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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. [ 1 ] [ a ] The hertz is an SI derived unit whose formal expression in terms of SI base units is s −1 , meaning that one hertz is one per second or the reciprocal of one second . [ 2 ]
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: =. In the special case of electromagnetic waves in vacuum , then v = c , where c is the speed of light in vacuum, and this expression becomes f = c λ . {\displaystyle f={\frac {c}{\lambda }}.}
The cycle per second is a once-common English name for the unit of frequency now known as the hertz (Hz). Cycles per second may be denoted by c.p.s., c/s, or, ambiguously, just "cycles" (Cyc., Cy., C, or c).
Heinrich Rudolf Hertz (/ h ɜːr t s / HURTS; German: [ˈhaɪnʁɪç hɛʁts]; [1] [2] 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The SI unit of frequency, the hertz (Hz), is named after him ...
The phase velocity is the rate at which the phase of the wave propagates in space. The group velocity is the rate at which the wave envelope, i.e. the changes in amplitude, propagates. The wave envelope is the profile of the wave amplitudes; all transverse displacements are bound by the envelope profile.
Wave speed is a wave property, which may refer to absolute value of: . phase velocity, the velocity at which a wave phase propagates at a certain frequency; group velocity, the propagation velocity for the envelope of wave groups and often of wave energy, different from the phase velocity for dispersive waves
where is the speed of the wave, the fundamental frequency can be found in terms of the speed of the wave and the length of the pipe: f 0 = v 4 L {\displaystyle f_{0}={\frac {v}{4L}}} If the ends of the same pipe are now both closed or both opened, the wavelength of the fundamental harmonic becomes 2 L {\displaystyle 2L} .
In dry air, the speed of sound increases by about 0.1 m/s as the frequency rises from 10 Hz to 100 Hz. For audible frequencies above 100 Hz it is relatively constant. Standard values of the speed of sound are quoted in the limit of low frequencies, where the wavelength is large compared to the mean free path.