sound travels in waves. Imagine that if you could see sound waves–they might look something like ocean waves. Then, visualize those waves or ripples passing by an object, such as a piling standing in the water. The number of waves that pass by the piling per second would be the frequency. Frequency is usually measured by “hertz,” a unit of measurement which corresponds to one cycle per second. High-frequency sound waves such as ultrasonic waves are measured in thousands of cycles per second, or kilohertz (kHz). Ultrasound starts at the upper limit of human hearing, approximately 20 kHz. Many animals can hear above this range, such as dogs and bats, and some, such as dolphins, communicate using it.