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'[[Image:FrequencyAnimation.gif|thumb|right|150px|Three flashing lights, from lowest frequency (top) to highest frequency (bottom). f is the frequency in Hertz ("Hz"), meaning the number of flashes per second. T is the period in seconds ("s"), meaning the number of seconds per flash. T and f are [[reciprocal (mathematics)|reciprocals]].]] {{About|the unit of frequency|the rental car company|The Hertz Corporation|other uses}} {{Redirect2|MHz|MHZ||MHZ (disambiguation)}} The '''hertz''' (symbol: '''Hz''') is the [[International System of Units|SI]] unit of [[frequency]] defined as the number of [[cycle per second|cycles per second]] of a periodic phenomenon.<ref>"hertz". (1992). ''American Heritage Dictionary of the English Language'', 3rded. Boston: Houghton Mifflin.</ref> One of its most common uses is the description of [[sine wave]], particularly those used in radio and audio applications. == Definition == The hertz is equivalent to [[cycle per second|cycles per second]].<ref>{{cite web | url = http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html | title = SI brochure: Table 3. Coherent derived units in the SI with special names and symbols | authorlink = [[International Bureau of Weights and Measures|BIPM]] | accessdate = 20102025 }}</ref> In defining the [[second]] the [[Comité International des Poids et Mesures|CIPM]] declared that "''the standard to be employed is the transition between the hyperfine levels F&nbsp;=&nbsp;4, M&nbsp;= &nbsp;0 and F&nbsp;=&nbsp;3, M&nbsp;=&nbsp;0 of the ground state 2S_1/2 of the [[caesium]] 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9&nbsp;192&nbsp;631&nbsp;770 hertz''"<ref>{{cite web | url = http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf | title = [Resolutions of the] CIPM, 1964 - Atomic and molecular frequency standards | publication = SI brochure, Appendix 1 | authorlink = [[International Bureau of Weights and Measures|BIPM]] | accessdate = 2010-20-26 }}</ref> thereby effectively defining the hertz and the second simultaneously. In English, hertz is used as plural. As an SI unit, Hz can be [[SI prefix|prefixed]]; commonly used multiples are kHz (kilohertz, 10³&nbsp;Hz), MHz (megahertz, 10⁶&nbsp;Hz), GHz (gigahertz, 10⁹&nbsp;Hz) and THz (terahertz, 10¹²&nbsp;Hz). One hertz simply means "one cycle per [[second]]" (typically that which is being counted is a complete cycle); 100&nbsp;Hz means "one hundred cycles per second", and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at 1&nbsp;Hz, or a human heart might be said to [[heart rate|beat]] at 1.2&nbsp;Hz. The "frequency" (activity) of aperiodic or [[stochastic]] events, such as [[radioactive decay]], is expressed in [[becquerel]]s. {{Infobox Unit | bgcolour = | name = Hertz | image = | caption = | standard = [[SI derived unit]] | quantity = [[Frequency]] | symbol = Hz | namedafter = [[Heinrich Hertz]] | units1 = [[SI base unit]]s | inunits1 = 1/[[second|s]] }} Even though [[angular velocity]], [[angular frequency]] and hertz all have the dimensions of ''1/s'', angular velocity and angular frequency are ''not'' expressed in hertz <ref>{{cite web | url = http://www.bipm.org/en/si/derived_units/2-2-2.html | title = SI brochure, Section 2.2.2, paragraph 6 | authorlink = [[International Bureau of Weights and Measures|BIPM]] }}</ref>, but rather in an appropriate angular unit such as [[radian]]s per second. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2π&nbsp;rad/s ''or'' 1&nbsp;Hz, where the former measures the [[angular velocity]] and latter reflects the number of ''complete'' revolutions per second. The conversion between a frequency ''f'' measured in hertz and an angular velocity ''ω'' measured in radians per second are: :<math> \omega = 2\pi f</math> and <math>f = \omega/(2\pi) \, </math>. {{SI unit lowercase|Heinrich Hertz|hertz|Hz}} ==History== The hertz is named after the [[Germany|German]] physicist [[Heinrich Hertz]], who made important scientific contributions to the study of [[electromagnetism]]. The name was established by the [[International Electrotechnical Commission]] (IEC) in 1930.