# Notes / Frequency Units

The basic frequency unit is 1 *Hertz* (`Hz`

).
If the frequency of some event is n `Hz`

, it means that the event occurs n times per second
(see
Time Units).
Thus, $1~\textrm{Hz} = 1 / \textrm{second} = 1~\textrm{second}^{-1}$.
Each frequency value corresponds to a time period.
For example, `20 Hz`

corresponds to `50 ms`

because

Some additional useful frequency units (with corresponding time periods) are presented in the below table:

Unit | Symbol | Value in Hz | Time period |
---|---|---|---|

Terahertz | THz | $10^{12}$ | 1ps |

Gigahertz | GHz | $10^{9}$ | 1ns |

Megahertz | MHz | $10^{6}$ | 1us |

Kilohertz | kHz | $10^{3}$ | 1ms |

Hertz | Hz | 1 | 1s |

Millihertz | mHz | $10^{-3}$ | $10^{3}$s |

Microhertz | uHz ($\mu$Hz) | $10^{-6}$ | $10^{6}$s |

Nanohertz | nHz | $10^{-9}$ | $10^{9}$s |

The common symbol for frequency is `f`

.
If we want to calculate the frequency of an event,
we should divide the number of events by a time interval that contains all these events.
For example, if something happens 42 times per day,
it means that the frequency of the event is:

The *frequency* term is widely used in many physics and engineering disciplines.
Here are some famous examples:

- Humans can hear sounds with frequencies between
`20 Hz`

and`20 kHz`

. - Visible spectrum (a part of the electromagnetic spectrum that is visible to the human eye) is about
`430`

..`770 THz`

. The frequency range for the yellow color is about`508`

..`526 THz`

. - 440
`Hz`

is the frequency of the musical note of`A`

above middle`C`

(`A440`

, the pitch standard). - Communication with submarines uses extremely low frequency: from
`3`

to`30 Hz`

. - Shortwave radio uses frequencies in the range
`1.6`

..`30 MHz`

. - Frequency of a typical microwave oven is about
`2.45 GHz`

. - The most popular WiFi frequencies are about
`2.4 GHz`

(802.11b/g/n/ax) and`5 GHz`

(802.11a/h/j/n/ac/ax).

If we are talking about waves and we want to draw these waves on a plot, the frequency can be easily compared by a glance. Let us consider the following figure:

In this figure, we can observe three waves with different frequencies:

- (a) Let’s say that the first wave is a “reference” wave with a frequency
`1x`

- (b) The second wave frequency is twice (
`2x`

) that of the reference one - (c) The third wave has frequency =
`8x`

(eight times more than the reference one; four times more than the second frequency)