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The hertz (symbol: Hz) is the
unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in ...
of
frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...

frequency
in the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI) and is defined as one cycle per
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
. The hertz is an
SI derived unit SI derived units are units of measurement derived from the seven SI base unit, base units specified by the International System of Units (SI). They are either dimensionless quantity, dimensionless or can be expressed as a product of one or more o ...
whose expression in terms of
SI base unit The SI base units are the standard defined by the (SI) for the seven of what is now known as the : they are notably a basic set from which all other can be . The units and their physical quantities are the for , the for , the for , the f ...

SI base unit
s is s−1, meaning that one hertz is the reciprocal of one second. It is named after
Heinrich Rudolf Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of scien ...

Heinrich Rudolf Hertz
(1857–1894), the first person to provide conclusive proof of the existence of
electromagnetic wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...

electromagnetic wave
s. 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
sine wave A sine wave or sinusoid is any of certain mathematical curves that describe a smooth periodic oscillation Oscillation is the repetitive variation, typically in time Time is the indefinite continued sequence, progress of existence and ev ...

sine wave
s and
musical tone Traditionally in Western music, a musical tone is a steady periodic sound. A musical tone is characterized by its duration, pitch, intensity (or loudness), and timbre In music, timbre ( ), also known as tone color or tone quality (from p ...
s, particularly those used in
radio Radio is the technology of signaling and communicating Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (philosophy), entities or Organization, groups through the use ...

radio
- and audio-related applications. It is also used to describe the
clock speed In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwar ...
s 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 Max Karl Ernst Ludwig Planck, (; ; 23 April 1858 – 4 October 1947) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citi ...
''E''=''h''ν, where ''E'' is the photon's energy, ν is its frequency, and the proportionality constant ''h'' is
Planck's constant
Planck's constant
.


Definition

The hertz is defined as one
cycle per second The cycle per second was a once-common English name for the unit of frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequenc ...
. The
International Committee for Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in ...
defined the second as "the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the
caesium Caesium (IUPAC spelling) (American and British English spelling differences, also spelled cesium in American English) is a chemical element with the Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali ...

caesium
-133 atom" and then adds: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly hertz, ν(hfs Cs) = ." The dimension of the unit hertz is 1/time (1/T). Expressed in base SI units, the unit is 1/second (1/s). Problems can arise because the unit of angular measure (radian) is sometimes omitted in SI. In English, "hertz" is also used as the plural form. As an SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, ), MHz (megahertz, ), GHz (gigahertz, ) and THz (terahertz, ). One hertz simply means "one cycle per second" (typically that which is being counted is a complete cycle); 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 , or a human heart might be said to
beat Beat, beats or beating may refer to: Common meanings Assigned activity or area * Patrol, an area (usually geographic) that one is responsible to monitor, including: ** Beat (police), the territory and time that a police officer patrols ** Beat ...

beat
at . The occurrence rate of aperiodic or
stochastic Stochastic () refers to the property of being well described by a random In common parlance, randomness is the apparent or actual lack of pattern or predictability in events. A random sequence of events, symbols or steps often has no :wi ...
events is expressed in
reciprocal secondThe inverse second, reciprocal second, or per second (s−1) is a unit of frequency, defined as the multiplicative inverse of the second (a unit of time). It is dimensional analysis, dimensionally equivalent to: * the unit hertz – the SI unit for f ...
or inverse second (1/s or s−1) in general or, in the specific case of
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

radioactive decay
, in
becquerel The becquerel (; symbol: Bq) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or eve ...
s."(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." Whereas is one cycle per second, is one aperiodic radionuclide event per second. Even though
angular velocity In physics, angular velocity (\boldsymbol or \boldsymbol), also known as angular frequency vector,(UP1) is a vector measure of rotation rate, that refers to how fast an object rotates or revolves relative to another point, i.e. how fast the angu ...

