Qu'est-ce qu'un/une Kilohertz (kHz) ?
Formal Definition
The kilohertz (symbol: kHz) is a unit of frequency equal to 1,000 hertz (10³ Hz), or 1,000 cycles per second. The prefix "kilo-" denotes a factor of one thousand in the SI system. The kilohertz is widely used in audio engineering, radio communications, and electronics to describe frequencies that fall between the upper limit of infrasound and the lower range of radio frequencies.
The kilohertz range (1 kHz to 999 kHz) encompasses the entire audible spectrum above 1,000 Hz, the AM radio broadcast band, and many industrial and scientific applications. Frequencies in this range are directly perceptible as sound (up to about 20 kHz) and are used for communication signals that can propagate over long distances via ground waves and sky waves.
Practical Significance
The kilohertz provides convenient numerical values for frequencies commonly encountered in audio and radio work. Rather than writing 44,100 Hz for a CD sampling rate, engineers write 44.1 kHz. AM radio frequencies from 530,000 Hz to 1,700,000 Hz are expressed as 530 kHz to 1,700 kHz. This scale avoids unwieldy large numbers while maintaining precision.
Etymology
Origin of the Prefix
The prefix "kilo-" derives from the Greek word "chilioi" (χίλιοι), meaning "thousand." It was adopted as part of the metric system's prefix scheme during the French Revolution in the 1790s. Combined with "hertz" (named after Heinrich Hertz), "kilohertz" simply means "one thousand hertz." The abbreviation "kHz" uses the lowercase "k" for kilo (following SI convention) and uppercase "Hz" for hertz (following the convention for units named after people).
Historical Notation
Before the adoption of "hertz" in 1960, the equivalent unit was "kilocycles per second" (abbreviated kc/s or kc). Radio enthusiasts and engineers of the 1940s through 1960s spoke of stations broadcasting at "1200 kilocycles" rather than "1200 kilohertz." The transition to the hertz-based nomenclature was completed by the late 1970s in most countries.
Precise Definition
Exact Definition
One kilohertz equals exactly 1,000 hertz, or 1,000 cycles per second. In SI base units, 1 kHz = 10³ s⁻¹. The conversion is exact by definition of the SI prefix "kilo-".
Key Conversions
1 kHz = 1,000 Hz; 1 kHz = 0.001 MHz; 1 kHz = 0.000001 GHz; 1 kHz = 60,000 RPM (revolutions per minute). The kilohertz-to-RPM conversion is useful in mechanical engineering: a motor spinning at 1,000 RPM has a rotational frequency of approximately 16.67 Hz, not 1 kHz.
Frequency Measurement at kHz Scale
Frequencies in the kilohertz range are measured using oscilloscopes, frequency counters, audio analyzers, and spectrum analyzers. Audio frequency analyzers typically cover the range from 20 Hz to 20 kHz with resolution of 1 Hz or better. Radio frequency receivers for the AM band measure frequencies from 530 kHz to 1,700 kHz with precision of 1 kHz or finer.
Histoire
The Radio Age
The kilohertz range became critically important with the development of radio communication in the early 20th century. The first commercial AM radio broadcasts in the 1920s operated in the medium-frequency band (300 kHz to 3 MHz). Radio stations were assigned specific frequencies in kilocycles per second, and the ability to precisely control and measure these frequencies drove advances in crystal oscillator technology.
Audio Recording and Telephony
The development of telephone systems in the late 19th and early 20th centuries established the kilohertz range as central to audio communication. Telephone bandwidth was standardized at 300 Hz to 3.4 kHz, capturing the frequency range essential for speech intelligibility. The Nyquist sampling theorem (1928) established that audio signals must be sampled at least twice per period of the highest frequency — leading to telephone sampling at 8 kHz for the 4 kHz bandwidth.
Digital Audio
The compact disc (CD), introduced in 1982, established 44.1 kHz as the standard sampling rate for high-fidelity digital audio. This rate was chosen to capture the full audible spectrum up to 20 kHz (per the Nyquist theorem, requiring sampling above 40 kHz) with a small margin for anti-aliasing filter rolloff. Professional audio recording often uses 48 kHz, 96 kHz, or 192 kHz sampling rates.
