Frequency: Difference between revisions
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[[Image:Sine waves different frequencies.svg|thumb|right|360px|[[Sinusoidal wave]]s of various frequencies; the bottom waves have higher frequencies than those above.]] |
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'''Frequency''' is a [[measurement|measure]] of the number of occurrences of a repeating event per unit [[time]]. It is also referred to as '''temporal frequency'''. |
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== Definition and units == |
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Sound is a wave associated with the transmission of mechanical energy through a supporting medium. It can be shown experimentally that sound cannot travel through a vacuum. The energy available in a sound wave disturbs the medium in a periodic manner. Periodicity is important if a sound wave is to carry information. In air, the disturbance propagates as the successive compression and decompression (the latter sometimes called rarefaction) of small regions in the medium. If we generate a pure note and place a detector (our ear, for example) at a point in the surrounding medium, a distance from the source, the number of compression-decompression sequences arriving at the detector during a chosen time interval is called the frequency. The time interval between successive maximal compressions is called the period. The product of the frequency and the wavelength is the velocity. |
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For [[cycle|cyclical]] processes, such as [[rotation]], [[oscillation]]s, or [[wave]]s, it is defined as a number of [[cycle]]s, or [[period]]s, per unit time. In [[physics]] and [[engineering]] disciplines, such as [[optics]], [[acoustics]], and [[radio]], frequency is usually denoted by a roman letter ''f'' or by a greek letter ''ν'' (nu). |
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The number of wavelengths per second of a particular radiation. |
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In [[SI derived unit|SI]] system, the unit of frequency is [[Hertz|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 <ref>Accidentally, 1 [[hertz]] is the approximate frequency of a human heart (Herz in [[German language]])</ref>. 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 [[hertz|Hz]]: thus, 60 BPM translates into 1 Hz. |
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A related measure of frequency, called [[angular frequency]] ω, is often introduced. It is defined as the rate of change in the orientation angle (during rotation), or in the [[phase (waves)|phase]] of a [[sinusoid]]al waveform (e.g. in oscillations and waves): <math>\omega=2\pi f</math>. [[Angular frequency]] is measured in [[radian]]s per [[second]] (s<sup>-1</sup>). |
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== Measurement == |
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===By counting=== |
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To calculate the frequency of the 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. |
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To calculate the frequency of an event in experimental work however (for example, calculating the frequency of an oscillating pendulum) it is crucial that the time taken for a fixed number of occurrences is recorded, rather than the number of occurrences within a fixed time. This is because your [[random error]] is significantly increased performing the experiment the other way around. It [the frequency] is still calculated by dividing the number of occurrences by the time interval, however, the number of occurrences is fixed, not the time interval. |
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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 ''f'' as the reciprocal of this time: |
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:<math> |
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f = \frac{1}{T} |
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</math> |
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where |
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:''T= time'' |
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A more accurate measurement takes many cycles into account and averages the period between each. |
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===By stroboscope effect, or frequency beats=== |
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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 ''f<sub>0</sub>'', that must be tunable or very close to the measured frequency ''f''. Both the observed frequency and the reference frequency are simultaneously produced, and [[Beat (acoustics)|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 <math>f=f_0\pm \Delta f</math>. |
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==Frequency of waves== |
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<!-- This section is linked from [[Hearing impairment]] --> |
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Frequency has an inverse relationship to the concept of [[wavelength]],simply,frequency is inversely proportional to wavelength λ. The frequency ''f'' is equal to the [[speed]] ''v'' of the [[wave]] [[division (mathematics)|divided]] by the [[wavelength]] λ (lambda) of the wave: |
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:<math> |
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f = \frac{v}{\lambda}. |
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</math> |
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In the [[special case]] of electromagnetic waves moving through a [[vacuum]], then '''v = c''', where '''c''' is the [[speed of light]] in a vacuum, and this expression becomes: |
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:<math> |
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f = \frac{c}{\lambda}. |
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</math> |
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When [[waves]] travel from one [[Medium (optics)|medium]] to another, their frequency remains exactly the same — only their [[wavelength]] and [[speed]] change. |
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Apart from being modified by the [[Doppler effect]] or any other nonlinear process, frequency is an invariant quantity in the universe. That is, it cannot be changed by any linearly physical process unlike velocity of propagation or wavelength. |
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==Examples== |
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*In [[music]] and [[acoustics]], the frequency of the [[note|standard pitch]] A above [[middle C]] on a [[piano]] is usually defined as [[A440|440 Hz]], that is, 440 cycles per second ({{Audio|Media-440Hz.ogg|Listen}}) and known as concert [[Pitch (music)|pitch]], to which an [[orchestra]] tunes. |
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*A baby can hear tones with oscillations up to approximately 20,000 Hz, but these frequencies become more difficult to hear as people age. |
