Frequency
Encyclopedia
Frequency is the number of occurrences of a repeating event per unit time
. It is also referred to as temporal frequency.
The period is the duration of one cycle
in a repeating event, so the period is the reciprocal of the frequency. For example, if a newborn baby's heart beats at a frequency of 120 times a minute, its period (the interval between beats) is half a second.
processes, such as rotation
, oscillation
s, or wave
s, frequency is defined as a number of cycles per unit time. In physics
and engineering
disciplines, such as optics
, acoustics
, and radio
, frequency is usually denoted by a Latin letter f or by a Greek letter ν (nu)
.
In SI
units, the unit of frequency is the hertz
(Hz), named after the German physicist Heinrich Hertz: 1 Hz means that an event repeats once per second
. A previous name for this unit was cycles per second.
A traditional unit of measure used with rotating mechanical devices is revolutions per minute
, abbreviated RPM. 60 RPM equals one hertz.
The period, usually denoted by T, is the length of time taken by one cycle, and is the reciprocal of the frequency f:
The SI
unit for period is the second.
If the number of counts is not very large, it is more accurate to measure the time interval for a predetermined number of occurrences, rather than the number of occurrences within a specified time. The latter method introduces a random error
into the count of between zero and one count, so on average
half a count.
. This is an intense repetitively flashing light (strobe light
) whose frequency can be adjusted with a calibrated timing circuit. The strobe light is pointed at the rotating object and the frequency adjusted up and down. When the frequency of the strobe equals the frequency of the rotating or vibrating object, the object completes one cycle of oscillation and returns to its original position between the flashes of light, so when illuminated by the strobe the object appears stationary. Then the frequency can be read from the calibrated readout on the stroboscope. A downside of this method is that an object rotating at an integer multiple of the strobing frequency will also appear stationary.
. This is an electronic instrument
which measures the frequency of an applied repetitive electronic signal and displays the result in hertz on a digital display. It uses digital logic to count the number of cycles during a time interval established by a precision quartz
time base. Cyclic processes that are not electrical in nature, such as the rotation rate of a shaft, mechanical vibrations, or sound waves, can be converted to a repetitive electronic signal by transducer
s and the signal applied to a frequency counter. Frequency counters can currently cover the range up to about 100 GHz. This represents the limit of direct counting methods; frequencies above this must be measured by indirect methods.
). A reference signal of a known frequency near the unknown frequency is mixed with the unknown frequency in a nonlinear mixing device such as a diode
. This creates a heterodyne
or "beat" signal at the difference between the two frequencies. If the two signals are close together in frequency the heterodyne is low enough to be measured by a frequency counter. Of course, this process just measures the unknown frequency by its offset from the reference frequency, which must be determined by some other method. To reach higher frequencies, several stages of heterodyning can be used. Current research is extending this method to infrared and light frequencies (optical heterodyne detection
).
; simply, frequency is inversely proportional to wavelength λ (lambda
). The frequency f is equal to the phase velocity
v of the wave
divided
by the wavelength λ of the wave:
In the special case
of electromagnetic waves moving through a vacuum
, then v = c, where c is the speed of light
in a vacuum, and this expression becomes:
When waves
from a monochrome
source travel from one medium
to another, their frequency remains exactly the same — only their wavelength
and speed change.
Visible light is an electromagnetic wave, consisting of oscillating electric
and magnetic field
s traveling through space. The frequency of the wave determines its color: is red light, is violet light, and between these (in the range 4-) are all the other colors of the rainbow
. An electromagnetic wave can have a frequency less than , but it will be invisible to the human eye; such waves are called infrared
(IR) radiation. At even lower frequency, the wave is called a microwave
, and at still lower frequencies it is called a radio wave
. Likewise, an electromagnetic wave can have a frequency higher than , but it will be invisible to the human eye; such waves are called ultraviolet
(UV) radiation. Even higher-frequency waves are called X-ray
s, and higher still are gamma ray
s.