angular velocity
,
angular frequency In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...
and the unit hertz all have the dimension 1/T, angular velocity and angular frequency are not expressed in hertz, but rather in an appropriate angular unit such as the
radian per second The radian per second (symbol: rad⋅s−1 or rad/s) is the unit of , commonly denoted by the Greek letter ''ω'' (omega). The per second is also the SI unit of . The radian per second is defined as the change in the of an object, in s, every ...
. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2 rad/s ''or'' , where the former measures the
angular velocity In physics, angular velocity (\boldsymbol or \boldsymbol), also known as angular frequency vector,(UP1) is a vector measure of rotation rate, that refers to how fast an object rotates or revolves relative to another point, i.e. how fast the angu ...

angular velocity
and the 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 The radian, denoted by the symbol \text, is the SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric sys ...

radians
per second is : \omega = 2\pi f \, and f = \frac \, .


History

The hertz is named after the German physicist
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empi ...

Heinrich Hertz
(1857–1894), who made important scientific contributions to the study of
electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagnet ...

electromagnetism
. The name was established by the
International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French language, French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, e ...
(IEC) in 1935. It was adopted by the
General Conference on Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures The International Bureau of Weights and Measures (fr ...
(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. Sometimes the "per second" was omitted, so that "megacycles" (Mc) was used as an abbreviation of "megacycles per second" (that is, megahertz (MHz)).


Applications


Vibration

Sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the b ...

Sound
is a traveling
longitudinal wave Longitudinal waves are waves in which the vibration of the medium is parallel to the direction the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical wave, Mechanical longitudinal ...

longitudinal wave
which is an oscillation of
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

pressure
. Humans perceive frequency of sound waves as
pitch Pitch may refer to: Acoustic frequency * Pitch (music), the perceived frequency of sound including "definite pitch" and "indefinite pitch" ** Absolute pitch or "perfect pitch" ** Pitch class, a set of all pitches that are a whole number of octaves ...
. Each
musical note In music, a note is a symbol denoting a musical sound. In English usage, a note is also the sound itself. Notes can represent the Pitch (music), pitch and Duration (music), duration of a sound in musical notation. A note can also represent a pitch ...
corresponds to a particular frequency which can be measured in hertz. An infant's ear is able to perceive frequencies ranging from to ; the average
adult human
adult human
can hear sounds between and . The range of
ultrasound Ultrasound is s with higher than the upper audible limit of human . Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies from person to person and is appro ...

ultrasound
,
infrasound Infrasound, sometimes referred to as low status sound , describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to per ...
and other physical vibrations such as
molecular A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atom An atom is the smallest unit of ordinary matter In ...
and atomic vibrations extends from a few
femto ''Femto'' (symbol f) is a unit prefixA unit prefix is a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units. Units of various order of magnitude, sizes are commonly formed by the use of ...
hertz into the
tera Tera or TERA may refer to: People * Tera Patrick Tera Patrick (born Linda Ann Hopkins July 25, 1976) is an American former pornographic film actor, pornographic actress and model who was the ''Penthouse (magazine), Penthouse'' Pet of the M ...
hertz range and beyond.


Electromagnetic radiation

Electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

Electromagnetic radiation
is often described by its frequency—the number of
oscillation Oscillation is the repetitive variation, typically in time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversible succession from the pas ...

oscillation
s 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 Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

Light
is electromagnetic radiation that is even higher in frequency, and has frequencies in the range of tens (
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

infrared
) to thousands (
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that stud ...

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 Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic wave Electromagnetism is a branch of physic ...
. Even higher frequencies exist, such as that of
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s, 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
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adja ...

wavelength
s or
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

photon
energies In physics, energy is the physical quantity, quantitative physical property, property that must be #Energy transfer, transferred to an Physical body, object in order to perform Work (thermodynamics), work on, or to heat, the object.The second ...

energies
: for a more detailed treatment of this and the above frequency ranges, see
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existe ...

electromagnetic spectrum
.)


