The hertz (Hz) is one of the simplest units in the entire SI system: it describes an event that repeats exactly once per second (1 Hz = 1/s). There is nothing exotic about it — and yet it is hard to name another unit that binds together such different fields. The same hertz measures the pitch of a sound, the frequency of the power in your outlet, the reach of radio waves, the smoothness of your smartphone display, and the clock of your processor. So let's travel from a single tick per second all the way up to billions of cycles in silicon — and see why a count of repetitions per second became the common denominator of modern technology.
The man behind the name
The unit honors the German physicist Heinrich Rudolf Hertz (born 22 February 1857 in Hamburg, died 1 January 1894 in Bonn). He died young, at 36, but left behind work that still underpins wireless communication. He was a wide-ranging scholar — beyond physics he studied Arabic and Sanskrit — and his name is attached to the classical theory of contact stresses (Hertzian contact), still used in mechanics today.
But something else mattered most. In the second half of the 1880s, Hertz experimentally proved the existence of electromagnetic waves, confirming James Clerk Maxwell's theory and showing that these waves are of the same nature as light. Along the way, in 1887, he observed the external photoelectric effect — a phenomenon that three decades later Albert Einstein explained theoretically (and won a Nobel Prize for).
Hertz himself considered his discovery of no practical use. Asked what it was good for, he reportedly answered: nothing at all. He was spectacularly wrong: on his results, Oliver Lodge and Guglielmo Marconi built the first radio. Today the scholar's name marks not only the unit of frequency but also a crater on the Moon.
The hertz you can hear
In acoustics, the hertz describes how fast the air of a sound wave vibrates — which translates directly into pitch. The audible range of a healthy young ear stretches roughly from 20 Hz to 20,000 Hz (20 kHz). That upper limit is not fixed for life, though: it falls with age. After your twenties the very top of the band slowly fades; a forty-year-old often can no longer hear tones above 15 kHz, and past sixty the limit can drop to 8–10 kHz.
The ear is also not equally sensitive across the whole range. We hear best around 2,000–4,000 Hz — exactly where the key frequencies of speech lie. The closer to the edges of the band, the louder a sound must be for us to register it at all. That is why loudness (measured in decibels) and frequency have to be read together:
| Level (dB) | Example source | Effect on hearing |
|---|---|---|
| 30 dB | a whisper | completely safe, background noise |
| 50–60 dB | normal conversation | comfortable environment |
| 70–80 dB | street traffic | prolonged exposure can be tiring |
| 85 dB | continuous industrial noise | threshold of hearing-damage risk under steady exposure |
| 100–110 dB | concert / club | high risk of hearing damage without protection |
| 120–130 dB | threshold of pain (jet takeoff) | risk of immediate, permanent hearing loss |
Frequency also governs how loudspeakers are built. Bass (roughly 16–300 Hz) means long waves that must be "pushed" by a large mass of air — which is why woofers and subwoofers are big. Treble (above ~3,000 Hz) requires very fast vibrations, so tweeters are small and light. Audiophile gear deliberately reaches far beyond audibility, even to 40 kHz — manufacturers credit ultrasound with adding to the perceived fullness of the sound, though the research on that effect remains inconclusive.
Mains power and flickering light
Homes are fed alternating current (AC), whose direction and voltage change cyclically. In Europe the standard is 50 Hz, in North America 60 Hz. A higher frequency allows smaller, lighter transformers — which is why aviation uses as much as 400 Hz, where every kilogram counts. Lower 50 Hz, in turn, means smaller transmission losses.
Alternating current sets the cores of transformers and chokes vibrating, which you can hear as a characteristic hum. Interestingly, its pitch is twice the mains frequency — in Europe 100 Hz (the core is pulled in twice per cycle, regardless of current direction). That is a tone sitting roughly between the notes G2 and G#2.
The same voltage swings make lamps and LEDs flicker. The eye rarely sees it directly, but the brain registers the changes in brightness, which can tire the eyes and trigger headaches. The phenomenon even shaped television standards: PAL (Europe) runs to the rhythm of the 50 Hz grid and a 25/50 frame rate, while NTSC (the Americas) runs at 60 Hz and 30/60 frames. When a camera's frame rate doesn't match the lighting frequency, scrolling dark bands appear in the footage. That is why camera operators choose exposure times that are multiples of the mains period (e.g. 1/50 s or 1/100 s in Europe), and modern cameras have Anti-flicker modes.
Radio waves: frequency governs range
Radio waves are the part of the electromagnetic spectrum stretching from 3 kHz to 3 THz. They all travel through a vacuum at the same speed — the speed of light (c ≈ 299,792,458 m/s). A simple formula ties them together:
v = λ · f
that is, speed equals wavelength times frequency. Since the speed is fixed, the higher the frequency, the shorter the wave — and that completely changes how the wave propagates through the environment:
| Band (abbr.) | Frequency | Wavelength | How it propagates | Use |
|---|---|---|---|---|
| ELF (extremely low) | 3–30 Hz | 100,000–10,000 km | penetrates earth and seawater | contact with submerged submarines |
| LF (low) | 30–300 kHz | 10–1 km | ground wave along the Earth's curvature | longwave AM radio |
| MF (medium) | 300–3,000 kHz | 1,000–100 m | ground wave; ionospheric reflections at night | local AM stations, radionavigation |
| HF (high) | 3–30 MHz | 100–10 m | ionospheric reflections — global reach | amateur radio, international broadcasting |
| VHF (very high) | 30–300 MHz | 10–1 m | line of sight, poor at bending around obstacles | FM radio (87.5–108 MHz), TV |
| UHF (ultra high) | 300–3,000 MHz | 1–0.1 m | line of sight, attenuated by buildings | Wi-Fi, Bluetooth, LTE, GPS, terrestrial TV |
| SHF (centimeter) | 3–30 GHz | 10–1 cm | straight-line, attenuated by rain | radar, satellite TV, 5G |
A clear trade-off shows up here. Low frequencies carry as ground waves over thousands of kilometers, even across the sea — but the band is so narrow that they carry little information. High frequencies (microwaves) travel only in a straight line and are absorbed by rain and water vapor, but they offer enormous bandwidth, which is how we transmit gigabits per second. The whole art of designing wireless networks is maneuvering between range and throughput.
