Hertz are used to measure sound waves, or frequencies. 1 Hertz is 1 cycle per second and is represented as seen in this picture, with one peak and one troph. Humans can audibly sense from about 20 to 20,000 Hertz. Dolphins can hear nearly 8 fold of the human range, elephants can communicate with infrasound, and some bat species can hear up to 200,000 Hz, what we term ultrasonic. These are on the same spectrum as the ultrasonic waves used in medical diagnostics!