Band-stop filter

A bandstop filter, also known as a bandstop filter or bathtub filter in audio engineering, is a filter in electrical engineering that attenuates a specific, usually broad frequency band and does not allow it to pass in the limit case. In analogue circuit structures, series and parallel resonant circuits are reversed in relation to the bandpass filter, resulting in the opposite behaviour.

In contrast to a notch filter, which has only a single zero point in the transfer function, which is ideally placed exactly on a single frequency to be suppressed, the transfer function of the bandstop can also include different zero points and thus cover a wider bandwidth in the stopband.

The cut-off frequencies of a bandstop are defined as those frequencies at which the ratio of output to input voltage is

or -3 dB respectively.

The center frequency is the geometric mean of the upper and lower cutoff frequency:

In the case of the static frequency response change, the emphasis and the de-emphasis, the time constant is usually specified instead of the cut-off frequency.