Cutoff frequency

In telecommunication, the term cutoff frequency 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% to the reference 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.

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.