The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second.["hertz". (1992). American Heritage Dictionary of the English Language (3rd ed.), Boston: Houghton Mifflin.] The hertz is an SI derived unit whose formal expression in terms of SI base units is 1/s or s−1, meaning that one hertz is one per second or the Inverse second.
Some of the unit's most common uses are in the description of periodic waveforms and , particularly those used in radio- and audio-related applications. It is also used to describe the at which computers and other electronics are driven. The units are sometimes also used as a representation of the photon energy, via the Planck relation E = hν, where E is the photon's energy, ν is its frequency, and h is the Planck constant.
Definition
The hertz is defined as one per second for periodic events. The International Committee for Weights and Measures defined the second as "the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the
caesium-133 atom"
and then adds: "It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly , ." 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.[ NIST Guide to SI Units – 9 Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology] As an SI unit, Hz can be Metric prefix; commonly used multiples are kHz (kilohertz, ), MHz (megahertz, ), GHz (gigahertz, ) and THz (terahertz, ). One hertz (i.e. one per second) simply means "one periodic event occurs per second" (where the event being counted may be a complete cycle); means "one hundred periodic events occur per second", and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at , or a human heart might be said to heart rate at .
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 . Whereas (one per second) specifically refers to one cycle (or periodic event) per second, (also one per second) specifically refers to 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.
- and
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 1935.
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.
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)).
Applications
Sound and vibration
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 to ; the average
Adult can hear sounds between and .
The range of
ultrasound,
infrasound and other physical vibrations such as molecular and
atom vibrations extends from a few
into the
tera- range and 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), with the latter known as . Light is electromagnetic radiation that is even higher in frequency, and has frequencies in the range of tens of terahertz (THz, infrared) to a few petahertz (PHz, ultraviolet), with the visible spectrum being 400–790 THz. 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 and , 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 or Photon energy: for a more detailed treatment of this and the above frequency ranges, see Electromagnetic spectrum.
Gravitational waves
Current observations of gravitational waves are conducted in the 30–7000 Hz range by laser
like
LIGO, and the nanohertz (1–1000 nHz) range by pulsar timing arrays. Future space-based detectors are planned to fill in the gap, with LISA operating from 0.1–10 mHz (with some sensitivity from 10 μHz to 100 mHz), and DECIGO in the 0.1–10 Hz range.
Computers
In computers, most central processing units (CPU) are labeled in terms of their
clock rate expressed in megahertz () or gigahertz (). 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
Megahertz myth. Some processors use multiple clock cycles to perform a single operation, while others can perform multiple operations in a single cycle.
For personal computers, CPU clock speeds have ranged from approximately in the late 1970s (
Atari, Commodore,
Apple computers) to up to 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
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
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").
-
(ヘルツ, )
-
(Hz)
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(kHz)
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(MHz)
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(GHz)
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(THz)
See also
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Alternating current
-
Bandwidth (signal processing)
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Electronic tuner
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FLOPS
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Frequency changer
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Normalized frequency (signal processing)
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Orders of magnitude (frequency)
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Orders of magnitude (rotational speed)
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Periodic function
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Radian per second
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Rate
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Sampling rate
Notes
External links
