: f = 1/T and T = 1/f.]]
The hertz (symbol: Hz) is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications.
Definition
The hertz is equivalent to
cycles per second. 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
caesium 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9 192 631 770 hertz" thereby effectively defining the hertz and the second simultaneously.
In English, hertz is used as a plural. 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 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" (activity) of aperiodic or stochastic events, such as radioactive decay, is expressed in becquerels.
Even though
angular velocity,
angular frequency and hertz all have the dimensions of
1/s, angular velocity and angular frequency are
not expressed in hertz, 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 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 are:
:
and
.
History
The hertz is named after the German physicist
Heinrich Hertz, who made important scientific contributions to the study of
electromagnetism. The name was established by the
International Electrotechnical Commission (IEC) in 1930. 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 clock rates and radio frequency (RF) applications, can be pronounced either , with a hard sound, or , with a soft (see giga-#Pronunciation). The prefix "giga-" is derived directly from the Greek "."
Applications
as an example of a non-
sinusoidal periodic phenomenon that can be described in terms of hertz. Two complete cycles are illustrated.]]
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 20 Hz to 20,000 Hz; the average adult human can hear sounds between 20 Hz and 16,000 Hz. The range of
ultrasound, high-intensity
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 rate expressed in megahertz or gigahertz (10
9 hertz). This number refers to the frequency of the CPU's master
clock signal ("
Clock rate"). This signal is simply an electrical voltage which changes from low to high and back again at regular intervals. This signal is also referred to as a
square wave. 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. For home-based personal computers, the CPU has ranged from approximately 1 megahertz in the late 1970s (Atari, Commodore, Apple computers) to up to 6 GHz in the present (IBM POWER processors).
Various computer buses, such as the front-side bus connecting the CPU and northbridge, also operate at different frequencies in the megahertz range (for modern products).
CRT television and monitor refresh rates are measured in hertz.
SI multiples
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 quantum energy, which is proportional to the frequency by the factor of
Planck's constant.
See also
Alternating current
Electronic tuner
Frequency changer
Normalized frequency
Orders of magnitude (frequency)
Radian per second
Signal bandwidth
References
External links
BIPM Cesium ion fCs definition
National Research Council of Canada: Generation of the Hz
National Research Council of Canada: Cesium fountain clock
National Physical Laboratory: Trapped ion optical frequency standards
National Research Council of Canada: Optical frequency standard based on a single trapped ion
National Research Council of Canada: Optical frequency comb
One Hertz in Radians per Second (Google). Note, as of 06 May 2009 there is an error of 2.
Category:SI derived units
Category:Units of frequency
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