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 * 1Definition
 * 2History
 * 3Applications
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   * 3.1Sound and vibration
   * 3.2Electromagnetic radiation
   * 3.3Computers
 * 4SI multiples
 * 5Unicode
 * 6See also
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HERTZ

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From Wikipedia, the free encyclopedia

SI unit for frequency
This article is about the unit measure. For the car rental company, see The
Hertz Corporation. For other uses, see Hertz (disambiguation).
"Hz" and "Megahertz" redirect here. For other uses, see Hz (disambiguation) and
Megahertz (disambiguation).



hertz
Top to bottom: Lights flashing at frequencies f = 0.5 Hz, 1.0 Hz and 2.0 Hz;
that is, at 0.5, 1.0 and 2.0 flashes per second, respectively. The time between
each flash – the period T – is given by 1⁄f (the reciprocal of f ); that is, 2,
1 and 0.5 seconds, respectively.
General informationUnit systemSIUnit offrequencySymbolHzNamed afterHeinrich
HertzIn SI base unitss−1

The hertz (symbol: Hz) is the unit of frequency in the International System of
Units (SI), equivalent to one event (or cycle) per second.[1][3] The hertz is an
SI derived unit whose expression in terms of SI base units is s−1, meaning that
one hertz is the reciprocal of one second.[2] It is named after Heinrich Rudolf
Hertz (1857–1894), the first person to provide conclusive proof of the existence
of electromagnetic waves. Hertz are commonly expressed in multiples: kilohertz
(kHz), megahertz (MHz), gigahertz (GHz), terahertz (THz).

Some of the unit's most common uses are in the description of periodic waveforms
and musical tones, particularly those used in radio- and audio-related
applications. It is also used to describe the clock speeds at which computers
and other electronics are driven. The units are sometimes also used as a
representation of the energy of a photon, via the Planck relation E = hν, where
E is the photon's energy, ν is its frequency, and h is the Planck constant.


DEFINITION[EDIT]

The hertz is equivalent to one cycle per second. The International Committee for
Weights and Measures defined the second as "the duration of 9192631770 periods
of the radiation corresponding to the transition between the two hyperfine
levels of the ground state of the caesium-133 atom"[4][5] and then adds: "It
follows that the hyperfine splitting in the ground state of the caesium 133 atom
is exactly 9192631770 hertz, νhfs Cs = 9192631770 Hz." 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.[6] As an SI unit, Hz can be
prefixed; commonly used multiples are kHz (kilohertz, 103 Hz), MHz (megahertz,
106 Hz), GHz (gigahertz, 109 Hz) and THz (terahertz, 1012 Hz). One hertz simply
means "one event per second" (where the event being counted may be a complete
cycle); 100 Hz means "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 at
1 Hz, or a human heart might be said to beat at 1.2 Hz.

The occurrence rate of aperiodic or stochastic events is expressed in reciprocal
second or inverse second (1/s or s−1) in general or, in the specific case of
radioactivity, in becquerels.[7] Whereas 1 Hz is one cycle (or periodic event)
per second, 1 Bq is one radionuclide event per second on average.

Even though frequency, angular velocity, angular frequency and radioactivity all
have the dimension T−1, of these only frequency is expressed using the unit
hertz.[8] Thus a disc rotating at 60 revolutions per minute (rpm) is said to
have an angular velocity of 2π rad/s and a frequency of rotation of 1 Hz. The
correspondence between a frequency f with the unit hertz and an angular velocity
ω with the unit radians per second is

ω = 2 π f {\displaystyle \omega =2\pi f} and f = ω 2 π . {\displaystyle f={\frac
{\omega }{2\pi }}.}

The hertz is named after Heinrich Hertz. As with every SI unit named for a
person, its symbol starts with an upper case letter (Hz), but when written in
full it follows the rules for capitalisation of a common noun; i.e., "hertz"
becomes 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 physicist Heinrich Hertz (1857–1894), who
made important scientific contributions to the study of electromagnetism. The
name was established by the International Electrotechnical Commission (IEC) in
1935.[9] 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, "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.[10][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)).[11]


APPLICATIONS[EDIT]


A sine wave with varying frequency

A heartbeat is an example of a non-sinusoidal periodic phenomenon that may be
analyzed in terms of frequency. Two cycles are illustrated.


