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Frequency

For other uses, see Frequency (disambiguation).

"Frequencies" redirects here. For the film, seeFrequencies (film). For the album, see Frequencies (album).

For a broader coverage related to this topic, seeTemporal rate.

Frequency is the number of occurrences of a repeating event per unit of time.^[1] It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequencyand angular frequency. The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency.^[2] For example, if a newborn baby's heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second (that is, 60 seconds divided by 120 beats). Frequency is an important parameter used in science and engineering to specify the rate of oscillatoryand vibratory phenomena, such as mechanical vibrations, audio (sound) signals,radio waves, and light.

frequency
Common symbols	f, ν
SI unit	hertz
In SI base units	s−1

DefinitionsEdit

These three dots are flashing, orcycling, periodically—from lowest frequency (0.5 hertz) to highest frequency (2.0 hertz), top to bottom. For each flashing dot: "f" is the frequency in hertz, (Hz)—or the number of events per second (cycles per second)—that the dot flashes; while "T" is the period, or time, in seconds (s) of each cycle, (the number of seconds per cycle). NoteT and f are reciprocal values to each other.

As time elapses—here moving left to right on the horizontal axis—the fivesinusoidal waves vary, or cycle, regularly at different rates. The redwave (top) has the lowest frequency (cycles at the slowest rate) while the purple wave (bottom) has the highest frequency (cycles at the fastest rate).

For cyclical processes, such as rotation,oscillations, or waves, frequency is defined as a number of cycles per unit time. In physicsand engineering disciplines, such as optics,acoustics, and radio, frequency is usually denoted by a Latin letter f or by the Greek letter ${\displaystyle \nu }$  or ν (nu) (see e.g. Planck's formula).

The relation between the frequency and the period ${\displaystyle T}$  of a repeating event or oscillation is given by

${\displaystyle f={\frac {1}{T}}.}$ 

UnitsEdit

The SI unit of frequency is the hertz (Hz), named after the German physicist Heinrich Hertz; one hertz means that an event repeats once per second. A previous name for this unit was cycles per second (cps). The SI unit for period is the second.

A traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or rpm. 60 rpm equals one hertz.^[3]

Period versus frequencyEdit

As a matter of convenience, longer and slower waves, such as ocean surface waves, tend to be described by wave period rather than frequency. Short and fast waves, like audioand radio, are usually described by their frequency instead of period. These commonly used conversions are listed below:

Frequency	1 mHz (10−3 Hz)	1 Hz (100 Hz)	1 kHz (103 Hz)	1 MHz (106 Hz)	1 GHz (109 Hz)	1 THz (1012 Hz)
Period	1 ks (103 s)	1 s (100 s)	1 ms (10−3 s)	1 µs (10−6 s)	1 ns (10−9 s)	1 ps (10−12 s)

Related types of frequencyEdit

For other uses, see Frequency (disambiguation).

Diagram of the relationship between the different types of frequency and other wave properties.

• Angular frequency, usually denoted by the Greek letter ω (omega), is defined as the rate of change of angular displacement, θ, (during rotation), or the rate of change of the phase of a sinusoidal waveform (notably in oscillations and waves), or as the rate of change of the argument to thesine function:

${\displaystyle y(t)=\sin \left(\theta (t)\right)=\sin(\omega t)=\sin(2\mathrm {\pi } ft)}$

${\displaystyle {\frac {\mathrm {d} \theta }{\mathrm {d} t}}=\omega =2\mathrm {\pi } f}$

Angular frequency is commonly measured in radians per second (rad/s) but, fordiscrete-time signals, can also be expressed as radians per sample time, which is a dimensionless quantity. Angular frequency (in radians) is larger than regular frequency (in Hz) by a factor of 2π.

• Spatial frequency is analogous to temporal frequency, but the time axis is replaced by one or more spatial displacement axes. E.g.:

${\displaystyle y(t)=\sin \left(\theta (t,x)\right)=\sin(\omega t+kx)}$

${\displaystyle {\frac {\mathrm {d} \theta }{\mathrm {d} x}}=k}$

Wavenumber, k, is the spatial frequency analogue of angular temporal frequency and is measured in radians per meter. In the case of more than one spatial dimension, wavenumber is a vector quantity.

In wave propagationEdit

Further information: Wave propagation

For periodic waves in nondispersive media(that is, media in which the wave speed is independent of frequency), frequency has an inverse relationship to the wavelength, λ(lambda). Even in dispersive media, the frequency f of a sinusoidal wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave:

${\displaystyle f={\frac {v}{\lambda }}.}$

In the special case of electromagnetic waves moving through a vacuum, then v = c, where cis the speed of light in a vacuum, and this expression becomes:

${\displaystyle f={\frac {c}{\lambda }}.}$

When waves from a monochrome source travel from one medium to another, their frequency remains the same—only theirwavelength and speed change.

MeasurementEdit

See also: Frequency meter

Measurement of frequency can done in the following ways,

CountingEdit

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:

${\displaystyle f={\frac {71}{15\,{\text{s}}}}\approx 4.7\,{\text{Hz}}}$

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.^[4] The latter method introduces a random error into the count of between zero and one count, so on averagehalf a count. This is called gating error and causes an average error in the calculated frequency of Δf = 1/(2 T_m), or a fractional error of Δf / f = 1/(2 f T_m) where T_m is the timing interval and f is the measured frequency. This error decreases with frequency, so it is a problem at low frequencies where the number of counts N is small.

 

A resonant-reed frequency meter, an obsolete device used from about 1900 to the 1940s for measuring the frequency of alternating current. It consists of a strip of metal with reeds of graduated lengths, vibrated by an electromagnet. When the unknown frequency is applied to the electromagnet, the reed which is resonant at that frequency will vibrate with large amplitude, visible next to the scale.

StroboscopeEdit

An older method of measuring the frequency of rotating or vibrating objects is to use astroboscope. 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 integral multiple of the strobing frequency will also appear stationary.

Frequency counterEdit

Main article: Frequency counter

 

Modern frequency counter

Higher frequencies are usually measured with a frequency counter. 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 transducers 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 methodsEdit

Above the range of frequency counters, frequencies of electromagnetic signals are often measured indirectly by means ofheterodyning (frequency conversion). 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 aheterodyne 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. This process only measures the difference between the unknown frequency and the reference frequency. 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).