# Cutoff frequency

In telecommunications, the term cutoff frequency fc has the following meanings:

1. The frequency either above which or below which the output of a circuit, such as a line, amplifier, or filter, is reduced to the specified level of - 3 dB = 70.7 % of the reference voltage level of 0 dB = 100 %. See also time constant and center frequency. The calculation of the center frequency of such a frequency band is not the arithmetic mean but the geometric mean. See external link.  (http://www.sengpielaudio.com/calculator-geommean.htm)

A level drop of (-)3 dB means a voltage reduction to 70,7 % and also a power reduction to 50 %, because voltage V2 is proportional to power P.
2. The frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by reflection from the layer.

A bandpass has two cutoff frequencies and a center frequency.

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