The

**hertz** (symbol:

**Hz**) is a unit
of

frequency. It is defined as the number
of complete

cycles 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 uses for hertz are to describe radio and audio frequencies,
more or less sinusoidal contexts in which case a frequency of
1 Hz is equal to one

cycle per
second.

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 9,192,531,770 hertz, \nu
(

*hfs* Cs) = 9,192,631,770 Hz. Equivalently, 1 Hz
= \nu (

*hfs* Cs). This definition is derived from the

SI definition of the

second. Hertz are inverse, s

^{−1}. In
practice, the hertz simply replaced the older

cycle per second.

In English, hertz is used as both singular and plural. 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 cycle per second and the hertz are, however, not
regularly used in nonsinusoidal contexts. 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 per
second are:\omega = 2\pi f and f = \omega/(2\pi) \,.

## 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. 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 , with a hard sound, or ,
with a soft . The prefix "giga-" is derived directly from the

Greek " ."

## 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. 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 (10

^{9} 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 accepted 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. 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 bus, such as the

front-side bus connecting the CPU and

northbridge, also operate at
different frequencies in the megahertz range (for modern
products).

## 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

## References

- BIPM definition
- IEC
History
- Dominant spectral region
- Good Riddance, Gigahertz

## External links