| Millihertz (mHz) | Gigahertz (GHz) |
|---|---|
| 1 Millihertz | 1 × 10-12 GHz |
| 2 Millihertz | 2 × 10-12 GHz |
| 3 Millihertz | 3 × 10-12 GHz |
| 4 Millihertz | 4 × 10-12 GHz |
| 5 Millihertz | 5 × 10-12 GHz |
| 10 Millihertz | 1 × 10-11 GHz |
| 20 Millihertz | 2 × 10-11 GHz |
| 25 Millihertz | 2.5 × 10-11 GHz |
| 50 Millihertz | 5 × 10-11 GHz |
| 100 Millihertz | 1 × 10-10 GHz |
| Reference | Millihertz (mHz) | Gigahertz (GHz) |
|---|---|---|
| European mains electricity | 50000 mHz | 0.00000005 GHz |
| Concert pitch A above middle C | 440000 mHz | 0.00000044 GHz |
| An FM radio station | 1 × 1011 mHz | 0.1 GHz |
| A Wi-Fi band | 2.4 × 1012 mHz | 2.4 GHz |
| A desktop processor clock | 3 × 1012 mHz | 3 GHz |
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 gigahertz is a unit of frequency equal to one thousand million hertz. Its symbol is GHz. It is the range of modern wireless communication and of the clock inside every current computer, and almost nothing in it was in everyday use before the 1990s.
Wi-Fi occupies two main bands, at 2.4 and 5 gigahertz, with a newer band near 6. The 2.4 gigahertz band is crowded because it is licence-free almost everywhere and shared with Bluetooth, cordless phones and microwave ovens, which operate at 2.45 gigahertz because water molecules absorb energy efficiently there. The 5 gigahertz band offers more channels and higher rates but is absorbed more strongly by walls, so it covers a smaller area.
Mobile telephony spans the range. Earlier generations used bands below 1 gigahertz for their long reach into buildings and across countryside, while newer allocations at 1.8, 2.6 and 3.5 gigahertz carry more data over shorter distances. The higher a frequency, the more bandwidth is available and the less it penetrates, and every generation of network design is a negotiation between those two facts.
Computer clock speeds crossed into gigahertz around the year 2000 and stalled near four a few years later. The limit is thermal: power dissipation rises steeply with frequency, and beyond about four gigahertz a conventional silicon processor cannot shed the heat. Performance gains since then have come from adding cores, widening instruction issue and improving memory hierarchies rather than from raising the clock.
Satellite links, radar and radio astronomy all work here. Weather radar typically runs between 2.7 and 5.6 gigahertz, air traffic control radar in similar bands, and the microwave background radiation that fills the universe peaks near 160 gigahertz, well above the range of terrestrial communication.
Millimetre-wave systems at 24 gigahertz and above are used for automotive collision-avoidance radar and for the highest-capacity mobile network bands. Their very short wavelengths allow small antennas and enormous data rates but are blocked by almost anything, including rain and foliage.
One gigahertz equals one thousand million hertz, 1000 megahertz, or 0.001 terahertz.