All of these waves, from the lowest-frequency radio waves to the highest-frequency gamma rays, are fundamentally the same, and they are all called electromagnetic radiation
. They all travel through a vacuum at the speed of light
.
Another property of an electromagnetic wave is its wavelength
. The wavelength is inversely proportional to the frequency, so an electromagnetic wave with a higher frequency has a shorter wavelength, and vice-versa.
is made up of changes in air pressure in the form of waves. Frequency is the property of sound
that most determines pitch
. The frequencies an ear can hear are limited to a specific range of frequencies.
Mechanical vibrations perceived as sound travel through all forms of matter
: gas
es, liquid
s, solid
s, and plasma
s. The matter that supports the sound is called the medium
. Sound cannot travel through a vacuum
.
The audible frequency range for humans is typically given as being between about 20 Hz
and 20,000 Hz (20 kHz). High frequencies often become more difficult to hear with age. Other species
have different hearing ranges. For example, some dog breeds can perceive vibrations up to 60,000 Hz.
, Africa
, Australia
, Southern South America
, most of Asia
, and Russia
, the frequency of the alternating current
in household electrical outlets
is 50 Hz (close to the tone
G), whereas in North America
and Northern South America
, the frequency of the alternating current in household electrical outlets is 60 Hz (between the tones
B♭ and B; that is, a minor third
above the European frequency). The frequency of the 'hum
' in an audio recording can show where the recording was made, in countries using a European, or an American, grid frequency.
s, tend to be described by wave period rather than frequency. Short and fast waves, like audio
and radio
, are usually described by their frequency instead of period. These commonly used conversions are listed below:
Many systems are characterized by the range of frequencies to which they respond. Musical instruments produce different ranges of notes
within the hearing range
. The electromagnetic spectrum
can be divided into many different ranges such as visible light
, infrared
or ultraviolet
radiation, radio
waves, X-ray
s and so on, and each of these ranges can in turn be divided into smaller ranges. A radio communications signal must occupy a range of frequencies carrying most of its energy, called its bandwidth. Allocation of radio frequency ranges to different uses is a major function of radio spectrum allocation.
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
. It is also referred to as temporal frequency.
The period is the duration of one cycle
Turn (geometry)
A turn is an angle equal to a 360° or 2 radians or \tau radians. A turn is also referred to as a revolution or complete rotation or full circle or cycle or rev or rot....
in a repeating event, so the period is the reciprocal of the frequency. For example, if a newborn baby's heart beats at a frequency of 120 times a minute, its period (the interval between beats) is half a second.
Definitions and units
For cyclicalTurn (geometry)
A turn is an angle equal to a 360° or 2 radians or \tau radians. A turn is also referred to as a revolution or complete rotation or full circle or cycle or rev or rot....
processes, such as rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
, oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
s, or wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
s, frequency is defined as a number of cycles per unit time. In physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
and engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
disciplines, such as optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
, acoustics
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
, and radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
, frequency is usually denoted by a Latin letter f or by a Greek letter ν (nu)
Nu (letter)
Nu , is the 13th letter of the Greek alphabet. In the system of Greek numerals it has a value of 50...
.
In SI
SI derived unit
The International System of Units specifies a set of seven base units from which all other units of measurement are formed, by products of the powers of base units. These other units are called SI derived units, for example, the SI derived unit of area is square metre , and of density is...
units, the unit of frequency is the hertz
Hertz
The hertz 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....
(Hz), named after the German physicist Heinrich Hertz: 1 Hz means that an event repeats once per second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
. A previous name for this unit was cycles per second.
A traditional unit of measure used with rotating mechanical devices is revolutions per minute
Revolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
, abbreviated RPM. 60 RPM equals one hertz.
The period, usually denoted by T, is the length of time taken by one cycle, and is the reciprocal of the frequency f:
The SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
unit for period is the second.
