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A sine wave with varying frequency

The hertz (symbol: Hz, definition: 1/s) is a unit derived from time which measures frequency in the International System of Units (SI). Frequency is how often something happens. A frequency of 1 hertz means that something happens once a second. The note Middle C (the C in the middle of the piano) is 261.65 (262) Hz. This means there are 262 vibrations every second, which is heard as the note Middle C, when that piano key is played.

Humans are able to hear sounds between 20-20 000 Hz.[1] Owls are able to hear sounds between 200–12000 Hz.[2]

The basic unit is 1/second. The unit is named after a German physicist Heinrich Rudolf Hertz.

Multiple units

1000 hertz is 1 kilohertz. 1000 kilohertz is 1 megahertz, 1000 megahertz is 1 gigahertz: (see table)

Unit Equal to:
Kilohertz (kHz) 1000 Hz
Megahertz (MHz) 1000 kHz
Gigahertz (GHz) 1000 MHz
Terahertz (THz) 1000 GHz
Petahertz (PHz) 1000 THz
Exahertz (EHz) 1000 PHz
Zettahertz (ZHz) 1000 EHz
Yottahertz (YHz) 1000 ZHz