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A stopband is a band of frequencies, between specified limits, that a circuit, such as a filter or telephone circuit, does not allow to pass. A band-stop filter or bandpass filter has a range of frequencies that are attenuated to very low levels; this range is the stopband. It is the opposite of the passband, and the presence of one implies the presence of the other.

Specifications
An ideal filter completely blocks all frequencies inside of the stopband. However, due to the limitations of physically realizable circuits, frequencies inside of the stopband instead are attenuated to very low levels. The stopband of a filter specifically refers to the portion of a frequency signal that is attenuated by a filter or a combination of filters.

The lower and upper limiting frequencies, also denoted lower and upper cutoff frequencies, are the boundaries of the stopband. Cutoff frequencies are also referred to as -3dB frequencies. This term refers to the fact that at this frequency, the power delivered is halved, resulting in a loss of 3dB. In a low pass filter, the stopband ranges from the a lower cutoff frequency up to an infinite frequency. Conversely, in a high-pass filter, the stopband consists of the frequencies from 0 hertz to an upper cutoff frequency. A band-stop filter has one stopband, specified by two non-zero and non-infinite cutoff frequencies. The difference between the limits in the band-stop filter is the stopband bandwidth, which usually is expressed in hertz. A bandpass filter typically has two stopbands. The stopband of a filter can characterized by its shape factor. Stopband shape factor is the ratio between the bandwidth and the difference between the two frequencies that have a specified attenuation level. In other words, it is the ratio difference between the -3dB frequencies and the difference between the -ndB frequencies, where n is an arbitrary number. The phenomenon of stopband ripple refers to slight fluctuations in a filter's frequency response, caused by imperfections within a chosen filter.

Examples
Frequently, the stopband of a filter will be matched with the frequency ranges of a source of constant Interference or noise. This will subsequently eliminate the noise, while leaving most of the signal intact. If the frequency range is to be very specific, a Notch filter can be applied. The stopbands of many Notch filters are often set to attenuate power line hum, referred to as Mains hum. The specific frequency of hum differs depending of the line, and is most often between 50 and 60 Hz.