Measurement
By counting
Calculating the frequency of a repeating event is accomplished by counting the number of times that event occurs within a specific time period, then dividing the count by the length of the time period. For example, if 71 events occur within 15 seconds the frequency is:If the number of counts is not very large, it is more accurate to measure the time interval for a predetermined number of occurrences, rather than the number of occurrences within a specified time. The latter method introduces a random error
Random error
Random errors are errors in measurement that lead to measurable values being inconsistent when repeated measures of a constant attribute or quantity are taken...
into the count of between zero and one count, so on average
Average
In mathematics, an average, or central tendency of a data set is a measure of the "middle" value of the data set. Average is one form of central tendency. Not all central tendencies should be considered definitions of average....
half a count.
By stroboscope
An older method of measuring the frequency of rotating or vibrating objects is to use a stroboscopeStroboscope
A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects...
. This is an intense repetitively flashing light (strobe light
Strobe light
A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope...
) whose frequency can be adjusted with a calibrated timing circuit. The strobe light is pointed at the rotating object and the frequency adjusted up and down. When the frequency of the strobe equals the frequency of the rotating or vibrating object, the object completes one cycle of oscillation and returns to its original position between the flashes of light, so when illuminated by the strobe the object appears stationary. Then the frequency can be read from the calibrated readout on the stroboscope. A downside of this method is that an object rotating at an integer multiple of the strobing frequency will also appear stationary.
By frequency counter
Higher frequencies are usually measured with a frequency counterFrequency counter
A frequency counter is an electronic instrument, or component of one, that is used for measuring frequency. Frequency is defined as the number of events of a particular sort occurring in a set period of time. Frequency counters usually measure the number of oscillations or pulses per second in a...
. This is an electronic instrument
Electronic instrumentation
Electronic instrumentation refers to measuring instruments used to measure the properties of electrical devices.Electronics Instrumentation is a subject which deals with various types of electrical instruments with their functionality and operation....
which measures the frequency of an applied repetitive electronic signal and displays the result in hertz on a digital display. It uses digital logic to count the number of cycles during a time interval established by a precision quartz
Quartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...
time base. Cyclic processes that are not electrical in nature, such as the rotation rate of a shaft, mechanical vibrations, or sound waves, can be converted to a repetitive electronic signal by transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
s and the signal applied to a frequency counter. Frequency counters can currently cover the range up to about 100 GHz. This represents the limit of direct counting methods; frequencies above this must be measured by indirect methods.
Heterodyne methods
Above the range of frequency counters, frequencies of electromagnetic signals are often measured indirectly by means of heterodyning (frequency conversionFrequency changer
A frequency changer or frequency converter is an electronic device that converts alternating current of one frequency to alternating current of another frequency. The device may also change the voltage, but if it does, that is incidental to its principal purpose.Traditionally, these devices were...
). A reference signal of a known frequency near the unknown frequency is mixed with the unknown frequency in a nonlinear mixing device such as a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
. This creates a heterodyne
Heterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...
or "beat" signal at the difference between the two frequencies. If the two signals are close together in frequency the heterodyne is low enough to be measured by a frequency counter. Of course, this process just measures the unknown frequency by its offset from the reference frequency, which must be determined by some other method. To reach higher frequencies, several stages of heterodyning can be used. Current research is extending this method to infrared and light frequencies (optical heterodyne detection
Optical heterodyne detection
Optical heterodyne detection is an important special case of heterodyne detection. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" that is set at a close-by frequency...
).
Frequency of waves
For periodic waves, frequency has an inverse relationship to the concept of wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
; simply, frequency is inversely proportional to wavelength λ (lambda
Lambda
Lambda is the 11th letter of the Greek alphabet. In the system of Greek numerals lambda has a value of 30. Lambda is related to the Phoenician letter Lamed . Letters in other alphabets that stemmed from lambda include the Roman L and the Cyrillic letter El...
