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Hertz

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This article is about the unit measure. For the car rental company, seeHertz Global Holdings.For other uses, seeHertz (disambiguation).
"Hz" and "Megahertz" redirect here. For other uses, seeHz (disambiguation)andMegahertz (disambiguation).

hertz
Top to bottom: Lights flashing atfrequenciesf=0.5 Hz,1.0 Hzand2.0 Hz;that is, at 0.5, 1.0 and 2.0 flashes per second, respectively. The time between each flash – theperiodT– is given by1⁄f(thereciprocaloff ); that is, 2, 1 and 0.5 seconds, respectively.
General information
Unit systemSI
Unit offrequency
SymbolHz
Named afterHeinrich Hertz
InSI base unitss−1

Thehertz(symbol:Hz) is the unit offrequencyin theInternational System of Units(SI), equivalent to one event (orcycle) persecond.[1][a]The hertz is anSI derived unitwhose expression in terms ofSI base unitsis s−1,meaning that one hertz is thereciprocal of one second.[2]It is named afterHeinrich Rudolf Hertz(1857–1894), the first person to provide conclusive proof of the existence ofelectromagnetic waves.Hertz are commonly expressed inmultiples:kilohertz (kHz), megahertz (MHz), gigahertz (GHz), terahertz (THz).

Some of the unit's most common uses are in the description ofperiodic waveformsandmusical tones,particularly those used inradio- and audio-related applications. It is also used to describe theclock speedsat which computers and other electronics are driven. The units are sometimes also used as a representation of theenergy of a photon,via thePlanck relationE=hν,whereEis the photon's energy,νis its frequency, andhis thePlanck constant.

Definition[edit]

The hertz is equivalent to onecycle per second.TheInternational Committee for Weights and Measuresdefined the second as "the duration of9192631770periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of thecaesium-133 atom "[3][4]and then adds: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly9192631770hertz,νhfs Cs=9192631770Hz."The dimension of the unit hertz is 1/time (T−1). Expressed in base SI units, the unit is the reciprocal second (1/s).

In English, "hertz" is also used as the plural form.[5]As an SI unit, Hz can beprefixed;commonly used multiples are kHz (kilohertz,103Hz), MHz (megahertz,106Hz), GHz (gigahertz,109Hz) and THz (terahertz,1012Hz). One hertz simply means "one event per second" (where the event being counted may be a complete cycle);100 Hzmeans "one hundred events per second", and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at1 Hz,or a human heart might be said tobeatat1.2 Hz.

The occurrencerate of aperiodicorstochasticevents is expressed inreciprocal secondorinverse second(1/s or s−1) in general or, in the specific case ofradioactivity,inbecquerels.[b]Whereas1 Hzis one cycle (or periodic event) per second,1 Bqis one radionuclide event per second on average.

Even though frequency,angular velocity,angular frequencyand radioactivity all have the dimension T−1,of these only frequency is expressed using the unit hertz.[7]Thus a disc rotating at 60 revolutions per minute (rpm) is said to have an angular velocity of 2πrad/s and afrequency of rotationof1 Hz.The correspondence between a frequencyfwith the unit hertz and an angular velocityωwith the unitradiansper second is

and

The hertz is named afterHeinrich Hertz.As with everySIunit named for a person, its symbol starts with anupper caseletter (Hz), but when written in full, it follows the rules for capitalisation of acommon noun;i.e.,hertzbecomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

History[edit]

Further information:Cycle per second

The hertz is named after the German physicistHeinrich Hertz(1857–1894), who made important scientific contributions to the study ofelectromagnetism.The name was established by theInternational Electrotechnical Commission(IEC) in 1935.[8]It was adopted by theGeneral Conference on Weights and Measures(CGPM) (Conférence générale des poids et mesures) in 1960, replacing the previous name for the unit, "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.[9][failed verification]

In some usage, the "per second" was omitted, so that "megacycles" (Mc) was used as an abbreviation of "megacycles per second" (that is, megahertz (MHz)).[10]

Applications[edit]

Asine wavewith varying frequency
A heartbeatis an example of a non-sinusoidalperiodic phenomenon that may be analyzed in terms of frequency. Two cycles are illustrated.

Sound and vibration[edit]

Soundis a travelinglongitudinal wave,which is anoscillationofpressure.Humans perceive the frequency of a sound as itspitch.Eachmusical notecorresponds to a particular frequency. An infant's ear is able to perceive frequencies ranging from20 Hzto20000Hz;the averageadult humancan hear sounds between20 Hzand16000Hz.[11]The range ofultrasound,infrasoundand other physical vibrations such asmolecularandatomic vibrationsextends from a fewfemtohertz[12]into theterahertzrange[c]and beyond.[13]

Electromagnetic radiation[edit]

Electromagnetic radiationis often described by its frequency—the number of oscillations of the perpendicular electric and magnetic fields per second—expressed in hertz.

Radio frequency radiation is usually measured in kilohertz (kHz), megahertz (MHz), or gigahertz (GHz).Lightis 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 calledterahertz radiation.Even higher frequencies exist, such as that ofgamma rays,which can be measured in exahertz (EHz). (For historical reasons, the frequencies of light and higher frequency electromagnetic radiation are more commonly specified in terms of theirwavelengthsorphotonenergies:for a more detailed treatment of this and the above frequency ranges, seeElectromagnetic spectrum.)

