In telecommunications, the term cutoff frequency f_c 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 - 3dB = 70.1% of the reference voltage level of 0dB = 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. [1] (http://www.sengpielaudio.com/calculator-geommean.htm)

A level drop of - 3dB means a voltage reduction to 70,1 % and also a power reduction to 50%, because V² is proportional to 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.
See also: low-pass filter and high-pass filter.

External link

• Calculation of the center frequency with geometric mean and comparison to the arithmetic solution (http://www.sengpielaudio.com/calculator-geommean.htm)