). The frequency f is equal to the phase velocity
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
v of the wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
divided
Division (mathematics)
right|thumb|200px|20 \div 4=5In mathematics, especially in elementary arithmetic, division is an arithmetic operation.Specifically, if c times b equals a, written:c \times b = a\,...
by the wavelength λ of the wave:
In the special case
Special case
In logic, especially as applied in mathematics, concept A is a special case or specialization of concept B precisely if every instance of A is also an instance of B, or equivalently, B is a generalization of A. For example, all circles are ellipses ; therefore the circle is a special case of the...
of electromagnetic waves moving through a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, then v = c, where c is the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in a vacuum, and this expression becomes:
When waves
WAVES
The WAVES were a World War II-era division of the U.S. Navy that consisted entirely of women. The name of this group is an acronym for "Women Accepted for Volunteer Emergency Service" ; the word "emergency" implied that the acceptance of women was due to the unusual circumstances of the war and...
from a monochrome
Monochrome
Monochrome describes paintings, drawings, design, or photographs in one color or shades of one color. A monochromatic object or image has colors in shades of limited colors or hues. Images using only shades of grey are called grayscale or black-and-white...
source travel from one medium
Medium (optics)
An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it...
to another, their frequency remains exactly the same — only their wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
and speed change.
Physics of light
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
and magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s traveling through space. The frequency of the wave determines its color: is red light, is violet light, and between these (in the range 4-) are all the other colors of the rainbow
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
. An electromagnetic wave can have a frequency less than , but it will be invisible to the human eye; such waves are called infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
(IR) radiation. At even lower frequency, the wave is called a microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
, and at still lower frequencies it is called a radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
. Likewise, an electromagnetic wave can have a frequency higher than , but it will be invisible to the human eye; such waves are called ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
(UV) radiation. Even higher-frequency waves are called X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, and higher still are gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s.
All of these waves, from the lowest-frequency radio waves to the highest-frequency gamma rays, are fundamentally the same, and they are all called electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
. They all travel through a vacuum at the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
.
Another property of an electromagnetic wave is its wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
. The wavelength is inversely proportional to the frequency, so an electromagnetic wave with a higher frequency has a shorter wavelength, and vice-versa.
Physics of sound
SoundSound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
is made up of changes in air pressure in the form of waves. Frequency is the property of sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
that most determines pitch
Pitch (music)
Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...
. The frequencies an ear can hear are limited to a specific range of frequencies.
Mechanical vibrations perceived as sound travel through all forms of matter
State of matter
States of matter are the distinct forms that different phases of matter take on. Solid, liquid and gas are the most common states of matter on Earth. However, much of the baryonic matter of the universe is in the form of hot plasma, both as rarefied interstellar medium and as dense...
: gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es, liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s, solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
s, and plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
s. The matter that supports the sound is called the medium
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
. Sound cannot travel through a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
.
The audible frequency range for humans is typically given as being between about 20 Hz
Hertz
The hertz 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....
and 20,000 Hz (20 kHz). High frequencies often become more difficult to hear with age. Other species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
have different hearing ranges. For example, some dog breeds can perceive vibrations up to 60,000 Hz.
Line current
In EuropeEurope
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, Southern South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, most of Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
, and Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, the frequency of the alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
in household electrical outlets
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...
is 50 Hz (close to the tone
Note
In music, the term note has two primary meanings:#A sign used in musical notation to represent the relative duration and pitch of a sound;#A pitched sound itself....
G), whereas in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Northern South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, the frequency of the alternating current in household electrical outlets is 60 Hz (between the tones
Note
In music, the term note has two primary meanings:#A sign used in musical notation to represent the relative duration and pitch of a sound;#A pitched sound itself....
B♭ and B; that is, a minor third
Minor third
In classical music from Western culture, a third is a musical interval encompassing three staff positions , and the minor third is one of two commonly occurring thirds. The minor quality specification identifies it as being the smallest of the two: the minor third spans three semitones, the major...
above the European frequency). The frequency of the 'hum
Mains hum
Mains hum, electric hum, or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity, which is usually 50 Hz or 60 Hz, depending on the local power line frequency...
