Hertz

The hertz (symbol: Hz) is a measure of frequency, informally defined as the number of events occurring per second. It is the basic unit of frequency in the International System of Units (SI), and is used worldwide in both general-purpose and scientific contexts. "Hertz" can be used to measure any periodic event; the most common use for "hertz" is to describe frequency of rotation, in which case a speed of 1 Hz is equal to one cycle per second.

More formally, the unit "hertz" is defined by the International System of Units (SI) such that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, $u$ ("hfs Cs") = 9 192 631 770 Hz. [ [http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html BIPM definition] ] . Equivalently, 1 Hz = frac|9,192,631,770 $u$ ("hfs Cs"). This definition is derived from the SI definition of the second. The base unit for "hertz" is s^-1 (also called inverse seconds, reciprocal seconds). As "hertz" is commonly used to measure rotational speed, the base unit is sometimes represented as cycle/s.

In English, "hertz" is used as both singular and plural. As any 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 cycle per second" (typically that which is being counted is a complete "cycle"); 100 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 1Hz, or a human heart might be said to beat at 1.2 Hz. The "frequency" (activity) of aperiodic or stochastic events, especially radioactive decay, is expressed in becquerels.

To avoid confusion, periodically varying angles are typically "not" expressed in hertz, but rather in an appropriate angular unit such as radians per second. A disc rotating at 60 revolutions per minute (RPM) can thus be said to be rotating at ≈6.283 rad/s "or" 1 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::$omega\; =\; 2pi\; f$ and $f\; =\; omega/(2pi)\; ,$.

History

The hertz is named after the German physicist Heinrich Hertz, who made important scientific contributions to electromagnetism. The name was established by the International Electrotechnical Commission (IEC) in 1930. [ [http://www.iec.ch/about/history/ IEC History] ] 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."

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 "Polytonic|γιγας" and hence the preferred pronunciation is IPA|/ˈgɪga/. Some 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. 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 Hz to 20,000 Hz; the average human can hear sounds between 20 Hz and 16,000 Hz. [ [http://www.mmk.e-technik.tu-muenchen.de/persons/ter/top/dominant.html Dominant spectral region] ] The range of ultrasound, infrasound and other physical vibrations such as molecular vibrations extends into the megahertz range and well beyond.

Electromagnetic radiation

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, 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 rays, 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 wavelengths or photon energies: for a more detailed treatment of this and the above frequency ranges, see electromagnetic spectrum.)

Computing

In computing, most central processing units (CPU) are labeled in terms of their clock speed expressed in megahertz or gigahertz (10⁹ hertz). This number refers to the frequency of the CPU's master clock signal ("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. [ [http://www.wired.com/news/business/0,1367,62851,00.html Good Riddance, Gigahertz] ] 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 increasing the frequency of the CPU (overclocking) in the BIOS or other software. (Likewise, speed can also be decreased, or underclocked.)

Various computer buses, such as 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

Frequencies not expressed in hertz

Even higher frequencies are believed to occur naturally, in the frequencies of the quantum-mechanical wave functions 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

See also

*Frequency changer
*Orders of magnitude (frequency)
*Signal bandwidth
*Radian per second
*Electronic tuner

External links

* [http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html BIPM Cesium ion f_Cs 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"]