SI unit for frequency
hertz
|
---|
Top to bottom: Lights flashing at
frequencies
f
=
0.5 Hz
,
1.0 Hz
and
2.0 Hz
; that is, at 0.5, 1.0 and 2.0 flashes per second, respectively. The time between each flash ? the
period
T
? is given by
1
⁄
f
(the
reciprocal
of
f
); that is, 2, 1 and 0.5 seconds, respectively.
|
|
Unit system
| SI
|
---|
Unit of
| frequency
|
---|
Symbol
| Hz
|
---|
Named after
| Heinrich Hertz
|
---|
In
SI base units
| s
?1
|
---|
The
hertz
(symbol:
Hz
) is the unit of
frequency
in the
International System of Units
(SI), equivalent to one event (or
cycle
) per
second
.
[1]
[a]
The hertz is an
SI derived unit
whose expression in terms of
SI base units
is s
?1
, meaning that one hertz is the
reciprocal of one second
.
[2]
It is named after
Heinrich Rudolf Hertz
(1857?1894), the first person to provide conclusive proof of the existence of
electromagnetic waves
. Hertz are commonly expressed in
multiples
: kilohertz (kHz), megahertz (MHz), gigahertz (GHz), terahertz (THz).
Some of the unit's most common uses are in the description of
periodic waveforms
and
musical tones
, particularly those used in
radio
- and audio-related applications. It is also used to describe the
clock speeds
at which computers and other electronics are driven. The units are sometimes also used as a representation of the
energy of a photon
, via the
Planck relation
E
=
hν
, where
E
is the photon's energy,
ν
is its frequency, and
h
is the
Planck constant
.
Definition
[
edit
]
The hertz is equivalent to one
cycle per second
. The
International Committee for Weights and Measures
defined the second as "the duration of
9
192
631
770
periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the
caesium
-133 atom"
[3]
[4]
and then adds: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly
9
192
631
770
hertz
,
ν
hfs Cs
=
9
192
631
770
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 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 event per second" (where the event being counted may be a complete cycle);
100 Hz
means "one hundred events 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 occurrence
rate of aperiodic
or
stochastic
events is expressed in
reciprocal second
or
inverse second
(1/s or s
?1
) in general or, in the specific case of
radioactivity
, in
becquerels
.
[b]
Whereas
1 Hz
is one cycle (or periodic event) per second,
1 Bq
is 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 2
π
rad/s and a
frequency of rotation
of
1 Hz
. The correspondence between a frequency
f
with the unit hertz and an angular velocity
ω
with the unit
radians
per second is
- and
The hertz is named after
Heinrich Hertz
. As with every
SI
unit named for a person, its symbol starts with an
upper case
letter (Hz), but when written in full, it follows the rules for capitalisation of a
common noun
; i.e.,
hertz
becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.
History
[
edit
]
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 1935.
[8]
It was adopted by the
General Conference on Weights and Measures
(CGPM) (
Conference generale 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]
Applications
[
edit
]
Sound and vibration
[
edit
]
Sound
is a traveling
longitudinal wave
, which is an
oscillation
of
pressure
. Humans perceive the frequency of a sound as its
pitch
. Each
musical note
corresponds to a particular frequency. 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
.
[11]
The range of
ultrasound
,
infrasound
and other physical vibrations such as
molecular
and
atomic vibrations
extends from a few
femtohertz
[12]
into the
terahertz
range
[c]
and beyond.
[13]
Electromagnetic radiation
[
edit
]
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 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 (EHz). (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
.)
Computers
[
edit
]
Further information on why the frequency, including for gigahertz (GHz) etc., is a flawed speed indicator for computers:
Megahertz myth
In computers, most
central processing units
(CPU) are labeled in terms of their
clock rate
expressed in megahertz (
MHz
) or gigahertz (
GHz
). This specification refers to the frequency of the CPU's master
clock signal
. This signal is nominally a
square 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 an
easily manipulable benchmark
. Some processors use multiple clock cycles to perform a single operation, while others can perform multiple operations in a single cycle.
[14]
For personal computers, CPU clock speeds have ranged from approximately
1 MHz
in the late 1970s (
Atari
,
Commodore
,
Apple computers
) to up to
6 GHz
in
IBM Power microprocessors
.
Various
computer buses
, such as the
front-side bus
connecting the CPU and
northbridge
, also operate at various frequencies in the megahertz range.
SI multiples
[
edit
]
Higher frequencies than the
International 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 the
Planck constant
.
Unicode
[
edit
]
The
CJK Compatibility
block in
Unicode
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").
[15]
- U+3339
㌹
SQUARE HERUTU
(
ヘルツ
,
herutsu
)
- U+3390
㎐
SQUARE HZ
(Hz)
- U+3391
㎑
SQUARE KHZ
(kHz)
- U+3392
㎒
SQUARE MHZ
(MHz)
- U+3393
㎓
SQUARE GHZ
(GHz)
- U+3394
㎔
SQUARE THZ
(THz)
See also
[
edit
]
Notes
[
edit
]
- ^
Although hertz is equivalent to 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]
- ^
Atomic vibrations
are typically on the order of tens of terahertz
References
[
edit
]
- ^
"hertz". (1992).
American Heritage Dictionary of the English Language
(3rd ed.), Boston: Houghton Mifflin.
- ^
a
b
"SI Brochure: The International System of Units (SI) ? 9th edition"
(PDF)
.
BIPM
: 26
. Retrieved
7 August
2022
.
- ^
"SI Brochure: The International System of Units (SI) § 2.3.1 Base units"
(PDF)
(in British English and French) (9th ed.).
BIPM
. 2019. p. 130
. Retrieved
2 February
2021
.
- ^
"SI Brochure: The International System of Units (SI) § Appendix 1. Decisions of the CGPM and the CIPM"
(PDF)
(in British English and French) (9th ed.).
BIPM
. 2019. p. 169
. Retrieved
2 February
2021
.
- ^
NIST Guide to SI Units ? 9 Rules and Style Conventions for Spelling Unit Names
, National Institute of Standards and Technology
- ^
"BIPM ? Table 3"
.
BIPM
. Retrieved
24 October
2012
.
- ^
"SI brochure, Section 2.2.2, paragraph 6"
. Archived from
the original
on 1 October 2009.
- ^
"IEC History"
. Iec.ch. Archived from
the original
on 19 May 2013
. Retrieved
6 January
2021
.
- ^
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 from
the original
on 26 April 2012
. Retrieved
28 April
2012
.
- ^
"Black Hole Sound Waves ? Science Mission Directorate"
. science.nasa.go.
- ^
"Black Hole Sound Waves ? Science Mission Directorate"
. science.nasa.go.
- ^
Asaravala, Amit (30 March 2004).
"Good Riddance, Gigahertz"
.
Wired
. Retrieved
28 April
2012
.
- ^
Unicode Consortium
(2019).
"The Unicode Standard 12.0 ? CJK Compatibility ? Range: 3300?33FF ?"
(PDF)
.
Unicode.org
. Retrieved
24 May
2019
.
External links
[
edit
]