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Frequency is the measurement of the number of times that a repeated event occurs per unit time. To calculate the frequency, one fixes a time interval, counts the number of occurrences of the event within that interval, and then divides this count by the length of the time interval.

In SI units, the result is measured in hertz (Hz) after the German physicist, Heinrich Rudolf Hertz. 1 Hz means that an event repeats once per second. Other units that have been used to measure frequency include: cycles per second and revolutions per minute (rpm).

An alternative method to calculate frequency is to measure the time between two consecutive occurrences of the event (the period) and then compute the frequency as the reciprocal of this time:

where T is the period.

Table of contents
1 Frequency of Waves
2 Statistical Frequency
3 Examples
4 See also
5 External link

Frequency of Waves

In measuring the frequency of sound, electromagnetic waves (such as radio or light), electrical signals, or other waves, the frequency in hertz is the number of cycles of the repetitive waveform per second.

Frequency has an inverse relationship to the concept of wavelength. The frequency f is equal to the speed of sound c (or v) of the wave divided by the wavelength λ (lambda) of the wave:

Statistical Frequency

In statistics, the frequency of an event is simply the number of times the event occurred in the experiment or the study. These frequencies are often graphically represented in histograms.


Is a period 10 milliseconds, that is 0.01 seconds, you get a frequency of:

The frequency of the standard pitch tone A above middle C is nowadays set at that is 440 cycles per second (or slightly higher) and known as concert pitch, after which an orchestra is tuned.
A baby can hear tones with oscillations up to approximately 20,000 cycles per second. Adults do not hear this high frequency any longer.
The frequency of the alternating current in European electricity mains is 230 (!) volts of rated voltage and has exactly 50 cycles per second. 50 Hz is close to the tone G.
In the US-American net we have 117 (!) volts of rated voltage and has exactly 60 cycles per second. 60 Hz is close to the tone B flat. From the height of the net-hum tone it is to be recognized whether a sound recording was made e.g. in Europe. In Europe it hums a small third lower than in the USA. An analyzer will find rather the double and the three-fold of that electricity mains frequency and hardly the fundamental in the recording.

See also

amplitude. List of frequencies of radio waves, light and infrared is called electromagnetic spectrum.

External link