Hertz 14121 224461831 2008-07-08T23:06:08Z The Anome 76 /* History */ and occasionally ''kilomegacycles per second'' (kMc/s). {{intro-rewrite}} {{otheruses2|Hertz}} {{redirect-acronym|MHZ|the [[IATA]] airport code for [[RAF Mildenhall]]}} {{redirect-acronym|MHz|[[MHz Networks]]}}The '''hertz''' (symbol: '''Hz''') is the [[International System of Units]] (SI) [[SI base unit|base unit]] of [[frequency]]. The definition of the hertz is based upon that for the [[second]], namely: the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, <math>\nu</math> (''hfs Cs'') = 9 192 631 770 Hz.<ref>[http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html BIPM definition]</ref> Its base unit is cycle/s or s<sup>-1</sup> (also called inverse seconds, reciprocal seconds). In English, ''hertz'' is used as both singular and plural. As any SI unit, Hz can be [[SI prefix|prefixed]]; commonly used multiples are kHz (kilohertz, 10<sup>3</sup> Hz), MHz (megahertz, 10<sup>6</sup> Hz), GHz (gigahertz, 10<sup>9</sup> Hz) and THz (terahertz, 10<sup>12</sup> 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&#160;Hz, or a human heart might be said to [[heart rate|beat]] at 1.2&nbsp;Hz. The frequencies of aperiodic events, such as [[radioactive decay]], are expressed in [[becquerel]]s. To avoid confusion, periodically varying angles are typically ''not'' expressed in hertz, but rather in an appropriate angular unit such as [[radian]]s per second. A disc rotating at 60 revolutions per minute (RPM) can thus be said to be rotating at ≈6.283&nbsp;rad/s ''or'' 1&nbsp;Hz, where the latter reflects the number of ''complete'' revolutions per second. The conversion between a frequency ''f'' measured in Hertz and an angular frequency ''ω'' measured in radians/s 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 [[electromagnetism]]. The name was established by the [[International Electrotechnical Commission]] (IEC) in 1930.<ref>[http://www.iec.ch/about/history/ 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 speed and [[radio frequency]] (RF) applications, can be pronounced either {{IPA|/ˈgigaˌhɝts/}}, with a hard {{IPA|/g/}} sound or {{IPA|/ˈʒɪgaˌhɝts/}} or {{IPA|/ˈdʒɪgaˌhɝts/}}, with a soft {{IPA|/ʒ/}} sound at the beginning of the word. The prefix "giga-" is derived directly from the [[Greek language|Greek]] "{{Polytonic|γιγας}}" and hence the preferred pronunciation is {{IPA|/ˈgɪga/}}. Some [[Electrical engineering|electrical engineers]] use {{IPA|/ˈdʒɪga/}}, by analogy with "gigantic". == Applications == === 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 16&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 speed expressed in megahertz or gigahertz (10<sup>9</sup> hertz). The number of megahertz refers to the frequency of the CPU's [[Clock signal|master clock signal]] ("[[Clock rate|clock speed]]"). This signal is simply an electrical voltage which changes from low to high and back again at regular intervals. Hertz has become the primary unit of measurement used by the general populace to determine the speed 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 nearly 4 GHz in the present. This can be increased even further by [[overclocking|increasing the frequency of the CPU]] in the BIOS or other software. Various [[Bus (computing)|computer bus]]es, such as [[Front side bus|memory buses]] connecting the CPU and system [[random access memory]] (RAM), also transfer data using clock signals operating at different frequencies in the megahertz ranges (for modern products). ==Order of magnitude== {| class="prettytable" ! Frequency (Hz) ! Name ! Unit !rowspan="12"|&nbsp;&nbsp; ! Frequency (Hz) ! Name ! Unit |- |10<sup>0</sup> | '''Hertz''' |Hz | | | |- |10<sup>1</sup> | Dekahertz |daHz |10<sup>–1</sup> | Decihertz |dHz |- |10<sup>2</sup> | Hektohertz |hHz |10<sup>–2</sup> | Centihertz |cHz |- |10<sup>3</sup> | Kilohertz |kHz |10<sup>–3</sup> | Millihertz |mHz |- |10<sup>6</sup> | Megahertz |MHz |10<sup>–6</sup> | Microhertz |µHz |- |10<sup>9</sup> | Gigahertz |GHz |10<sup>–9</sup> | Nanohertz |nHz |- |10<sup>12</sup> | Terahertz |THz |10<sup>–12</sup> | Picohertz |pHz |- |10<sup>15</sup> | Petahertz |PHz |10<sup>–15</sup> | Femtohertz |fHz |- |10<sup>18</sup> | Exahertz |EHz |10<sup>–18</sup> | Attohertz |aHz |- |10<sup>21</sup> | Zettahertz |ZHz |10<sup>–21</sup> | Zeptohertz |zHz |- |10<sup>24</sup> | Yottahertz |YHz |10<sup>–24</sup> | Yoktohertz |yHz |- |} ===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 energy. ==References== {{reflist|2}} == See also == *[[Frequency changer]] *[[Orders of magnitude (frequency)]] *[[Signal bandwidth]] *[[Radian per second]] ==External links== *[http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html BIPM Cesium ion f<sub>Cs</sub> 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''] [[Category:SI derived units]] [[Category:Units of frequency]] [[ar:هرتز]] [[ast:Herciu]] [[zh-min-nan:Hz]] [[be-x-old:Гэрц (адзінка вымярэньня)]] [[bs:Herc]] [[br:Hertz]] [[bg:Херц (единица)]] [[ca:Hertz]] [[cs:Hertz]] [[da:Hertz]] [[de:Hertz (Einheit)]] [[et:Herts]] [[es:Hercio]] [[eo:Herco]] [[eu:Hertz]] [[fa:هرتز]] [[fr:Hertz]] [[gl:Hertz]] [[ko:헤르츠]] [[hi:हर्ट्ज़]] [[hr:Herc]] [[id:Hertz]] [[ia:Hertz]] [[it:Hertz]] [[he:הרץ]] [[lv:Hercs (mērvienība)]] [[lt:Hercas]] [[hu:Hertz]] [[mk:Херц]] [[ms:Hertz]] [[nl:Hertz (eenheid)]] [[ja:ヘルツ]] [[no:Hertz]] [[nn:Hertz]] [[pl:Herc]] [[pt:Hertz]] [[ro:Hertz]] [[ru:Герц (единица измерения)]] [[sq:Herci]] [[simple:Hertz]] [[sk:Hertz (jednotka)]] [[sl:Herc]] [[sr:Херц]] [[fi:Hertsi]] [[sv:Hertz]] [[th:เฮิรตซ์]] [[vi:Hertz]] [[tr:Hertz (birim)]] [[uk:Герц]] [[ur:ہرٹز]] [[zh-yue:赫茲]] [[zh:赫兹]]