Sound waves are alternating zones of high and low pressure traveling through air or water and characterized by their frequency or intensity. Frequency is measured in hertz (Hz), which represents cycles per second (cps). The frequency range of human hearing is from 20 to 20,000 Hz. Sounds with a frequency lower than 20 Hz cannot be heard by humans and are referred to as infrasonic. Such signals start below 20 Hz, but can be detected at frequencies as low as a hundredth or even a thousandth of a hertz. Human ears are most sensitive to frequencies between 1,500 and 4,000 Hz. Within that range, an individual can distinguish frequencies that differ by only 2 or 3 Hz.