' in an audio recording can show where the recording was made, in countries using a European, or an American, grid frequency.
Period versus frequency
As a matter of convenience, longer and slower waves, such as ocean surface waveOcean surface wave
In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Waves in the...
s, tend to be described by wave period rather than frequency. Short and fast waves, like audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
and radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
, are usually described by their frequency instead of period. These commonly used conversions are listed below:
| Frequency | 1 mHz (10−3) | 1 Hz (100) | 1 kHz (103) | 1 MHz (106) | 1 GHz (109) | 1 THz (1012) |
|---|---|---|---|---|---|---|
| Period (time) | 1 ks (103) | 1 s (100) | 1 ms (10−3) | 1 µs (10−6) | 1 ns (10−9) | 1 ps (10−12) |
Other types of frequency
- Angular frequencyAngular frequencyIn physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...
ω is defined as the rate of change of angular displacementAngular displacementAngular displacement of a body is the angle in radians through which a point or line has been rotated in a specified sense about a specified axis....
(during rotation), or in the phasePhase (waves)Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
of a sinusoidal waveform (e.g. in oscillations and waves):
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- In other words,
- Angular frequency is measured in radianRadianRadian is the ratio between the length of an arc and its radius. The radian is the standard unit of angular measure, used in many areas of mathematics. The unit was formerly a SI supplementary unit, but this category was abolished in 1995 and the radian is now considered a SI derived unit...
s per second (rad/s).- Spatial frequencySpatial frequencyIn mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space. The spatial frequency is a measure of how often sinusoidal components of the structure repeat per unit of distance. The SI unit of spatial frequency is...
is analogous to temporal frequency, but the time axis is replaced by one or more spatial displacement axes. - WavenumberWavenumberIn the physical sciences, the wavenumber is a property of a wave, its spatial frequency, that is proportional to the reciprocal of the wavelength. It is also the magnitude of the wave vector...
is the spatial analogue of angular frequency. In case of more than one spatial dimension, wavenumber is a vector quantity.
- Spatial frequency
Frequency ranges
The frequency range of a system is the range over which it is considered to provide a useful level of signal with acceptable distortion characteristics. A listing of the upper and lower limits of frequency limits for a system is not useful without a criterion for what the range represents.Many systems are characterized by the range of frequencies to which they respond. Musical instruments produce different ranges of notes
Ambitus (music)
Ambitus is a Latin term literally meaning "the going round", and in Medieval Latin means the "course" of a melodic line, most usually referring to the range of scale degrees attributed to a given mode, particularly in Gregorian chant. It may also refer to the range of a voice, instrument, or piece...
within the hearing range
Hearing range
For more detail on human hearing see Audiogram, Equal loudness contours and Hearing impairment.Hearing range usually describes the range of frequencies that can be heard by an animal or human, though it can also refer to the range of levels...
. The electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
can be divided into many different ranges such as visible light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
, infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
or ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation, radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
waves, X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s and so on, and each of these ranges can in turn be divided into smaller ranges. A radio communications signal must occupy a range of frequencies carrying most of its energy, called its bandwidth. Allocation of radio frequency ranges to different uses is a major function of radio spectrum allocation.
See also
- Absolute threshold of hearingAbsolute threshold of hearingThe absolute threshold of hearing is the minimum sound level of a pure tone that an average ear with normal hearing can hear with no other sound present. The absolute threshold relates to the sound that can just be heard by the organism...
- Audible range
- Bandwidth (signal processing)
- Bandwidth extensionBandwidth extensionBandwidth extension of signal is defined as the deliberate process of expanding the frequency range of a signal in which it contains an appreciable and useful content, and/or the frequency range in which its effects are such...
- Bass (sound)
- Coherence bandwidthCoherence bandwidthCoherence bandwidth is a statistical measurement of the range of frequencies over which the channel can be considered "flat", or in other words the approximate maximum bandwidth or frequency interval over which two frequencies of a signal are likely to experience comparable or correlated amplitude...