<ref>[http://www.iec.ch/about/history/overview/summary.htm IEC History]</ref> It was adopted by the [[General Conference on Weights and Measures]] (CGPM) (''Conférence générale des poids et mesures'') in 1960, replacing the previous name for the unit, ''[[cycle per second|cycles per second]]'' (cps), along with its related multiples, primarily ''kilocycles per second'' (kc/s) and ''megacycles per second'' (Mc/s), and occasionally ''kilomegacycles per second'' (kMc/s). The term ''cycles per second'' was largely replaced by ''hertz'' by the 1970s. The term "gigahertz", most commonly used in computer processor clock rates and [[radio frequency]] (RF) applications, can be pronounced either {{IPA-en|ˈɡɪɡəhɜrts|}}, with a hard {{IPA|/ɡ/}} sound, or {{IPA-en|ˈdʒɪɡəhɜrts|}}, with a soft {{IPA|/dʒ/}}.<ref>"[http://dictionary.reference.com/browse/gigahertz Gigahertz]" in Dictionary.com Unabridged. ''Dictionary''.</ref> The prefix "giga-" is derived directly from the [[Greek language|Greek]] "{{Polytonic|γιγας}}." ==Applications== [[Image:Cardiac Cycle Left Ventricle.PNG|thumb|right|400px|Details of a [[heartbeat]] as an example of a non-sinusoidal periodic phenomenon that can be described in terms of hertz. Two complete cycles are illustrated.]] === Vibration === [[Sound]] is a traveling wave which is an oscillation of [[pressure]]. Humans perceive frequency of sound waves as [[Pitch (music)|pitch]]. Each musical [[note]] corresponds to a particular frequency which can be measured in hertz. An infant's ear is able to perceive frequencies ranging from 20&nbsp;Hz to 20,000&nbsp;Hz; the average human can hear sounds between 20&nbsp;Hz and 16,000&nbsp;Hz.<ref>[http://www.mmk.e-technik.tu-muenchen.de/persons/ter/top/dominant.html Dominant spectral region]</ref> The range of [[ultrasound]], [[infrasound]] and other physical vibrations such as [[molecular vibration]]s extends into the megahertz range and well beyond. ===Electromagnetic radiation=== [[Electromagnetic radiation]] is often described by its frequency—the number of [[oscillation]]s of the perpendicular electric and magnetic fields per second—expressed in hertz. Radio frequency radiation is usually measured in kilohertz, megahertz, or gigahertz; this is why radio dials are commonly labeled with kHz, MHz, and GHz. [[Light]] is electromagnetic radiation that is even higher in frequency, and has frequencies in the range of tens ([[infrared]]) to thousands ([[ultraviolet]]) of terahertz. Electromagnetic radiation with frequencies in the low terahertz range, (intermediate between those of the highest normally usable radio frequencies and long-wave infrared light), is often called [[terahertz radiation]]. Even higher frequencies exist, such as that of [[gamma ray]]s, which can be measured in exahertz. (For historical reasons, the frequencies of light and higher frequency electromagnetic radiation are more commonly specified in terms of their [[wavelength]]s or [[photon]] [[Energy|energies]]: for a more detailed treatment of this and the above frequency ranges, see [[electromagnetic spectrum]].) ===Computing=== In computing, most [[central processing unit]]s (CPU) are labeled in terms of their [[clock rate]] expressed in megahertz or gigahertz (10⁹ hertz). This number refers to the frequency of the CPU's master [[clock signal]] ("[[Clock rate]]"). This signal is simply an electrical voltage which changes from low to high and back again at regular intervals. This signal is also referred to as a [[square wave]]. Hertz has become the primary unit of measurement accepted by the general populace to determine the performance of a CPU, but many experts have criticized this approach, which they claim is an easily manipulable benchmark.