| Reference | Hertz (Hz) | Kilohertz (kHz) |
|---|---|---|
| European mains electricity | 50 Hz | 0.05 kHz |
| Concert pitch A above middle C | 440 Hz | 0.44 kHz |
| An FM radio station | 100000000 Hz | 100000 kHz |
| A Wi-Fi band | 2.4 × 109 Hz | 2400000 kHz |
| A desktop processor clock | 3 × 109 Hz | 3000000 kHz |
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.
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.