- Critical band
- Cumulative frequency analysisCumulative frequency analysisCumulative frequency analysis is the applcation of estimation theory to exceedance probability . The complement, the non-exceedance probability concerns the frequency of occurrence of values of a phenomenon staying below a reference value. The phenomenon may be time or space dependent...
- Cutoff frequencyCutoff frequencyIn physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
- DownsamplingDownsamplingIn signal processing, downsampling is the process of reducing the sampling rate of a signal. This is usually done to reduce the data rate or the size of the data....
- Electronic filterElectronic filterElectronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
- FalsettoFalsettoFalsetto is the vocal register occupying the frequency range just above the modal voice register and overlapping with it by approximately one octave. It is produced by the vibration of the ligamentous edges of the vocal folds, in whole or in part...
- Flashes Per MinuteFlashes Per MinuteFlashes Per Minute, or FPM, is simply the number of times a flashing light blinks per minute. Police and emergency vehicles use warning lights with flashing rates that typically fall in a range from 60 to 240 FPM....
- Frequency converter
- Frequency domainFrequency domainIn electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
- Frequency distributionFrequency distributionIn statistics, a frequency distribution is an arrangement of the values that one or more variables take in a sample. Each entry in the table contains the frequency or count of the occurrences of values within a particular group or interval, and in this way, the table summarizes the distribution of...
- Frequency extenderFrequency extenderIn broadcast engineering, a frequency extender is an electronic device that allows high-fidelity analogue audio to be sent over regular POTS telephone lines, without the loss of higher audio frequencies . It is an extended concept of a telephone hybrid.The concept uses frequency shifting, in order...
- Frequency gridFrequency gridThe most common frequency grid is that used for channel spacing in Dense Wavelength Division Multiplexing for Fiber-optic communication at wavelengths around 1550 nm and defined by ITU-T G.694.1 . The grid is defined relative to 193.1THz and extends from 191.7 THz to 196.1 THz with...
- Free spectral rangeFree spectral rangeFree spectral range is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima of an interferometer or diffractive optical element.- Diffraction gratings :...
- Frequency deviationFrequency deviationFrequency deviation is used in FM radio to describe the maximum instantaneous difference between an FM modulated frequency and the nominal carrier frequency...
- Frequency spectrumFrequency spectrumThe frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
- Hearing rangeHearing rangeFor more detail on human hearing see Audiogram, Equal loudness contours and Hearing impairment.Hearing range usually describes the range of frequencies that can be heard by an animal or human, though it can also refer to the range of levels...
- High frequency limit
- HyperacusisHyperacusisHyperacusis is a health condition characterized by an over-sensitivity to certain frequency ranges of sound...
- Interaction frequencyInteraction frequencyInteraction frequency is a sociological concept referring to the total number of social interactions per unit time. Interactions, or what Georg Simmel in his pioneering work called Wechselwirkungen, are the basis for society itself, according to Herbert Blumer....
- PassbandPassbandA passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
- Musical acousticsMusical acousticsMusical acoustics or music acoustics is the branch of acoustics concerned with researching and describing the physics of music – how sounds employed as music work...
- MVDDS disputeMVDDS disputeThe MVDDS dispute refers to several legal disputes and political controversies that surrounded Federal Communications Commission approval of MVDDS terrestrial wireless broadband technology. The controversy arose over the lobbying efforts of Northpoint Technology, a developer of MVDDS, to receive...
- Natural frequency
- Negative frequencyNegative frequencyThe concept of negative and positive frequency can be as simple as a wheel rotating one way or the other way. A signed value of frequency indicates both the rate and direction of rotation...
- Normalized frequencyNormalized frequency (digital signal processing)In digital signal processing, the normalized frequency of a periodic signal is its frequency expressed in units of cycles per sample, rather than in the usual SI units of hertz...
