Top to bottom: Lights flashing atfrequenciesf =0.5 Hz,1.0 Hz and2.0 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 (thereciprocal off ); that is, 2, 1 and 0.5 seconds, respectively.
Thehertz (symbol:Hz) is the unit offrequency in theInternational System of Units (SI), often described as being equivalent to one event (orcycle) persecond.[1][a] The hertz is anSI derived unit whose formal expression in terms ofSI base units is s−1, meaning that one hertz is one per second or thereciprocal of one second.[2] It is used only in the case of periodic events. It is named afterHeinrich Rudolf Hertz (1857–1894), the first person to provide conclusive proof of the existence ofelectromagnetic waves. For high frequencies, the unit is commonly expressed inmultiples: kilohertz (kHz), megahertz (MHz), gigahertz (GHz), terahertz (THz).
Some of the unit's most common uses are in the description ofperiodic waveforms andmusical tones, particularly those used inradio- and audio-related applications. It is also used to describe theclock speeds at 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, whereE is the photon's energy,ν is its frequency, andh is thePlanck constant.
The hertz is defined as one per second for periodic events. TheInternational Committee for Weights and Measures defined the second as "the duration of9192631770 periods 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 exactly9192631770 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.[5] As an SI unit, Hz can beprefixed; commonly used multiples are kHz (kilohertz,103 Hz), MHz (megahertz,106 Hz), GHz (gigahertz,109 Hz) and THz (terahertz,1012 Hz). One hertz (i.e. one per second) simply means "one periodic event occurs per second" (where the event being counted may be a complete cycle);100 Hz means "one hundred periodic events occur 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 tobeat at1.2 Hz.
The occurrencerate of aperiodic orstochastic events is expressed inreciprocal second orinverse second (1/s or s−1) in general or, in the specific case ofradioactivity, inbecquerels.[b] Whereas1 Hz (one per second) specifically refers to one cycle (or periodic event) per second,1 Bq (also one per second) specifically refers to 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.[7] Thus a disc rotating at 60 revolutions per minute (rpm) is said to have an angular velocity of 2π rad/s and afrequency of rotation of1 Hz. The correspondence between a frequencyf with the unit hertz and an angular velocityω with the unitradians per second is
and
The hertz is named afterHeinrich Hertz. As with everySI unit named for a person, its symbol starts with anupper case letter (Hz), but when written in full, it follows the rules for capitalisation of acommon noun; i.e.,hertz becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.
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]
Asine wave with varying frequencyA heartbeat is an example of a non-sinusoidal periodic phenomenon that may be analyzed in terms of frequency. Two cycles are illustrated.
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). with the latter known asmicrowaves.Light is electromagnetic radiation that is even higher in frequency, and has frequencies in the range of tens of terahertz (THz,infrared) to a few petahertz (PHz,ultraviolet), with thevisible spectrum being 400–790 THz. 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 ofX-rays andgamma 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 theirwavelengths orphoton energies: for a more detailed treatment of this and the above frequency ranges, seeElectromagnetic spectrum.
Gravitational waves are also described in Hertz. Current observations are conducted in the 30–7000 Hz range by laserinterferometers likeLIGO, and the nanohertz (1–1000 nHz) range bypulsar timing arrays. Future space-based detectors are planned to fill in the gap, withLISA operating from 0.1–10 mHz (with some sensitivity from 10 μHz to 100 mHz), andDECIGO in the 0.1–10 Hz range.
Further information on why frequency is a flawed speed indicator for computers:Megahertz myth
In computers, mostcentral processing units (CPU) are labeled in terms of theirclock rate expressed 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.[13] For personal computers, CPU clock speeds have ranged from approximately1 MHz in the late 1970s (Atari,Commodore,Apple computers) to up to6 GHz inIBM Power microprocessors.
Higher frequencies than theInternational 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 thePlanck constant.
TheCJK Compatibility block inUnicode 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").[14]
^Although hertz is often said to imply 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]
^"(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]
^Cartwright, Rufus (March 1967). Beason, Robert G. (ed.)."Will Success Spoil Heinrich Hertz?"(PDF).Electronics Illustrated. Fawcett Publications, Inc. pp. 98–99.
^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.
^Ernst Terhardt (20 February 2000)."Dominant spectral region". Mmk.e-technik.tu-muenchen.de. Archived fromthe original on 26 April 2012. Retrieved28 April 2012.
