Frequency
In case when the frequency is so high that counting is difficult or impossible with the available means, another method is used, based on a source (such as a laser, a tuning fork, or a waveform generator) of a known reference frequency f0, that must be tunable or very close to the measured frequency f. Both the observed frequency and the reference frequency are simultaneously produced, and frequency beats are observed at a much lower frequency Δf, which can be measured by counting. This is sometimes referred to as a stroboscope effect. The unknown frequency is then found from 
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Frequency of waves
Frequency has an inverse relationship to the concept of wavelength, simply, frequency is inversely proportional to wavelength λ (lambda). The frequency f is equal to the speed 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 = c0 , where c0 is the speed of light in a vacuum, and this expression becomes:
When waves from a monochromatic source travel from one medium to another, their frequency remains exactly the same — only their wavelength and speed change.
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Examples
- In music and acoustics, the frequency of the standard pitch A above middle C on a piano is usually defined as 440 Hz, that is, 440 cycles per second (Listen ) and known as concert pitch, to which an orchestra tunes.
- A baby can hear tones with oscillations up to approximately 20,000 Hz, but these frequencies become more difficult to hear as people age.
- In Europe, Africa, Australia, Southern South America, most of Asia, and in Russia, the frequency of the alternating current in household electrical outlets is 50 Hz (close to the tone G), however, in North America and Northern South America, the frequency of the alternating current 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 utilizing the European, or the American grid frequency.
- Visible light from deep red to violet has frequencies of 430 to 750 THz.
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Period versus frequency
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 audio and radio, 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) |
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Other types of frequency
- Angular frequency ω is defined as the rate of change in the orientation angle (during rotation), or in the phase of a sinusoidal waveform (e.g. in oscillations and waves):
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- Angular frequency is measured in radians per second (rad/s).
- Spatial frequency is analogous to temporal frequency, but the time axis is replaced by one or more spatial displacement axes.
- Wavenumber is the spatial analogue of angular frequency. In case of more than one space dimension, wavenumber is a vector quantity.
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See also
- Negative frequency
- Periodicity
- Frequency converter
- Frequency range or frequency band
- Letter frequencies
- Pitch (music)
- Natural frequency
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References
- ^ Accidentally, 1 hertz is the approximate frequency of a human heart (Herz in German language)
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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 - javascript calculation.
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