User Contributed Dictionary
Noun
frequencies Plural of frequency
Extensive Definition
Frequency is a measure of the number of
occurrences of a repeating event per unit time. It is also referred to as
temporal frequency. The period is the duration of one cycle in a repeating event, so the
period is the reciprocal
of the frequency.
Definition and units
For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles, or periods, per unit time. In physics and engineering disciplines, such as optics, acoustics, and radio, frequency is usually denoted by a Latin letter f or by a Greek letter ν (nu).In SI
units, the unit of frequency is hertz (Hz), named
after the German physicist Heinrich
Hertz. For example, 1 Hz means that an event repeats once per
second, 2 Hz is twice per
second, and so on . This unit was originally called a cycle per
second (cps), which is still sometimes used. Heart rate and
musical tempo are measured
in beats per
minute (BPM). Frequency of rotation is often expressed as a
number of revolutions
per minute (rpm). BPM and rpm values must be divided by 60 to
obtain the corresponding value in Hz: thus, 60 BPM translates into 1
Hz.
The period is usually denoted as T, and is the
reciprocal
of the frequency f: T = \frac.
The SI unit for period is the second (s).
Measurement
By timing
To calculate the frequency of an event, the number of occurrences of the event within a fixed time interval are counted, and then divided by the length of the time interval.In experimental work (for example, calculating
the frequency of an oscillating pendulum) it is more accurate to
measure the time taken for a fixed number of occurrences, rather
than the number of occurrences within a fixed time. The latter
method introduces a random error
of 1/f Hz into the result. Frequency is still calculated by
dividing the number of occurrences by the time interval; however it
is the number of occurrences that is fixed, not the time
interval.
An alternative method to calculate frequency is
to measure the time between two consecutive occurrences of the
event (the period T) and then compute the frequency f as the
reciprocal of this time: f = \frac.
A more accurate measurement can be obtained by
taking many cycles into account and averaging the periods between
each.
By stroboscope effect, or frequency beats
In case when the frequency is so high that counting is difficult or impossible with the available means, another method is used, based on a source (such as a laser, a tuning fork, or a waveform generator) of a known reference frequency f0, that must be tunable or very close to the measured frequency f. Both the observed frequency and the reference frequency are simultaneously produced, and frequency beats are observed at a much lower frequency Δf, which can be measured by counting. This is sometimes referred to as a stroboscope effect. The unknown frequency is then found from f=f_0\pm \Delta f.Frequency of waves
Frequency has an inverse relationship to the
concept of wavelength, simply, frequency
is inversely proportional to wavelength λ (lambda). The frequency f
is equal to the speed v of
the wave divided
by the wavelength λ of the wave: f = \frac.
In the special case
of electromagnetic waves moving through a vacuum, then v = c0 , where c0 is
the speed of
light in a vacuum, and this expression becomes: f =
\frac.
When waves from a monochromatic source travel
from one medium to
another, their frequency remains exactly the same — only their
wavelength and
speed change.
Examples
 In music and acoustics, the frequency of the standard pitch A above middle C on a piano is usually defined as 440 Hz, that is, 440 cycles per second (Listen) and known as concert pitch, to which an orchestra tunes.
 A baby can hear tones with oscillations up to approximately 20,000 Hz, but these frequencies become more difficult to hear as people age.
 In Europe, Africa, Australia, Southern South America, most of Asia, and in Russia, the frequency of the alternating current in household electrical outlets is 50 Hz (close to the tone G), however, in North America and Northern South America, the frequency of the alternating current is 60 Hz (between the tones B♭ and B — that is, a minor third above the European frequency). The frequency of the 'hum' in an audio recording can show where the recording was made — in countries utilizing the European, or the American grid frequency.
 Visible light from deep red to violet has frequencies of 430 to 750 THz.
Period versus frequency
As a matter of convenience, longer and slower waves, such as ocean surface waves, tend to be described by wave period rather than frequency. Short and fast waves, like audio and radio, are usually described by their frequency instead of period. These commonly used conversions are listed below:Other types of frequency
 Angular frequency ω is defined as the rate of change in the orientation angle (during rotation), or in the phase of a sinusoidal waveform (e.g. in oscillations and waves):

 \omega=2\pi f\,.
 Angular frequency is measured in radians per second (rad/s).
 Spatial frequency is analogous to temporal frequency, but the time axis is replaced by one or more spatial displacement axes.
 Wavenumber is the spatial analogue of angular frequency. In case of more than one space dimension, wavenumber is a vector quantity.
See also
References
External links
 National Research Council of Canada: Femtosecond comb; The measurement of optical frequencies
 Conversion: frequency to wavelength and back
 Conversion: period, cycle duration, periodic time to frequency
 Keyboard frequencies = naming of notes  The English and American system versus the German system
 Teaching resource for 1416yrs on sound including frequency
 A simple tutorial on how to build a frequency meter
 Frequency  diracdelta.co.uk  javascript calculation.
frequencies in Afrikaans: Frekwensie
frequencies in Arabic: تردد
frequencies in Asturian: Frecuencia
frequencies in Azerbaijani: Tezlik
frequencies in Bengali: কম্পাঙ্ক
frequencies in Min Nan: Pînlu̍t
frequencies in Bosnian: Frekvencija
frequencies in Bulgarian: Честота
frequencies in Catalan: Freqüència
frequencies in Czech: Frekvence
frequencies in Danish: Frekvens
frequencies in German: Frequenz
frequencies in Estonian: Sagedus
frequencies in Modern Greek (1453):
Συχνότητα
frequencies in Spanish: Frecuencia
frequencies in Esperanto: Frekvenco
frequencies in Basque: Maiztasun
frequencies in Persian: بسامد
frequencies in French: Fréquence
frequencies in Galician: Frecuencia
frequencies in Korean: 진동수
frequencies in Hindi: आवृत्ति
frequencies in Croatian: Frekvencija
frequencies in Ido: Frequeso
frequencies in Indonesian: Frekuensi
frequencies in Icelandic: Tíðni
frequencies in Italian: Frequenza
frequencies in Hebrew: תדירות
frequencies in Latin: Frequentia
frequencies in Latvian: Frekvence
frequencies in Lithuanian: Dažnis
frequencies in Hungarian: Frekvencia
frequencies in Macedonian: Фреквенција
frequencies in Malagasy: Hatetika
frequencies in Malay (macrolanguage):
Frekuensi
frequencies in Mongolian: Давтамж
frequencies in Dutch: Frequentie
frequencies in Japanese: 周波数
frequencies in Norwegian: Frekvens
frequencies in Norwegian Nynorsk: Frekvens i
fysikk
frequencies in Polish: Częstotliwość
frequencies in Portuguese: Frequência
frequencies in Romanian: Frecvenţă
frequencies in Russian: Частота
frequencies in Simple English: Frequency
frequencies in Slovak: Frekvencia (fyzika)
frequencies in Slovenian: Frekvenca
frequencies in Serbian: Фреквенција
frequencies in Sundanese: Frékuénsi
frequencies in Finnish: Taajuus
frequencies in Swedish: Frekvens
frequencies in Tamil: அதிர்வெண்
frequencies in Thai: ความถี่
frequencies in Vietnamese: Tần số
frequencies in Cherokee: ᎠᏍᏓᏩᏛᏍᏙᏗ
frequencies in Turkish: Frekans
frequencies in Ukrainian: Частота
frequencies in Urdu: تعدد
frequencies in Samogitian: Tonkėnis
frequencies in Chinese: 頻率