| Millihertz (mHz) | Microhertz (µHz) |
|---|---|
| 1 Millihertz | 1000 µHz |
| 2 Millihertz | 2000 µHz |
| 3 Millihertz | 3000 µHz |
| 4 Millihertz | 4000 µHz |
| 5 Millihertz | 5000 µHz |
| 10 Millihertz | 10000 µHz |
| 20 Millihertz | 20000 µHz |
| 25 Millihertz | 25000 µHz |
| 50 Millihertz | 50000 µHz |
| 100 Millihertz | 100000 µHz |
| Reference | Millihertz (mHz) | Microhertz (µHz) |
|---|---|---|
| European mains electricity | 50000 mHz | 50000000 µHz |
| Concert pitch A above middle C | 440000 mHz | 440000000 µHz |
| An FM radio station | 1 × 1011 mHz | 1 × 1014 µHz |
| A Wi-Fi band | 2.4 × 1012 mHz | 2.4 × 1015 µHz |
| A desktop processor clock | 3 × 1012 mHz | 3 × 1015 µHz |
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 microhertz is a unit of frequency equal to one millionth of a hertz. Its symbol is µHz. One microhertz is one cycle per million seconds, which is a little over eleven and a half days, so the unit measures things that repeat on the scale of weeks, months or years.
Asteroseismology is where it is most used. Stars oscillate, and the frequencies of those oscillations depend on their internal structure in the same way the pitch of a bell depends on its shape and thickness. Sun-like stars ring at frequencies of a few thousand microhertz, red giants at a few tens, and the largest evolved stars below one. Space telescopes such as Kepler and TESS measured these frequencies for hundreds of thousands of stars by watching their brightness vary by a few parts per million.
The technique gives quantities no other method can supply. From the oscillation spectrum an astronomer can derive a star's mass, radius and age, and the age of a star is otherwise almost impossible to determine. This is how the ages of planet-hosting stars, and therefore of their planetary systems, are now established.
Gravitational-wave astronomy has claimed the band as well. The planned space-based observatory LISA will be sensitive between roughly 100 microhertz and one hertz, a range containing merging supermassive black holes and the many thousands of close binary white dwarfs in our own galaxy. Ground-based detectors cannot reach these frequencies because seismic noise overwhelms them.
Geophysics uses the unit for the slowest tidal constituents, including the fortnightly and monthly lunar tides and the semi-annual and annual solar ones, which together produce the long-period variations that tide predictions must include. Polar motion, the wobble of the Earth's rotation axis, has a fourteen-month period corresponding to about 0.8 microhertz.
Climate and ocean science reaches lower still. The El Nino Southern Oscillation recurs irregularly every two to seven years, well below a hundredth of a microhertz, and long ice-core records resolve cycles slower again.
The Earth itself supplies a textbook example. Its rotation axis wanders in a small circle with a period of about 433 days, the Chandler wobble discovered in 1891, which is a frequency of roughly 0.027 microhertz. Detecting it took decades of positional astronomy, because a signal that slow can only be separated from drift by observing for many cycles. That is the general constraint of the band: a measurement at one microhertz needs a record of at least a few million seconds, and preferably tens of them, so instruments must be stable for years before their data can be read. The unit therefore belongs to patient sciences rather than to fast ones.
One microhertz equals 0.000001 hertz, one cycle per million seconds, or 1000 nanohertz.