O que é um/uma Hertz (Hz)?
Formal Definition
The hertz (symbol: Hz) is the SI unit of frequency, defined as one cycle per second. In SI base units, one hertz equals one reciprocal second (1 Hz = 1 s⁻¹). Frequency describes the number of occurrences of a repeating event per unit of time. The hertz applies to any periodic phenomenon — sound waves, electromagnetic radiation, electrical signals, mechanical vibrations, or even biological rhythms.
The hertz is named after Heinrich Rudolf Hertz (1857–1894), the German physicist who first conclusively demonstrated the existence of electromagnetic waves in 1887. Before the adoption of "hertz" as the official unit name, frequency was expressed simply as "cycles per second" (cps). The 14th General Conference on Weights and Measures (CGPM) adopted the name "hertz" in 1960 as part of the International System of Units.
Multiples and Range
Frequency values in practical use span an enormous range: from fractions of a hertz (earthquake seismic waves, brain waves) to exahertz (10¹⁸ Hz, gamma rays). Common SI multiples include kilohertz (kHz, 10³ Hz), megahertz (MHz, 10⁶ Hz), gigahertz (GHz, 10⁹ Hz), and terahertz (THz, 10¹² Hz). Each prefix steps up by a factor of 1,000.
Etymology
Heinrich Hertz and Electromagnetic Waves
The unit is named in honor of Heinrich Rudolf Hertz (1857–1894), who was born in Hamburg, Germany, and studied under Hermann von Helmholtz in Berlin. Between 1886 and 1888, Hertz conducted a series of groundbreaking experiments that confirmed James Clerk Maxwell's theoretical prediction of electromagnetic waves. Using a spark-gap transmitter and a loop antenna receiver, Hertz demonstrated that invisible electromagnetic waves could be generated, transmitted through air, reflected, refracted, and polarized — all properties shared with visible light.
From "Cycles Per Second" to "Hertz"
Before 1960, frequency was universally expressed as "cycles per second" (abbreviated cps or c/s). The International Electrotechnical Commission (IEC) proposed the name "hertz" in 1935, and it was gradually adopted by national standards bodies. The 11th CGPM recognized the hertz in 1960 when the SI system was formally established. The transition from "cycles per second" to "hertz" took several decades: American engineering texts still commonly used "cps" into the 1970s.
Pronunciation and Plural
The word "hertz" is both singular and plural in English (one hertz, ten hertz), following the convention for units named after people. In German, Hertz's native language, the surname is pronounced approximately "hairts" with a rolled R, while in English it is typically pronounced "hurts."
Precise Definition
SI Base Unit Expression
One hertz is defined as exactly one cycle per second, or equivalently, 1 Hz = 1 s⁻¹. The hertz is a derived SI unit with a special name, expressible entirely in terms of the SI base unit of time (the second). The second is itself defined by the cesium-133 hyperfine transition frequency, which is fixed at exactly 9,192,631,770 Hz — creating a circular but self-consistent definition.
Relationship to Angular Frequency
The hertz measures ordinary (temporal) frequency, also called cyclical frequency. The related quantity angular frequency (ω) is measured in radians per second (rad/s) and relates to hertz by: ω = 2πf, where f is frequency in hertz. Thus, 1 Hz corresponds to approximately 6.2832 rad/s. Angular frequency is commonly used in physics and electrical engineering, while hertz is preferred in most practical applications.
Measurement Methods
Frequency in hertz is measured using frequency counters, oscilloscopes, spectrum analyzers, and digital signal processors. Modern frequency counters can measure frequencies from millihertz to tens of gigahertz with resolution of 0.001 Hz or better. The most precise frequency measurements in the world are performed at national metrology institutes using atomic clocks, achieving relative uncertainties of 10⁻¹⁸ — equivalent to less than one second of error over the age of the universe.
História
Hertz's Experiments (1886–1888)
In 1886, Heinrich Hertz set up an experiment in a lecture hall at the Karlsruhe Polytechnic in Germany. He used a Ruhmkorff coil (an induction coil) to generate sparks across a gap in a transmitting antenna. Across the room, a receiving loop antenna with its own small gap produced tiny sparks when the transmitter fired — demonstrating that electromagnetic energy had traveled through the air. Over the next two years, Hertz systematically measured the wavelength and speed of these waves, showing they traveled at the speed of light, as Maxwell had predicted.
From Radio Waves to Broadcasting
Hertz's discovery of radio waves led directly to the development of wireless telegraphy by Guglielmo Marconi in the 1890s and eventually to radio broadcasting in the 1920s. The need to describe the frequencies of radio transmissions made "cycles per second" a crucial measurement unit. Radio stations were assigned specific frequencies, and the ability to tune a receiver to a particular frequency became a defining feature of radio technology.