<ref>[http://www.wired.com/news/business/0,1367,62851,00.html Good Riddance, Gigahertz]</ref> For home-based personal computers, the CPU has ranged from approximately 1 megahertz in the late 1970s (Atari, Commodore, Apple computers) to up to 6&nbsp;GHz in the present (IBM POWER processors). Various [[Bus (computing)|computer bus]]es, such as the [[front-side bus]] connecting the CPU and [[northbridge (computing)|northbridge]], also operate at different frequencies in the megahertz range (for modern products). [[Cathode ray tube|CRT]] television and monitor [[Refresh rate|refresh rates]] are measured in hertz. ==SI multiples== {{SI multiples |symbol=Hz |unit=hertz |note=Common prefixed units are in bold face. |k=|M=|G=|T=| |anchor=SI_prefixed_forms_of_hertz |right= }} ===Frequencies not expressed in hertz=== Even higher frequencies are believed to occur naturally, in the frequencies of the quantum-mechanical [[wave function]]s of high-energy (or, equivalently, massive) particles, although these are not directly observable, and must be inferred from their interactions with other phenomena. For practical reasons, these are typically not expressed in hertz, but in terms of the equivalent quantum energy, which is proportional to the frequency by the factor of [[Planck constant|Planck's constant]]. Yo mama! ==See also== {{multicol}} * [[Alternating current]] *[[Electronic tuner]] *[[Frequency changer]] {{multicol-break}} *[[Orders of magnitude (frequency)]] *[[Radian per second]] *[[Signal bandwidth]] {{multicol-end}} ==References== {{reflist|2}} ==External links== *[http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html BIPM Cesium ion f_Cs definition] *[http://inms-ienm.nrc-cnrc.gc.ca/research/frequency_time_projects_e.html#gen National Research Council of Canada: ''Generation of the Hz''] *[http://inms-ienm.nrc-cnrc.gc.ca/research/cesium_clock_e.html National Research Council of Canada: ''Cesium fountain clock''] *[http://www.npl.co.uk/server.php?show=ConWebDoc.1086 National Physical Laboratory: ''Trapped ion optical frequency standards'' ] *[http://inms-ienm.nrc-cnrc.gc.ca/research/optical_frequency_projects_e.html#optical National Research Council of Canada: ''Optical frequency standard based on a single trapped ion''] *[http://inms-ienm.nrc-cnrc.gc.ca/research/optical_frequency_projects_e.html#femtosecond National Research Council of Canada: ''Optical frequency comb''] * [http://www.google.com/search?q=one+hertz+in+radians+per+second One Hertz in Radians per Second (Google). Note, as of 06 May 2009 there is an error of 2<math>\pi</math>.] {{SI derived units}} [[Category:SI derived units]] [[Category:Units of frequency]] [[ar:هرتز]] [[ast:Herciu]] [[zh-min-nan:Hz]] [[be-x-old:Гэрц (адзінка вымярэньня)]] [[bo:ཧེར་ཚི།]] [[bs:Herc]] [[br:Hertz]] [[bg:Херц]] [[ca:Hertz]] [[cs:Hertz]] [[cy:Hertz]] [[da:Hertz]] [[de:Hertz (Einheit)]] [[et:Herts]] [[el:Χερτζ]] [[es:Hercio]] [[eo:Herco]] [[eu:Hertz]] [[fa:هرتز]] [[fr:Hertz]] [[ga:Heirts]] [[gl:Hertz]] [[hak:Het-tsṳ̂]] [[ko:헤르츠]] [[hi:हर्ट्ज़]] [[hr:Herc]] [[id:Hertz]] [[ia:Hertz]] [[it:Hertz]] [[he:הרץ]] [[lv:Hercs]] [[lt:Hercas]] [[hu:Hertz]] [[mk:Херц]] [[mr:हर्ट्झ]] [[ms:Hertz]] [[nl:Hertz (eenheid)]] [[ja:ヘルツ]] [[no:Hertz]] [[nn:Hertz]] [[pnb:ہرٹز]] [[pl:Herc]] [[pt:Hertz]] [[ro:Hertz]] [[ru:Герц (единица измерения)]] [[sq:Herci]] [[simple:Hertz]] [[sk:Hertz (jednotka)]] [[sl:Herc]] [[sr:Херц]] [[sh:Herc]] [[fi:Hertsi]] [[sv:Hertz]] [[ta:ஏர்ட்சு]] [[th:เฮิรตซ์]] [[tg:Ҳертс]] [[tr:Hertz (birim)]] [[uk:Герц]] [[ur:ہرٹز]] [[vi:Hertz]] [[zh-yue:赫茲]] [[zh:赫兹]]'