Computers

In computers, most
central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of individual electroni ...

central processing unit
s (CPU) are labeled in terms of their
clock rate In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
expressed in megahertz () or gigahertz (). This specification refers to the frequency of the CPU's master
clock signal In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplificati ...

clock signal
. This signal is a
square wave A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Tim ...

square wave
, which is an electrical voltage that switches between low and high logic values 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 periods to perform a single operation, while others can perform multiple operations in a single cycle. For personal computers, CPU clock speeds have ranged from approximately in the late 1970s (
Atari Atari () is a brand name owned by several entities since its inception in 1972, currently by Atari Interactive Atari Interactive is a name used by several separate groups and corporations since the mid-1990s. In 1996, it was the name of Atar ...

Atari
,
Commodore Commodore may refer to: Ranks * Commodore (rank) Commodore is a senior naval rank used in many navies which is equivalent to brigadier and air commodore that is superior to a navy captain, but below a rear admiral. It is either regarded as ...
,
Apple computers Apple Inc. is an American multinational technology company headquartered in Cupertino, California Cupertino ( ) is a city in Santa Clara County, California Santa Clara County, officially the County of Santa Clara, is California's List o ...

Apple computers
) to up to in
IBM Power microprocessors IBM Power (originally POWER prior to Power10) is a line of microprocessors designed and sold by IBM for Server (computing), servers and supercomputers. The name "POWER" was originally presented as an acronym for "Performance Optimization With Enh ...
. Various
computer buses In computer architecture, a bus (a contraction of the Latin ''wikt:omnibus, omnibus'', and historically also called ''data highway'') is a communication system that transfers Data (computing), data between components inside a computer, or betw ...
, such as the
front-side bus A front-side bus (FSB) is a computer communication interface (bus trolleybus in Toronto Toronto is the capital city of the Provinces and territories of Canada, Canadian province of Ontario. With a recorded population of 2,731,571 in 2 ...
connecting the CPU and northbridge, also operate at various frequencies in the megahertz range.


Higher frequencies than the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
provides prefixes for are believed to occur naturally in the frequencies of the quantum-mechanical vibrations of high-energy, or, equivalently, massive particles, although these are not directly observable and must be inferred from their interactions with other phenomena. By convention, 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's constant.


Unicode

The
CJK Compatibility CJK Compatibility is a Unicode block containing square symbols (both CJK and Latin alphanumeric) encoded for compatibility with east Asian character sets. In Unicode 1.0, it was divided into two blocks, named CJK Squared Words (U+3300–U+337F) an ...
block in
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

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"). * * * * *


See also

*
Alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through ...
*
Bandwidth (signal processing) Bandwidth is the difference between the upper and lower frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequen ...
*
Electronic tuner chromatic LCD tuner, with simulated analog indicator needle In music, an electronic tuner is a device that detects and displays the Pitch (music), pitch of musical notes played on a musical instrument A musical instrument is a device created or ...
*
FLOPS In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest c ...

FLOPS
*
Frequency changer A frequency changer or frequency converter is an electronic or electromechanical device that converts alternating current ( AC) of one frequency to alternating current of another frequency. The device may also change the voltage, but if it d ...
*
Normalized frequency (unit) Normalized frequency is a unit of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), ...
*
Orders of magnitude (frequency) The following list illustrates various frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular freque ...
*
Periodic function A periodic function is a function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logica ...

Periodic function
*
Radian per second The radian per second (symbol: rad⋅s−1 or rad/s) is the unit of , commonly denoted by the Greek letter ''ω'' (omega). The per second is also the SI unit of . The radian per second is defined as the change in the of an object, in s, every ...
* Rate *
Sampling rate In , sampling is the reduction of a to a . A common example is the conversion of a (a continuous signal) to a sequence of samples (a discrete-time signal). A sample is a value or set of values at a point in time and/or space. A sampler is a su ...


Notes and references


External links


SI Brochure: Unit of time (second)

National Research Council of Canada: ''Cesium fountain clock''


* ttps://web.archive.org/web/20130627155632/http://archive.nrc-cnrc.gc.ca/eng/projects/inms/optical-comb.html National Research Council of Canada: ''Optical frequency comb''
National Physical Laboratory: ''Time and frequency Optical atomic clocks''

Online Tone Generator
{{SI units SI derived units Units of frequency Heinrich Hertz