stopband

In signal processing, the stopband of a filter is the frequency range in which the filter attenuates or rejects signals, in contrast to the passband where frequencies pass with minimal attenuation. The boundary between these regions is called the transition band, and the stopband begins at the stopband edge, the frequency at which the specified stopband attenuation is first met.

Filter specifications typically specify passband ripple and stopband attenuation, measured in decibels. Stopband attenuation indicates how much unwanted energy in the stopband is suppressed; higher dB values correspond to stronger rejection. In digital filters, frequencies are often expressed as normalized values relative to the sampling rate (0 to 0.5 in Nyquist units).

Stopbands are essential to all filter types, including low-pass, high-pass, band-pass, and band-stop (notch) filters. A band-stop filter is designed to suppress a particular frequency range, leaving the rest of the spectrum within the passband as intact as possible. The exact shape and width of the stopband depend on the design and order of the filter, as well as any chosen ripple constraints.

The sharpness of the stopband depends on the filter order and design method. Higher order filters or specialized design approaches produce steeper transitions. Common analog prototypes include Butterworth (maximally flat passband), Chebyshev (ripple in the passband or stopband), and elliptic (ripple in both). Digital filter design uses analogous concepts with attention to phase response and group delay, in addition to magnitude attenuation.