frequency

Measured in Hertz (cycles per second). A clock might be said to tick at 1 Hz.

The equation for frequency is $f=\frac{v}{\lambda }$ where

$$\begin{array}{rl}& f=\text{ frequency }\left({\mathrm{s}}^{-1},\text{ or }\mathrm{Hz}\right)\\ & v=\text{ velocity }\left(\mathrm{m}/\mathrm{s},{\mathrm{ms}}^{-1}\right)\\ & \lambda =\text{ wavelength }(\mathrm{m})\end{array}$$

Frequency is what differentiates noise from music:

• The frequency of a sound wave determines pitch
  • too low to hear: infrasound
  • too high: ultrasound
• Frequency ratios determine harmony