Adoption of the Hertz Unit
The IEC proposed the name "hertz" in 1935, but adoption was gradual. The CGPM formally recognized the hertz as an SI unit in 1960. Throughout the 1960s and 1970s, older publications continued to use "cycles per second" or "cps," while newer ones adopted "hertz." By the 1980s, "hertz" was universally standard in scientific and engineering literature worldwide.
The Digital Age
The rise of digital computing in the 1970s and 1980s brought a new context for frequency: processor clock speeds. The Intel 4004 (1971) operated at 740 kHz, the Intel 8086 (1978) at 5–10 MHz, and modern processors operate at 3–6 GHz. The hertz became a household term through computer marketing, where "more gigahertz" was (sometimes misleadingly) equated with "faster computer."
Uso atual
Telecommunications
The hertz and its multiples are fundamental to telecommunications. Radio frequencies are allocated in hertz: AM radio operates at 530–1700 kHz, FM radio at 87.5–108 MHz, Wi-Fi at 2.4 GHz and 5 GHz, and 5G cellular networks at 600 MHz to 39 GHz. The International Telecommunication Union (ITU) manages the global radio frequency spectrum, allocating bands in hertz for different services.
Audio and Music
In audio engineering, the hertz defines the pitch of sound. Human hearing spans approximately 20 Hz to 20,000 Hz (20 kHz). The standard tuning pitch A4 is set at 440 Hz by international convention (ISO 16). Middle C on a piano is approximately 261.6 Hz. Subwoofers reproduce frequencies down to 20–30 Hz, while tweeters handle 2–20 kHz.
Computing and Electronics
Processor clock speeds, memory bus frequencies, display refresh rates, and serial communication baud rates are all expressed in hertz. A modern desktop CPU operates at 3–6 GHz, DDR5 RAM at 4800–8000 MHz, and a gaming monitor at 144–360 Hz refresh rate. USB 3.2 operates at 5–20 GHz.
Science and Medicine
In physics, the hertz quantifies atomic transition frequencies, laser frequencies, and oscillation frequencies of mechanical systems. In medicine, electroencephalography (EEG) measures brain wave frequencies in hertz: delta waves (0.5–4 Hz), theta waves (4–8 Hz), alpha waves (8–13 Hz), and beta waves (13–30 Hz).
Everyday Use
Music and Sound
Every time you listen to music, you interact with hertz. The bass notes that you feel in your chest are frequencies around 40–100 Hz. The human voice typically ranges from 85–255 Hz for fundamental frequency (lower for men, higher for women). The highest note on a standard piano is C8 at 4,186 Hz. When your phone rings, the ringtone contains frequencies typically between 500 and 4,000 Hz.
Electrical Power
The electrical grid operates at a fixed frequency: 60 Hz in North America and parts of South America and Asia, and 50 Hz in Europe, Africa, and most of Asia. This frequency determines the speed of AC motors, the flicker rate of older fluorescent lights, and the hum you sometimes hear from transformers and power supplies. Clocks and timers built into many appliances rely on the grid frequency for timekeeping.
Display Technology
Your TV, computer monitor, and smartphone all have a refresh rate measured in hertz. Standard displays operate at 60 Hz (refreshing the image 60 times per second), while gaming monitors run at 144 Hz, 240 Hz, or even 360 Hz for smoother motion. Modern smartphones typically use 90 Hz or 120 Hz displays for fluid scrolling.
Health and Fitness
Heart rate monitors measure your pulse in beats per minute, but the underlying frequency is in hertz: 60 beats per minute equals 1 Hz. Brain wave monitors in sleep labs display frequency bands in hertz to classify sleep stages.
Interesting Facts
Heinrich Hertz tragically died at age 36 from Wegener's granulomatosis, just six years after his groundbreaking experiments. He never lived to see the explosive growth of radio technology that his work made possible.
The cesium-133 atom oscillates at exactly 9,192,631,770 Hz — this frequency defines the SI second. Atomic clocks based on this transition are accurate to about one second in 300 million years.
The human ear can detect sound frequencies from about 20 Hz to 20,000 Hz, but this range shrinks with age. By age 50, most people cannot hear frequencies above 12,000–14,000 Hz, and by age 65, the upper limit may drop to 8,000 Hz.
The lowest note on a standard piano (A0) vibrates at 27.5 Hz, while the highest note (C8) vibrates at 4,186 Hz. The entire audible range of music spans only about 7.5 octaves of the roughly 10 octaves humans can hear.
Gamma rays — the highest-frequency electromagnetic radiation — can reach frequencies above 10²⁴ Hz (1 yottahertz). At the other extreme, the theoretical lower limit of electromagnetic waves is determined by the size of the observable universe, corresponding to about 10⁻¹⁸ Hz.
A hummingbird's wings beat at 50–80 Hz, producing the characteristic humming sound. The fastest insect wing beat ever recorded belongs to a midge at approximately 1,046 Hz — over 1,000 wing beats per second.
When you tune a guitar string to concert pitch A (440 Hz), the string physically vibrates back and forth 440 times every second, displacing air molecules that carry this vibration to your ear as sound.