# Frequency

From Wikipedia, in a visual modern way
Frequency
A pendulum making 25 complete oscillations in 60 s, a frequency of 0.416 Hz
Common symbols
f, ν
SI unithertz (Hz)
Other units
In SI base unitss−1
Derivations from
other quantities
• f = 1 / T
Dimension${\displaystyle {\mathsf {T}}^{-1}}$

Frequency is the number of occurrences of a repeating event per unit of time.[1] It is also occasionally referred to as temporal frequency for clarity, and is distinct from angular frequency. Frequency is measured in hertz (Hz) which is equal to one event per second. The period is the interval of time between events, so the period is the reciprocal of the frequency.[2]

For example, if a heart beats at a frequency of 120 times a minute (2 hertz), the period, T—the interval at which the beats repeat—is half a second (60 seconds divided by 120 beats). Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light.

## Period versus frequency

As a matter of convenience, longer and slower waves, such as ocean surface waves, are more typically described by wave period rather than frequency.[8] Short and fast waves, like audio and radio, are usually described by their frequency. Some commonly used conversions are listed below:

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

## Measurement

Measurement of frequency can be done in the following ways:

### 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 period. For example, if 71 events occur within 15 seconds the frequency is:

${\displaystyle f={\frac {71}{15\,{\text{s}}}}\approx 4.73\,{\text{Hz}}.}$
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.[10] The latter method introduces a random error into the count of between zero and one count, so on average half a count. This is called gating error and causes an average error in the calculated frequency of ${\textstyle \Delta f={\frac {1}{2T_{\text{m}}}}}$, or a fractional error of ${\textstyle {\frac {\Delta f}{f}}={\frac {1}{2fT_{\text{m}}}}}$ where ${\displaystyle T_{\text{m}}}$ is the timing interval and ${\displaystyle f}$ is the measured frequency. This error decreases with frequency, so it is generally 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.

### Stroboscope

An old method of measuring the frequency of rotating or vibrating objects is to use a stroboscope. 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.

### 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, 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. As of 2018, frequency counters can 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 utilizing heterodyning (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 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. 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).

## Examples

### Light

Complete spectrum of electromagnetic radiation with the visible portion highlighted

Visible light is an electromagnetic wave, consisting of oscillating electric and magnetic fields traveling through space. The frequency of the wave determines its color: 400 THz (4×1014 Hz) is red light, 800 THz (8×1014 Hz) is violet light, and between these (in the range 400–800 THz) are all the other colors of the visible spectrum. An electromagnetic wave with a frequency less than 4×1014 Hz 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 with a frequency higher than 8×1014 Hz will also be invisible to the human eye; such waves are called ultraviolet (UV) radiation. Even higher-frequency waves are called X-rays, and higher still are gamma rays.

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 vacuum at the same speed (the speed of light), giving them wavelengths inversely proportional to their frequencies.

where c is the speed of light (c in vacuum or less in other media), f is the frequency and λ is the wavelength.

In dispersive media, such as glass, the speed depends somewhat on frequency, so the wavelength is not quite inversely proportional to frequency.

### Sound

The sound wave spectrum, with rough guide of some applications

Sound propagates as mechanical vibration waves of pressure and displacement, in air or other substances.[11] In general, frequency components of a sound determine its "color", its timbre. When speaking about the frequency (in singular) of a sound, it means the property that most determines its pitch.[12]

The frequencies an ear can hear are limited to a specific range of frequencies. The audible frequency range for humans is typically given as being between about 20 Hz and 20,000 Hz (20 kHz), though the high frequency limit usually reduces with age. Other species have different hearing ranges. For example, some dog breeds can perceive vibrations up to 60,000 Hz.[13]

In many media, such as air, the speed of sound is approximately independent of frequency, so the wavelength of the sound waves (distance between repetitions) is approximately inversely proportional to frequency.

### Line current

In Europe, 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.

### Discover more about Examples related topics

Source: "Frequency", Wikipedia, Wikimedia Foundation, (2023, January 31st), https://en.wikipedia.org/wiki/Frequency.

### Enjoying Wikiz?

#### Get our FREE extension now!

###### Notes
1. ^ The term spatial period, sometimes used in place of wavelength, analogously corresponds to the (temporal) period.[9]
###### References
1. ^ "Definition of FREQUENCY". Retrieved 3 October 2016.
2. ^ "Definition of PERIOD". Retrieved 3 October 2016.
3. ^ Serway & Faughn 1989, p. 346.
4. ^ a b Serway & Faughn 1989, p. 354.
5. ^ "Resolution 12 of the 11th CGPM (1960)". BIPM (International Bureau of Weights and Measures). Archived from the original on 8 April 2020. Retrieved 21 January 2021.
6. ^ "Special Publication 811: NIST Guide to the SI, Chapter 8". Nist. 28 January 2016. Retrieved 2022-11-08.
7. ^ Davies 1997, p. 275.
8. ^ Young 1999, p. 7.
9. ^ Boreman, Glenn D. "Spatial Frequency". SPIE. Retrieved 22 January 2021.
10. ^ Bakshi, K.A.; A.V. Bakshi; U.A. Bakshi (2008). Electronic Measurement Systems. US: Technical Publications. pp. 4–14. ISBN 978-81-8431-206-5.
11. ^ "Definition of SOUND". Retrieved 3 October 2016.
12. ^ Pilhofer, Michael (2007). Music Theory for Dummies. For Dummies. p. 97. ISBN 978-0-470-16794-6.
13. ^ Condon, Tim (2003). Elert, Glenn (ed.). "Frequency range of dog hearing". The Physics Factbook. Retrieved 2008-10-22.
14. ^ Lombardi, Michael A. (2007). "Fundamentals of Time and Frequency". In Bishop, Robert H. (ed.). Mechatronic Systems, Sensors, and Actuators: Fundamentals and Modeling. Austin: CRC Press. ISBN 9781420009002.
15. ^ Newell, David B; Tiesinga, Eite (2019). The international system of units (SI): (PDF) (Report). Gaithersburg, MD: National Institute of Standards and Technology. doi:10.6028/nist.sp.330-2019. sub§2.3.4, Table 4.