Frequency is the number of complete cycles per second; the SI unit is the hertz (Hz). Frequency applies broadly: electromagnetic waves (radio, microwave, infrared, visible light, X-rays), sound waves, electrical signals, and mechanical rotation are all described by frequency. Human hearing spans 20 Hz (low bass) to 20,000 Hz (20 kHz). Radio frequencies range from 3 kHz (maritime very low frequency) to 300 GHz (millimetre-wave). Wi-Fi operates at 2.4 GHz and 5 GHz; 5G millimetre-wave bands reach up to 100 GHz.
Revolutions per minute (RPM) measures rotational speed. Car engines idle at 600–900 RPM and redline at 5,000–8,000 RPM; electric motors can exceed 20,000 RPM. Converting RPM to Hz: divide by 60. A motor spinning at 3,600 RPM = 60 Hz, which is also the frequency of North American AC power — the reason synchronous motors were common in wall clocks. The wave–particle relationship E = hf links frequency to photon energy via Planck's constant h = 6.626 × 10⁻³⁴ J·s.
| 1 kHz | = 1,000 Hz |
| 1 MHz | = 1,000,000 Hz |
| 1 GHz | = 10⁹ Hz |
| 1 RPM | = 1/60 Hz |
| 60 Hz | = 3,600 RPM |
| Middle A (music) | = 440 Hz |