Frequency

Frequency is measured in hertz (Hz). 1 Hz means one complete wave per second.

$f = \frac{1}{T}$

where $T$ is the period (time for one complete wave).

For sound, higher frequency means higher pitch. The audible range for humans is approximately 20 Hz to 20 000 Hz. Below 20 Hz is infrasound; above 20 000 Hz is ultrasound.

For electromagnetic waves, higher frequency means shorter wavelength and more energy per photon.