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*In [[Europe]], [[Africa]], [[Australia]], Southern [[South America]], most of [[Asia]], and in [[Russia]], the frequency of the [[alternating current]] in [[mains electricity|household electrical outlets]] is 50 Hz (close to the [[note|tone]] G), however, in [[North America]] and Northern [[South America]], the frequency of the [[alternating current]] is 60 Hz (between the [[note|tones]] B♭ and B — that is, a [[minor third]] above the European frequency). The frequency of the '[[mains hum|hum]]' in an [[audio recording]] can show where the recording was made — in countries utilizing the European, or the American grid frequency. |
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== See also == |
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{{portalpar|Electronics|Nuvola_apps_ksim.png}} |
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*[[Negative frequency]] |
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*[[Angular frequency]] |
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*[[Frequency converter]] |
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*[[Frequency range]] or frequency band |
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*[[Letter frequencies]] |
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*[[Pitch (music)]] |
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*[[Natural frequency]] |
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*[[Wavelength]] |
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==Notes== |
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{{reflist}} |
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==External links== |
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{{Wiktionarypar2|frequency|often}} |
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*[http://www.sengpielaudio.com/calculator-wavelength.htm Conversion: frequency to wavelength and back] |
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*[http://www.sengpielaudio.com/calculator-period.htm Conversion: period, cycle duration, periodic time to frequency] |
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*[http://www.sengpielaudio.com/calculator-notenames.htm Keyboard frequencies = naming of notes - The English and American system versus the German system] |
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*[http://www.acoustics.salford.ac.uk/schools/index1.htm Teaching resource for 14-16yrs on sound including frequency] |
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*[http://www.ikalogic.com/freq_meter.php A simple tutorial on how to '''build a frequency meter'''] |
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*[http://www.diracdelta.co.uk/science/source/f/r/frequency/source.html Frequency - diracdelta.co.uk] - javascript calculation. |
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[[Category:Fundamental physics concepts]] |
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[[Category:Physical quantity]] |
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[[Category:Mechanical vibrations]] |
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[[Category:Acoustics]] |
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[[Category:Wave mechanics]] |
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[[Category:Qualities of thought]] |
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[[af:Frekwensie]] |
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[[ar:تردد]] |
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[[ast:Frecuencia]] |
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[[az:Tezlik]] |
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[[bn:কম্পাঙ্ক]] |
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[[zh-min-nan:Pîn-lu̍t]] |
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[[bs:Frekvencija]] |
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[[bg:Честота]] |
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[[ca:Freqüència]] |
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[[cs:Frekvence]] |
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[[da:Frekvens]] |
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[[de:Frequenz]] |
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[[et:Sagedus]] |
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[[el:Συχνότητα]] |
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[[es:Frecuencia]] |
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[[eo:Frekvenco]] |
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[[fa:بسامد]] |
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[[fr:Fréquence]] |
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[[gl:Frecuencia]] |
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[[ko:진동수]] |
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[[hi:आवृत्ति]] |
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[[hr:Frekvencija]] |
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[[io:Frequeso]] |
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[[id:Frekuensi]] |
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[[it:Frequenza]] |
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[[he:תדירות]] |
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[[lv:Frekvence]] |
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[[lt:Dažnis]] |
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[[hu:Frekvencia]] |
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[[mk:Фреквенција]] |
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[[mg:Hatetika]] |
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[[ms:Frekuensi]] |
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[[mn:Давтамж]] |
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[[nl:Frequentie]] |
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[[ja:周波数]] |
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[[no:Frekvens]] |
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[[nn:Frekvens i fysikk]] |
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[[pl:Częstotliwość]] |
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[[pt:Frequência]] |
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[[ro:Frecvenţă]] |
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[[ru:Частота]] |
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[[simple:Frequency]] |
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[[sk:Frekvencia (fyzika)]] |
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[[sl:Frekvenca]] |
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[[sr:Фреквенција]] |
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[[su:Frékuénsi]] |
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[[fi:Taajuus]] |
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[[sv:Frekvens]] |
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[[ta:அதிர்வெண்]] |
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[[th:ความถี่]] |
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[[vi:Tần số]] |
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[[chr:ᎠᏍᏓᏩᏛᏍᏙᏗ]] |
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[[tr:Frekans]] |
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[[uk:Частота]] |
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[[ur:تعدد]] |
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[[bat-smg:Tonkėnis]] |
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[[zh:頻率]] |
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Revision as of 16:44, 10 December 2007
hi madilne