Transition from Kilocycles
The term "kilocycles" remained in common use through the 1960s. The AM radio dial on cars and home receivers was marked in kilocycles well into the 1970s. The changeover to "kilohertz" in consumer products was gradual, with some manufacturers displaying both notations during the transition period.
Utilisation actuelle
AM Radio Broadcasting
The AM radio broadcast band extends from 530 kHz to 1,700 kHz (medium wave), with individual station assignments spaced 9 or 10 kHz apart depending on the region. When you tune to an AM station at "1020 AM" in the US, you are selecting a carrier frequency of 1,020 kHz.
Digital Audio
Audio sampling rates are universally expressed in kHz: 44.1 kHz for CDs, 48 kHz for professional video and broadcast audio, 96 kHz and 192 kHz for high-resolution audio. When audiophiles debate "hi-res audio," they are discussing sampling rates above 44.1 kHz, which theoretically capture frequencies beyond the human hearing range.
Ultrasound
Medical and industrial ultrasound uses frequencies in the upper kHz to MHz range. Therapeutic ultrasound operates at 20–40 kHz, dental cleaning tools at 25–30 kHz, and industrial cleaning baths at 20–80 kHz. These frequencies are just above the human audible range.
Navigation and Timing
Long-range navigation signals operate in the low kHz range. The WWVB time signal broadcast from Fort Collins, Colorado, transmits at 60 kHz, and radio-controlled clocks worldwide receive similar signals in the 40–77.5 kHz range. These low frequencies propagate reliably over continental distances.
Everyday Use
Hearing Tests
Audiologists test hearing across the kilohertz range, typically from 250 Hz to 8 kHz. The frequencies most critical for understanding speech — 1 kHz, 2 kHz, and 4 kHz — fall squarely in the kilohertz range. Age-related hearing loss typically begins at the higher kilohertz frequencies (4–8 kHz) and progresses toward lower frequencies.
Music and Hi-Fi
The treble range in music — cymbals, sibilance in vocals, the shimmer of acoustic guitars — occupies the 2–16 kHz range. Equalizers on home stereo systems and music apps let you boost or cut frequency bands labeled in kHz. The "presence" frequency range (2–5 kHz) is where human hearing is most sensitive, which is why boosting this range makes speech and vocals sound clearer.
Pest Deterrents
Ultrasonic pest deterrent devices emit sounds at 15–25 kHz, designed to be inaudible to adults but irritating to rodents and some insects. Young people can often hear these frequencies, leading to the development of the "Mosquito" anti-loitering device, which emits a tone at 17.4 kHz to deter teenagers from gathering in certain areas.
Dog Whistles
A dog whistle produces sound at approximately 23–54 kHz, beyond the range of human hearing but well within the range of dogs (which can hear up to about 65 kHz). The frequency is often adjustable and measured in kHz.
Interesting Facts
The standard CD sampling rate of 44.1 kHz was chosen not for audiological reasons but for practical video-tape-based recording: it matched the horizontal line rate of PAL and NTSC video systems used to store digital audio masters in the early 1980s.
AM radio stations are spaced 10 kHz apart in the Americas and 9 kHz apart in Europe and Asia. This seemingly trivial difference causes international interference issues and has resisted harmonization for nearly a century.
The WWVB time signal station in Fort Collins, Colorado, broadcasts at exactly 60 kHz with a power of 70 kilowatts. This single transmitter provides the time reference for over 100 million radio-controlled clocks in North America.
Dolphins communicate using clicks and whistles at frequencies up to 150 kHz — well above the human audible range. Researchers use hydrophones calibrated in kHz to study dolphin communication patterns.
The telephone system's 3.4 kHz bandwidth was established in the 1920s and remained unchanged for nearly a century. This narrow bandwidth is why telephone voices sound thinner and less natural than in-person speech, which contains frequencies up to 8 kHz and beyond.
Audio engineers use a test tone at exactly 1 kHz as the standard reference frequency for calibrating equipment. When you see a sine wave on a test screen, it is almost certainly at 1 kHz.