Computers[edit]

Further information on why the frequency, including for gigahertz (GHz) etc., is a flawed speed indicator for computers:Megahertz myth

In computers, mostcentral processing units(CPU) are labeled in terms of theirclock rateexpressed in megahertz (MHz) or gigahertz (GHz). This specification refers to the frequency of the CPU's masterclock signal.This signal is nominally asquare wave,which is an electrical voltage that switches between low and high logic levels at regular intervals. As the hertz has become the primary unit of measurement accepted by the general populace to determine the performance of a CPU, many experts have criticized this approach, which they claim is aneasily manipulable benchmark.Some processors use multiple clock cycles to perform a single operation, while others can perform multiple operations in a single cycle.[14]For personal computers, CPU clock speeds have ranged from approximately1 MHzin the late 1970s (Atari,Commodore,Apple computers) to up to6 GHzinIBM Power microprocessors.

Variouscomputer buses,such as thefront-side busconnecting the CPU andnorthbridge,also operate at various frequencies in the megahertz range.

SI multiples[edit]

For examples of various multiples, seeOrders of magnitude (frequency).
SI multiples of hertz (Hz)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1Hz dHz decihertz 101Hz daHz decahertz
10−2Hz cHz centihertz 102Hz hHz hectohertz
10−3Hz mHz millihertz 103Hz kHz kilohertz
10−6Hz μHz microhertz 106Hz MHz megahertz
10−9Hz nHz nanohertz 109Hz GHz gigahertz
10−12Hz pHz picohertz 1012Hz THz terahertz
10−15Hz fHz femtohertz 1015Hz PHz petahertz
10−18Hz aHz attohertz 1018Hz EHz exahertz
10−21Hz zHz zeptohertz 1021Hz ZHz zettahertz
10−24Hz yHz yoctohertz 1024Hz YHz yottahertz
10−27Hz rHz rontohertz 1027Hz RHz ronnahertz
10−30Hz qHz quectohertz 1030Hz QHz quettahertz
Common prefixed units are in bold face.

Higher frequencies than theInternational System of Unitsprovides prefixes for are believed to occur naturally in the frequencies of the quantum-mechanical vibrations of massive particles, although these are not directly observable and must be inferred through other phenomena. By convention, these are typically not expressed in hertz, but in terms of the equivalent energy, which is proportional to the frequency by the factor of thePlanck constant.

Unicode[edit]

TheCJK Compatibilityblock inUnicodecontains characters for common SI units for frequency. These are intended for compatibility with East Asian character encodings, and not for use in new documents (which would be expected to use Latin letters, e.g. "MHz" ).[15]

  • U+3339㌹SQUARE HERUTU(ヘルツ,herutsu)
  • U+3390㎐SQUARE HZ(Hz)
  • U+3391㎑SQUARE KHZ(kHz)
  • U+3392㎒SQUARE MHZ(MHz)
  • U+3393㎓SQUARE GHZ(GHz)
  • U+3394㎔SQUARE THZ(THz)

See also[edit]

Notes[edit]

  1. ^Although hertz is equivalent to cycle per second (cps), the SI explicitly states that "cycle" and "cps" are not units in the SI, likely due to ambiguity in the terms.[2]
  2. ^"(d) The hertz is used only for periodic phenomena, and the becquerel (Bq) is used only for stochastic processes in activity referred to a radionuclide."[6]
  3. ^Atomic vibrationsare typically on the order of tens of terahertz

References[edit]

  1. ^"hertz". (1992).American Heritage Dictionary of the English Language(3rd ed.), Boston: Houghton Mifflin.
  2. ^ab"SI Brochure: The International System of Units (SI) – 9th edition"(PDF).BIPM:26.Retrieved7 August2022.
  3. ^"SI Brochure: The International System of Units (SI) § 2.3.1 Base units"(PDF)(in British English and French) (9th ed.).BIPM.2019. p. 130.Retrieved2 February2021.
  4. ^"SI Brochure: The International System of Units (SI) § Appendix 1. Decisions of the CGPM and the CIPM"(PDF)(in British English and French) (9th ed.).BIPM.2019. p. 169.Retrieved2 February2021.
  5. ^NIST Guide to SI Units – 9 Rules and Style Conventions for Spelling Unit Names,National Institute of Standards and Technology
  6. ^"BIPM – Table 3".BIPM.Retrieved24 October2012.
  7. ^"SI brochure, Section 2.2.2, paragraph 6".Archived fromthe originalon 1 October 2009.
  8. ^"IEC History".Iec.ch. Archived fromthe originalon 19 May 2013.Retrieved6 January2021.
  9. ^Cartwright, Rufus (March 1967). Beason, Robert G. (ed.)."Will Success Spoil Heinrich Hertz?"(PDF).Electronics Illustrated.Fawcett Publications, Inc. pp. 98–99.
  10. ^Pellam, J. R.; Galt, J. K. (1946). "Ultrasonic Propagation in Liquids: I. Application of Pulse Technique to Velocity and Absorption Measurements at 15 Megacycles".The Journal of Chemical Physics.14(10): 608–614.Bibcode:1946JChPh..14..608P.doi:10.1063/1.1724072.hdl:1721.1/5042.
  11. ^Ernst Terhardt (20 February 2000)."Dominant spectral region".Mmk.e-technik.tu-muenchen.de. Archived fromthe originalon 26 April 2012.Retrieved28 April2012.
  12. ^"Black Hole Sound Waves – Science Mission Directorate".science.nasa.go.
  13. ^"Black Hole Sound Waves – Science Mission Directorate".science.nasa.go.
  14. ^Asaravala, Amit (30 March 2004)."Good Riddance, Gigahertz".Wired.Retrieved28 April2012.
  15. ^Unicode Consortium(2019)."The Unicode Standard 12.0 – CJK Compatibility ❰ Range: 3300—33FF ❱"(PDF).Unicode.org.Retrieved24 May2019.

External links[edit]

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