Conversion from Terahertz to Kilohertz

Information about the Terahertz (THz)

The terahertz is a unit of frequency equal to one million million hertz. Its symbol is THz. It occupies the gap between the highest radio frequencies and the lowest infrared light, a region that for most of the twentieth century was so hard to generate or detect that it was known as the terahertz gap.

The difficulty was practical rather than theoretical. Electronic oscillators run out of speed above a few hundred gigahertz because charge carriers cannot respond fast enough, while optical sources such as lasers become inefficient below the infrared. The band between them had no convenient technology on either side, and for decades it was the least explored part of the electromagnetic spectrum.

That changed with ultrafast lasers and photoconductive antennas, which generate terahertz pulses by illuminating a semiconductor with a laser pulse lasting a few femtoseconds. Terahertz time-domain spectroscopy now measures the response of materials across the band, and the technique has become a standard tool in materials science.

Security screening is the best-known application. Terahertz radiation passes through clothing, paper, plastic and ceramics but is stopped by metal and absorbed by water, and its photon energy is far too low to ionise anything, so it can image concealed objects without the risks of X-rays. Body scanners at airports work on this principle.

Molecular spectroscopy uses the band because many large molecules have rotational and vibrational modes here. Explosives and drugs have characteristic terahertz signatures, and the technique is used in pharmaceutical quality control to identify crystal forms that look identical by other methods. Art conservation applies it to see beneath paint layers without touching a canvas.

Astronomy observes at these frequencies from high, dry sites or from orbit, because atmospheric water vapour absorbs terahertz radiation strongly. Cold dust and molecular gas in star-forming regions radiate here, which is why observatories are built at high altitude in Chile and at the South Pole.

Communications research has begun to move into the band. Data rates rise with available bandwidth, and there is far more room above 100 gigahertz than below it, so laboratories have demonstrated wireless links carrying hundreds of gigabits a second at frequencies approaching a terahertz. The obstacle is that water vapour absorbs these waves strongly, limiting a link to a few hundred metres in clear air and much less in rain. That confines the technology to short hops: connections between rooftop antennas, links inside a data centre, or the last stretch between a lamp-post and a building rather than anything resembling a broadcast.

One terahertz equals one million million hertz, 1000 gigahertz, or a wavelength of about 0.3 millimetres.

Information about the Kilohertz (kHz)

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.