- Periodicity (disambiguation)
- Piano key frequenciesPiano key frequenciesThis is a virtual keyboard showing the absolute frequencies in hertz of the notes on a modern piano in twelve-tone equal temperament, with the 49th key, the fifth A , tuned to 440 Hz...
- Pink noisePink noisePink noise or 1/ƒ noise is a signal or process with a frequency spectrum such that the power spectral density is inversely proportional to the frequency. In pink noise, each octave carries an equal amount of noise power...
- Pitch (music)Pitch (music)Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...
- PreselectorPreselectorA preselector is a radio technological term for an electronic device that is inserted between the antenna and the receiver, limiting the range of frequencies that can be applied to it. Tuning to the desired frequency keeps the preselector's narrow bandwidth centered at the operating frequency,...
- Power bandwidthPower bandwidthThe power bandwidth of an amplifier is sometimes taken as the frequency range for which the rated power output of an amplifier can be maintained to at least half of the full rated power...
- Range (music)Range (music)In music, the range of a musical instrument is the distance from the lowest to the highest pitch it can play. For a singing voice, the equivalent is vocal range...
- Radar signal characteristicsRadar signal characteristicsA radar system uses a radio frequency electromagnetic signal reflected from a target to determine information about that target. In any radar system, the signal transmitted and received will exhibit many of the characteristics described below....
- Radio windowRadio windowThe radio window is the range of frequencies of electromagnetic radiation that the earth's atmosphere lets through. The wavelengths in the radio window run from about one centimetre to about eleven-metre waves.-See also:*Astronomical window...
- Rate (mathematics)
- Scientific pitch notationScientific pitch notationScientific pitch notation is one of several methods that name the notes of the standard Western chromatic scale by combining a letter-name, accidentals, and a number identifying the pitch's octave...
- Signaling (telecommunications)
- Spectral widthSpectral widthIn telecommunications, spectral width is the wavelength interval over which the magnitude of all spectral components is equal to or greater than a specified fraction of the magnitude of the component having the maximum value....
- Spread spectrumSpread spectrumSpread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
- Spectral componentSpectral componentIn telecommunications, spectral component is any of the waves that range outside the interval of frequencies assigned to a signal. Any waveform can be disassembled into its spectral components by Fourier analysis or Fourier transformation...
- Spectrum allocation
- Symbol rateSymbol rateIn digital communications, symbol rate is the number of symbol changes made to the transmission medium per second using a digitally modulated signal or a line code. The Symbol rate is measured in baud or symbols/second. In the case of a line code, the symbol rate is the pulse rate in pulses/second...
- Transition bandTransition bandThe transition band is a range of frequencies, that allows a transition between a passband and a stopband of a signal processing filter. The transition band is defined by a passband and a stopband cutoff frequency or corner frequency....
- TransverterTransverterA transverter is a radio frequency device that consists of an upconverter and a downconverter in one unit. Transverters are used in conjunction with transceivers to change the range of frequencies over which the transceiver can communicate....
- UltrasoundUltrasoundUltrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
- UpsamplingUpsamplingUpsampling is the process of increasing the sampling rate of a signal. For instance, upsampling raster images such as photographs means increasing the resolution of the image....
- WavelengthWavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
- Whistle registerWhistle registerThe whistle register is the highest register of the human voice, lying above the modal register and falsetto register...
- Wideband audioWideband AudioWideband audio is an audio technology used in telephony. It extends the frequency range of audio signals transmitted over telephone lines, resulting in higher quality speech. The range of the human voice extends from 80 Hz to 14 kHz but traditional, voiceband or narrowband telephone calls limit...
External links
- National Research Council of Canada: Femtosecond comb; The measurement of optical frequencies
- Conversion: frequency to wavelength and back
- Conversion: period, cycle duration, periodic time to frequency
- Keyboard frequencies = naming of notes - The English and American system versus the German system
- Teaching resource for 14-16yrs on sound including frequency
- A simple tutorial on how to build a frequency meter
- Frequency - diracdelta.co.uk – JavaScriptJavaScriptJavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....
calculation.