A filtering device that permits only those components in the frequency domain whose frequencies lie below one critical value or above another (higher) critical value to pass through with little attenuation; all components whose frequencies lie between the critical values are attenuated.

If the two critical values are very close together, the device is called a notch filter. Notch filters are used, for example, in modems in order to prevent certain components of the data signal from interfering with equipment in the telephone system. See also band-pass filter, high-pass filter, low-pass filter.