Hertz (Hz)

Hertz (Hz) is the SI unit of frequency — one cycle per second. Named after the German physicist Heinrich Hertz, who proved the existence of electromagnetic waves in 1887.

Common multiples and where you encounter them:

• Hz — seismic and infrasound (under 20 Hz, below human hearing)
• kHz (10³ Hz) — audio range (20 Hz to 20 kHz); AM radio
• MHz (10⁶ Hz) — FM radio (88-108 MHz); CPU clocks before 2000
• GHz (10⁹ Hz) — Wi-Fi, modern CPUs, microwave ovens (2.45 GHz)
• THz (10¹² Hz) — infrared, terahertz imaging
• PHz (10¹⁵ Hz) — visible light

Refresh rates for displays are quoted in Hz: standard LCD 60 Hz, gaming displays 120/144/240 Hz. Higher refresh rates reduce motion blur and input lag. The threshold where humans stop perceiving improvement varies but typically falls between 90 and 120 Hz for most viewers.

Worked example

A note played at A above middle C is 440 Hz — the orchestral tuning reference. The next octave up doubles the frequency: A5 = 880 Hz. The octave below halves: A3 = 220 Hz. Between A4 and A5 sit 12 equal semitones; each step multiplies frequency by 2^(1/12) ≈ 1.0595. So B4 (one semitone up from A) is 440 × 1.0595 ≈ 466.16 Hz; C5 (two semitones up): 440 × 1.0595² ≈ 493.88 Hz. Compare to a Wi-Fi router at 5 GHz = 5,000,000,000 Hz, ten million times higher frequency. Compare to visible green light at ~5.4 × 10¹⁴ Hz, another hundred-thousand times higher. The dynamic range of “things measured in Hz” spans more than 15 orders of magnitude — from sub-Hz seismic waves to PHz visible light — which is why prefixes (kilo-, mega-, giga-, tera-, peta-) get heavy use.

Period and frequency are reciprocals: a 60 Hz signal has a period of 1/60 = 16.67 ms; a 1 GHz signal has a period of 1 nanosecond; a 5 PHz light wave has a period of 0.2 femtoseconds. Engineers slip between “frequency” (Hz) and “period” (seconds) depending on context — audio designers think in Hz, oscilloscope users think in period, CPU architects use both interchangeably.

When and why it matters

Hz matters whenever a signal has a periodic structure: audio frequencies in music and recording, RF allocations in radio and Wi-Fi standards, refresh rates in displays, sample rates in digital audio, and clock speeds in electronics. The mistake to avoid in display purchases is treating refresh-rate Hz as the only factor — a 240 Hz monitor with a 5 ms response time has worse motion clarity than a 144 Hz monitor with a 1 ms response time, because the pixel transition limits actual frame distinguishability. The mistake in audio is conflating sample rate (44.1 kHz, 48 kHz, 96 kHz — the rate at which the signal is measured) with the audio frequencies present (typically 20 Hz to 20 kHz human hearing range). Sample rate must be at least twice the highest audio frequency (Nyquist theorem) to avoid aliasing — 44.1 kHz can faithfully represent audio up to ~22 kHz, which exceeds human hearing limits and is why CD-quality is the consumer standard. Reference: BIPM — SI derived units.

Why Hz replaced “cycles per second” in 1960: the unit was renamed by the General Conference on Weights and Measures (CGPM) to honour Heinrich Hertz, whose 1887 experiments produced the first artificial electromagnetic waves and confirmed Maxwell’s equations. Before the renaming, frequencies were written as “cps” or “c/s”. The change was purely terminological — the value remained one cycle per second — but it consolidated frequency under the SI naming convention shared by newton, watt, joule, and pascal. Hz is defined dimensionally as s⁻¹ (inverse seconds) in the modern SI.

The CPU-clock misconception: a CPU running at 3 GHz does not perform 3 billion operations per second — it performs 3 billion clock ticks. Modern superscalar pipelines retire multiple instructions per clock when conditions allow, while cache misses, branch mispredictions, and memory stalls drop the effective IPC (instructions per cycle) below 1. The headline GHz number has been a misleading marketing metric since the early 2000s; today, performance is far better described by SPEC CPU benchmarks or workload-specific scores. CPU frequency also varies dynamically — Intel Turbo Boost and AMD Precision Boost push frequencies above the nominal clock for short bursts when thermal headroom allows. Related: decibel, bps. Reference: BIPM SI Brochure — Derived units.