Revolutions per minute: frequency you can see
Not every frequency involves waves. When something spins, we also count repetitions over time — just conventionally per minute rather than per second. Hence revolutions per minute (rpm), the unit of engines, centrifuges, disk drives, and turntables. The conversion is trivial: 1 rpm = 1/60 Hz, because a minute has 60 seconds. It is the same frequency, only in different packaging.
| What's spinning | Revolutions | In hertz |
|---|---|---|
| vinyl LP record | 33⅓ rpm | ≈ 0.56 Hz |
| single | 45 rpm | 0.75 Hz |
| car engine (idle) | ~800 rpm | ~13 Hz |
| hard disk drive (HDD) | 7,200 rpm | 120 Hz |
| engine at high revs | 6,000 rpm | 100 Hz |
Notice that a vinyl record turns at a frequency below 1 Hz, while a hard-disk platter spins 120 times a second. It's the same physical quantity as a musical tone or the mains power, so you can convert one into the other with the same frequency converter.
The screen that can slow down to 1 Hz
On a phone or monitor, hertz describes the refresh rate — how many times per second the panel draws a new image. A higher value (90, 120, or 144 and 165 Hz for gaming) removes the choppiness of animation and the blur of text while scrolling. That is different from the touch sampling rate, which says how often the screen checks where your finger is — the higher it is, the shorter the reaction delay:
| Touch sampling | Interval between samples | Use |
|---|---|---|
| 60 Hz | 16.67 ms | older screens, basic use |
| 90 Hz | 11.11 ms | mid-range devices |
| 120 Hz | 8.33 ms | the standard on modern phones |
| 240 Hz | 4.17 ms | mobile gaming, low latency |
| 360 Hz | 2.78 ms | e-sports screens |
| 480 Hz | 2.08 ms | the highest available responsiveness |
The catch is that a constant 120 Hz drains the battery. The solution turned out to be LTPO (low-temperature polycrystalline oxide) technology — a hybrid that lets the refresh rate change smoothly depending on content. With a static clock in Always-On mode, the screen drops to as low as 1 Hz; during a film it syncs to the video frame rate (24, 30, or 60 Hz); and the moment you start scrolling, it jumps to a full 120 Hz. The idea was first tested in smartwatches (Apple Watch), and from 2021 it reached flagship phones (including the iPhone 13 Pro with ProMotion and the OnePlus 9 Pro with a full 1–120 Hz range). Its main drawback — high production cost — still reserves LTPO for pricier models.
Gigahertz in the processor: why more doesn't mean faster
A processor's clock is given in gigahertz — billions of clock cycles per second. For years the belief lingered that more gigahertz simply meant a faster computer. That is no longer true. Real performance is the product of clock speed and the IPC factor (instructions per cycle) — the number of instructions a core executes in a single cycle. A modern core clocked at 3.5 GHz easily beats an ancient Pentium 4 running at ~3 GHz, because it does far more work in each cycle.
What's more, pushing the clock much beyond about 5–6 GHz is blocked by hard limits of physics:
- Heat. Power draw rises nonlinearly with frequency, and a higher clock demands higher voltage. That produces more heat than conventional cooling can remove.
- Atom size. Transistors have shrunk to the scale of tens of silicon atoms (names like "7 nm" or "3 nm" are today more marketing labels for the process than the real gate width). Near the ~1 nm boundary, quantum tunneling kicks in — electrons spontaneously "jump" across barriers.
- Speed of light. At a hypothetical 40 GHz, a signal travels just ~7.5 mm through a vacuum in a single cycle — less than the width of a processor package. Synchronizing the clock across the whole chip becomes physically impossible.
That is why, for over a decade, Intel and AMD have not chased gigahertz but instead expanded IPC, cache, and core counts, while making brief clock bursts (Boost/Turbo modes) depend on temperature and power headroom. The scale is staggering: a core at 5 GHz reads L1 cache in about 1 nanosecond — in that time, light travels barely 30 centimeters.
One second, all of technology
The hertz is trivial to define and everywhere in its effects. From Hertz's spark experiments with electromagnetic waves, through the bands of sound and radio, all the way to gigahertz silicon clocks — it all comes down to a single question: how many times per second? Tellingly, today's technological progress no longer relies on cranking that number up at any cost. The thermal wall of processors and the power hunger of fast screens have forced the opposite move: intelligently lowering the frequency where it isn't needed — an LTPO screen slowing to 1 Hz, or a processor saving power when nothing is happening. Skillful management of hertz has become the foundation of the efficiency of the devices we use every day.
Further reading
- BIPM, The International System of Units (SI Brochure), 9th ed. (2019) — the official definition of the hertz and derived units.
- Heinrich Hertz — Wikipedia entry: biography and achievements.
- Radio spectrum / radio waves — Wikipedia entry: band divisions and propagation mechanisms.
- Display-manufacturer materials (e.g. explainers of LTPO technology) — context for variable refresh rate in smartphones.