| Kilohertz (kHz) | Millihertz (mHz) |
|---|---|
| 1 Kilohertz | 1000000 mHz |
| 2 Kilohertz | 2000000 mHz |
| 3 Kilohertz | 3000000 mHz |
| 4 Kilohertz | 4000000 mHz |
| 5 Kilohertz | 5000000 mHz |
| 10 Kilohertz | 10000000 mHz |
| 20 Kilohertz | 20000000 mHz |
| 25 Kilohertz | 25000000 mHz |
| 50 Kilohertz | 50000000 mHz |
| 100 Kilohertz | 100000000 mHz |
| Reference | Kilohertz (kHz) | Millihertz (mHz) |
|---|---|---|
| European mains electricity | 0.05 kHz | 50000 mHz |
| Concert pitch A above middle C | 0.44 kHz | 440000 mHz |
| An FM radio station | 100000 kHz | 1 × 1011 mHz |
| A Wi-Fi band | 2400000 kHz | 2.4 × 1012 mHz |
| A desktop processor clock | 3000000 kHz | 3 × 1012 mHz |
The kilohertz is a unit of frequency equal to 1000 hertz. Its symbol is kHz. It covers the upper part of human hearing and the lowest part of the radio spectrum, which makes it the meeting point of two very different technologies.
Audio lives here. The upper limit of human hearing is about 20 kilohertz in a young person, and the frequencies that carry speech intelligibility cluster between 1 and 4 kilohertz. Digital audio is sampled at 44.1 kilohertz for compact discs and 48 kilohertz for video work, rates chosen because the sampling theorem requires more than twice the highest frequency to be reproduced, with a margin for filtering.
Radio broadcasting began in this range. Long wave runs from 148 to 283 kilohertz and medium wave, the AM band, from 526 to 1606 kilohertz. These low frequencies propagate along the ground and reflect off the ionosphere at night, giving them ranges of hundreds or thousands of kilometres from a single transmitter, which is why they carried the first national broadcasting services and still serve maritime and aviation navigation beacons.
Submarine communication uses even lower frequencies because seawater absorbs radio energy, and only signals in the range of a few kilohertz and below penetrate more than a few metres. Naval very low frequency transmitters occupy enormous antenna arrays for this reason, and their data rates are correspondingly tiny.
Ultrasound for medical imaging sits far above the audible range, in megahertz, but industrial ultrasonic cleaning and animal deterrents work between 20 and 80 kilohertz, just above what people can hear. Bats echolocate in the same territory, from roughly 20 to well over 100 kilohertz.
Older computers had clock speeds in kilohertz. The processor in the original Apple II ran at just over one megahertz, but the earliest microprocessors and many microcontrollers still operate at a few hundred kilohertz where low power matters more than speed.
Digital audio put a specific number from this range into everyday use. The compact disc samples at 44.1 kilohertz, chosen because the sampling theorem requires more than twice the highest frequency to be reproduced, and twice 20 kilohertz leaves a margin for the filter that removes everything above it. Professional recording commonly uses 48 kilohertz to match video, and 96 or 192 for mastering. Telephony sits far lower, sampling at 8 kilohertz and so cutting off above about 3.4, which is why a voice on an ordinary phone line loses the sibilance that makes the letters s and f easy to tell apart.
One kilohertz equals 1000 hertz, 0.001 megahertz, or one millionth of a gigahertz.
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.