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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]
The SI unit for the period, as for all measurements of time, is the second. [5] A traditional unit of frequency used with rotating mechanical devices, where it is termed rotational frequency, is revolution per minute, abbreviated r/min or rpm. [6] 60 rpm is equivalent to one hertz. [7]
Symbolically, "cycle per second" units are "cycle/second", while hertz is "Hz" or "s −1 ". [2] For higher frequencies, kilocycles (kc), as an abbreviation of kilocycles per second were often used on components or devices. Other higher units like megacycle (Mc) and less commonly kilomegacycle (kMc) were used before 1960 [3] and in some later ...
2 Hz: 120 bpm, common tempo in music ~7.83 Hz: Fundamental frequency of the Schumann resonances: 10 1: 10 hertz 10 Hz: Cyclic rate of a typical automobile engine at idle (equivalent to 600 rpm) 12 Hz: Acoustic – the lowest possible frequency that a human can hear [3] 18 Hz: Average house cat's purr 24 Hz: Common frame rate of movies 27.5 Hz
The SI has special names for 22 of these coherent derived units (for example, hertz, the SI unit of measurement of frequency), but the rest merely reflect their derivation: for example, the square metre (m 2), the SI derived unit of area; and the kilogram per cubic metre (kg/m 3 or kg⋅m −3), the SI derived unit of density.
The TU (for time unit) is a unit of time defined as 1024 μs for use in engineering. The svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10 −13 seconds (100 fs). The galactic year, based on the rotation of the galaxy and usually measured in million years. [2]
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When a real number like .007 is denoted alternatively by 7.0 × 10 —3 then it is said that the number is represented in scientific notation.More generally, to write a number in the form a × 10 b, where 1 <= a < 10 and b is an integer, is to express it in scientific notation, and a is called the significand or the mantissa, and b is its exponent. [3]