Kilohertz
 
Unit system:  SI derived unit 
Unit of...  Frequency 
Symbol:  ㎐ 
Named after:  Heinrich Hertz 
In SI base units:  1 Hz = 1 s^{1} 
The hertz (symbol Hz) is the unit of frequency in the International System of Units (SI). It is defined as the number of cycles per second of a periodic phenomenon.^{[1]} One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications, such as the frequency of musical tones. The unit is named for Heinrich Rudolf Hertz, who was the first to conclusively prove the existence of electromagnetic waves.
Contents
Definition
The hertz is equivalent to cycles per second.^{[2]} In defining the second, the CIPM declared that "the standard to be employed is the transition between the hyperfine levels F = 4, M = 0 and F = 3, M = 0 of the ground state 2S_{1/2} of the cesium 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9 192 631 770 hertz"^{[3]} thereby effectively defining the hertz and the second simultaneously.
In English, "hertz" is also used as the plural form.^{[4]} As an SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, 10^{3} Hz), MHz (megahertz, 10^{6} Hz), GHz (gigahertz, 10^{9} Hz) and THz (terahertz, 10^{12} 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 1 Hz, or a human heart might be said to beat at 1.2 Hz. The "frequency" or activity of aperiodic or stochastic events, such as radioactive decay, is expressed in becquerels, not hertz.
Even though angular velocity, angular frequency and the unit hertz all have the dimension 1/s (reciprocal second), angular velocity and angular frequency are not expressed in hertz,^{[5]} but rather in an appropriate angular unit such as radians per second. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2π rad/s or 1 Hz, where the former measures the 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 per second is
 $$
\omega = 2\pi f \, and $f\; =\; \backslash frac\{\backslash omega\}\{2\backslash pi\}\; \backslash ,$. This The International System of Units, section 5.2.
History
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 1930.^{[6]} 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.
Applications
Vibration
Sound is a traveling longitudinal 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 20 Hz to 20,000 Hz; the average adult human can hear sounds between 20 Hz and 16,000 Hz.^{[7]} The range of ultrasound, highintensity 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 (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 longwave 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 rate expressed in megahertz or gigahertz (10^{6} or 10^{9} hertz, respectively). This number refers to the frequency of the CPU's master clock signal ("clock rate"). This signal is a square wave, which is an electrical voltage that switches between low and high values at regular intervals. Hertz has become the primary unit of measurement accepted by the general populace to determine the performance of a CPU, but many experts have criticized this approach, which they claim is an easily manipulable benchmark.^{[8]} For homebased personal computers, CPU clock speeds have ranged from approximately 1 megahertz in the late 1970s (Atari, Commodore, Apple computers) to up to 6 GHz in IBM POWER processors.
Various computer buses, such as the frontside bus connecting the CPU and northbridge, also operate at various frequencies in the megahertz range.
SI multiples
Submultiples  Multiples  

Value  Symbol  Name  Value  Symbol  Name  
10^{−1} Hz  dHz  decihertz  10^{1} Hz  daHz  decahertz  
10^{−2} Hz  cHz  centihertz  10^{2} Hz  hHz  hectohertz  
10^{−3} Hz  mHz  millihertz  10^{3} Hz  kHz  kilohertz  
10^{−6} Hz  µHz  microhertz  10^{6} Hz  MHz  megahertz  
10^{−9} Hz  nHz  nanohertz  10^{9} Hz  GHz  gigahertz  
10^{−12} Hz  pHz  picohertz  10^{12} Hz  THz  terahertz  
10^{−15} Hz  fHz  femtohertz  10^{15} Hz  PHz  petahertz  
10^{−18} Hz  aHz  attohertz  10^{18} Hz  EHz  exahertz  
10^{−21} Hz  zHz  zeptohertz  10^{21} Hz  ZHz  zettahertz  
10^{−24} Hz  yHz  yoctohertz  10^{24} Hz  YHz  yottahertz  
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 quantummechanical vibrations of highenergy, 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 quantum energy, which is proportional to the frequency by the factor of Planck's constant.
See also
Notes and references
External links
 definition