SOUND AND VIBRATION[EDIT]

Sound is a traveling longitudinal wave, which is an oscillation of pressure.
Humans perceive the frequency of a sound as its pitch. Each musical note
corresponds to a particular frequency. An infant's ear is able to perceive
frequencies ranging from 20 Hz to 20000 Hz; the average adult human can hear
sounds between 20 Hz and 16000 Hz.[12] The range of ultrasound, infrasound and
other physical vibrations such as molecular and atomic vibrations extends from a
few femtohertz[13] into the terahertz range[14] and beyond.[15]


ELECTROMAGNETIC RADIATION[EDIT]

Electromagnetic radiation is 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). 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 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 their wavelengths or photon energies: for a more detailed treatment of
this and the above frequency ranges, see Electromagnetic 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, most central processing units (CPU) are labeled in terms of their
clock rate expressed in megahertz (MHz) or gigahertz (GHz). This specification
refers to the frequency of the CPU's master clock signal. This signal is
nominally a square 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 an easily manipulable benchmark. Some processors use multiple clock
cycles to perform a single operation, while others can perform multiple
operations in a single cycle.[16] For personal computers, CPU clock speeds have
ranged from approximately 1 MHz in the late 1970s (Atari, Commodore, Apple
computers) to up to 6 GHz in IBM Power microprocessors.

Various computer buses, such as the front-side bus connecting the CPU and
northbridge, also operate at various frequencies in the megahertz range.


SI MULTIPLES[EDIT]

For examples of various multiples, see Orders of magnitude (frequency).

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

Higher frequencies than the International System of Units provides 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 the Planck constant.


UNICODE[EDIT]

The CJK Compatibility block in Unicode contains 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").[17]

 * U+3390 ㎐ SQUARE HZ
 * U+3391 ㎑ SQUARE KHZ
 * U+3392 ㎒ SQUARE MHZ
 * U+3393 ㎓ SQUARE GHZ
 * U+3394 ㎔ SQUARE THZ


SEE ALSO[EDIT]

 * Alternating current
 * Bandwidth (signal processing)
 * Electronic tuner
 * FLOPS
 * Frequency changer
 * Normalized frequency (signal processing)
 * Orders of magnitude (frequency)
 * Periodic function
 * Radian per second
 * Rate
 * Sampling rate


NOTES AND REFERENCES[EDIT]

 1.  ^ "hertz". (1992). American Heritage Dictionary of the English Language
     (3rd ed.), Boston: Houghton Mifflin.
 2.  ^ Jump up to: a b "SI Brochure: The International System of Units (SI) –
     9th edition" (PDF). BIPM: 26. Retrieved 7 August 2022.
 3.  ^ 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]
 4.  ^ "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.
     Retrieved 2 February 2021.
 5.  ^ "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. Retrieved 2 February 2021.
 6.  ^ NIST Guide to SI Units – 9 Rules and Style Conventions for Spelling Unit
     Names, National Institute of Standards and Technology
 7.  ^ "(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." "BIPM – Table 3". BIPM. Retrieved 24 October 2012.
 8.  ^ "SI brochure, Section 2.2.2, paragraph 6". Archived from the original on
     1 October 2009.
 9.  ^ "IEC History". Iec.ch. Archived from the original on 19 May 2013.
     Retrieved 6 January 2021.
 10. ^ Cartwright, Rufus (March 1967). Beason, Robert G. (ed.). "Will Success
     Spoil Heinrich Hertz?" (PDF). Electronics Illustrated. Fawcett
     Publications, Inc. pp. 98–99.
 11. ^ 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.
 12. ^ Ernst Terhardt (20 February 2000). "Dominant spectral region".
     Mmk.e-technik.tu-muenchen.de. Archived from the original on 26 April 2012.
     Retrieved 28 April 2012.
 13. ^ "Black Hole Sound Waves – Science Mission Directorate". science.nasa.go.
 14. ^ Atomic vibrations are typically on the order of tens of terahertz
 15. ^ "Black Hole Sound Waves – Science Mission Directorate". science.nasa.go.
 16. ^ Asaravala, Amit (30 March 2004). "Good Riddance, Gigahertz". Wired.
     Retrieved 28 April 2012.
 17. ^ Unicode Consortium (2019). "The Unicode Standard 12.0 – CJK Compatibility
     ❰ Range: 3300—33FF ❱" (PDF). Unicode.org. Retrieved 24 May 2019.


EXTERNAL LINKS[EDIT]

 * SI Brochure: Unit of time (second)
 * National Research Council of Canada: Cesium fountain clock
 * National Research Council of Canada: Optical frequency standard based on a
   single trapped ion (archived 23 December 2013)
 * National Research Council of Canada: Optical frequency comb (archived 27 June
   2013)
 * National Physical Laboratory: Time and frequency Optical atomic clocks
 * Online Tone Generator



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