| Millihertz (mHz) | Hertz (Hz) |
|---|---|
| 1 Millihertz | 0.001 Hz |
| 2 Millihertz | 0.002 Hz |
| 3 Millihertz | 0.003 Hz |
| 4 Millihertz | 0.004 Hz |
| 5 Millihertz | 0.005 Hz |
| 10 Millihertz | 0.01 Hz |
| 20 Millihertz | 0.02 Hz |
| 25 Millihertz | 0.025 Hz |
| 50 Millihertz | 0.05 Hz |
| 100 Millihertz | 0.1 Hz |
| Reference | Millihertz (mHz) | Hertz (Hz) |
|---|---|---|
| European mains electricity | 50000 mHz | 50 Hz |
| Concert pitch A above middle C | 440000 mHz | 440 Hz |
| An FM radio station | 1 × 1011 mHz | 100000000 Hz |
| A Wi-Fi band | 2.4 × 1012 mHz | 2.4 × 109 Hz |
| A desktop processor clock | 3 × 1012 mHz | 3 × 109 Hz |
The millihertz is a unit of frequency equal to one thousandth of a hertz. Its symbol is mHz. One millihertz is one cycle every thousand seconds, a little under seventeen minutes, so the unit describes things that repeat slowly enough to watch.
The lower-case m matters. A millihertz is a million times smaller than a megahertz, and the two are distinguished only by the case of the first letter. This is one of the few places in the SI where a typing error changes a quantity by six orders of magnitude, and it is a recurring source of confusion in documents that lose their capitalisation.
Seismology is the unit's natural home. A large earthquake sets the whole Earth ringing like a bell, and these free oscillations have periods of minutes to about an hour, placing them between roughly 0.3 and 5 millihertz. The gravest mode, in which the planet alternately expands and contracts as a sphere, has a period near twenty minutes. Measuring these frequencies is how the deep structure of the Earth was mapped before seismic tomography.
The Sun oscillates in the same range. Helioseismology studies pressure waves trapped inside the Sun, whose strongest signal is a five-minute oscillation at about 3.3 millihertz. Tracking millions of these modes reveals the Sun's internal rotation and the depth of its convection zone, information no telescope can obtain by looking at the surface.
Ocean tides sit far below even this. The principal lunar semidiurnal tide repeats every twelve hours and twenty-five minutes, which is about 0.022 millihertz, and the whole family of tidal constituents that tide tables are built from occupies the range below a tenth of a millihertz.
Engineering meets the unit in the sway of tall buildings and long bridges, whose fundamental modes fall between about 100 and 500 millihertz, and in the slow control loops of heating systems and chemical plant, where a process may take many minutes to respond to a change.
One millihertz equals 0.001 hertz, one cycle per 1000 seconds, or 1000 microhertz.
The hertz is the SI unit of frequency, equal to one cycle per second. Its symbol is Hz. It is a derived unit whose dimension is simply one divided by time, so a quantity in hertz counts how many times something repeats in a second.
It is named after Heinrich Hertz, who between 1886 and 1889 produced and detected radio waves in the laboratory and so confirmed Maxwell's prediction that light and electromagnetic waves are the same phenomenon. The unit was adopted internationally in 1960, replacing the older and more literal cycles per second, which is still occasionally seen on vintage equipment.
Sound occupies the low end of the scale. Human hearing runs from roughly 20 hertz to 20,000 hertz, though the upper limit falls with age and is usually well below 16,000 hertz by middle life. Musical pitch is frequency: the A above middle C is fixed at 440 hertz by convention, and each octave is a doubling, so the same note an octave higher is 880 hertz.
Mains electricity alternates at 50 hertz across most of the world and 60 hertz in North America and parts of Japan and South America. The split is historical rather than technical, dating from competing equipment standards in the late nineteenth century, and it is why appliances and clocks that depend on mains frequency cannot simply be moved between regions.
Screens and computing use it constantly. A display refreshing at 60 hertz redraws sixty times a second, and higher rates reduce visible motion blur. Processor clock speeds, once quoted in megahertz and now in gigahertz, count the same thing: how many times per second the internal clock ticks.
The unit also applies to anything periodic that is not a wave. Heart rate at 60 beats per minute is one hertz, a pendulum with a one-second period swings at one hertz, and the frequency of a rotating shaft in revolutions per second is expressed identically.
One hertz equals one cycle per second, 1000 millihertz, or one thousandth of a kilohertz.